CHAPTER
11 Sensors, Digital
Electronics, and
Multiplexing
Learning Objectives
After studying this chapter, you should be able to:
n List the various types of sensors used on a modern truck.
n Explain the differences between a digital and an analog signal.
n List the main logic gates and develop the truth table for each gate.
n Describe the types of memory commonly used in an electronic module.
n List the four main types of electronic module inputs.
n Explain the concept of a pull-up and pull-down resistor in an electronic module input circuit.
n Discuss the various forms of multiplexing used on a modern truck.
Key Terms flash memory non-volatile memory
floating parameter group number (PGN)
active-high gate parameter identifier (PID)
active-low Hall effect pull-down resistor
analog multiplexing idle validation switch (IVS) pull-up resistor
analog to digital (A/D) converter impedance random access memory (RAM)
backbone integrated circuit reference voltage
binary J1587 resistance temperature detector
bit J1708
bus J1939 (RTD)
byte logic sensor
controller area network (CAN) message identification (MID) shield
data link microprocessor signal conditioning
digital multiplexing source address
EEPROM stub
electronic service tool (EST)
339
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340 Chapter 11
suspect parameter number (SPN) thermocouple variable reluctance
terminating resistor time-division multiplexing volatile memory
thermistor transfer function Wheatstone bridge
INTRODUCTION SENSORS
Sensors are an important part of a modern truck’s Humans have five senses: sight, hearing, smell,
electrical system. Sensors convert some physical taste, and touch. These five senses permit your brain to
property such as pressure or temperature into an process what is occurring in the world around you. The
electrical signal. Sensors are also known as transducers eyes, ears, nose, taste buds, and skin could be thought
or sending units. The basic principles of some common of as being sensors. Sensors are used by the various
sensors will be examined in this chapter. electronic modules on a modern truck to gather a va-
riety of information.
This chapter will also introduce digital electronics,
which is the basis of modern truck electronic control Temperature Sensors
systems. Multiplexing is also presented.
Your skin provides your brain an indication of
AMPLIFIERS temperature. Nerves carry an electrical signal from the
skin to the brain, which you interpret as temperature.
An amplifier is utilized to increase the amplitude or In a similar manner, temperature sensors throughout a
strength of a low-level signal as shown in Figure 11-1. modern truck electrical system inform the various
Amplifiers are also known as amps, which is not to be electronic control modules the temperature of the en-
confused with the unit of measurement of electric cur- gine coolant, the manifold air temperature, the air
rent. An amplifier is typically drawn as a sideways tri- conditioning evaporator inlet temperature, and so
angle symbol, as illustrated in Figure 11-2. The input forth. A modern truck may have more than 20 different
side of the amplifier is the base of the triangle and may temperature sensors of various types. The most com-
be shown with one or two inputs. The output terminal is mon types of temperature sensors will be evaluated.
at the point of the triangle. The amplifier shown in
Figure 11-2 is called a differential amplifier. A differ- Thermistors. Thermistors are two-terminal
ential amplifier amplifies the difference in the voltage temperature-measurement sensors that are typically
between its two input terminals. This type of amplifier is made of semiconductor material. Thermistors are one
also known as an operational amplifier or op amp. An of the most common types of temperature measure-
operational amplifier is able to perform various mathe- ment sensors found on a modern truck. The resistance
matical operations such as addition, subtraction, multi- measured between the two terminals of the thermistor
plication, and even some calculus operations. A detailed changes proportionally to the temperature being mea-
study of operational amplifiers is outside the scope of sured. Most thermistors are constructed of semicon-
this book, although they are quite fascinating devices. ductor material and have a negative temperature
Some of the OEM-provided information in the drawings coefficient (NTC) as described in Chapter 6. You may
in this chapter show basic amplifier circuits. It is only recall from algebra that the graph of a line that points
important at this point to understand that an amplifier downhill from left to right has a negative slope. NTC
(amp) increases the amplitude of a low-level electrical indicates that the resistance of the thermistor decreases
signal. For more information on operational amplifiers, as the temperature being measured increases, as
see the Internet links at the end of this chapter. shown in Figure 11-3. This particular graph is the
Transistor © Cengage Learning 2014
amplifier
Input Output
signal signal
voltage voltage
Figure 11-1 An amplifier increases the amplitude of a low-level signal.
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 341
Differential Amplifier 50
+Vsupply
– 40
© Cengage Learning 2014
Input1
Resistance (K ohms)
Output
© Cengage Learning 2014
Input2 + 30
–Vsupply
20
Figure 11-2 A differential amplifier intensifies the 10
difference in voltage between its two inputs.
temperature-resistance curve for an air conditioning 0 25 50 75 100 125
evaporator temperature thermistor. The curved line –25 0
indicates that the sensor output is nonlinear, so the
change in resistance for a specific change in temper- Temperature (°F)
ature is not constant.
Figure 11-3 Air conditioning system thermistor
The resistance versus temperature curve of NTC resistance versus temperature.
thermistors varies depending upon the expected tem-
perature range that the thermistor will be measuring. output is nonlinear, thermistors have a limited tem-
The circuit shown in Figure 11-4 is an engine coolant perature range where the measurement system accu-
temperature (ECT) sensor circuit. The thermistor re- racy is acceptable. For example, an electronic module
sistance versus temperature curve for this ECT sensor could not accurately determine the difference between
is different from the curve shown for the air condi- 1238F (518C) and 1258F (528C) for the thermistor plot
tioning sensor shown in Figure 11-3. This is because shown in Figure 11-3 because the change in resistance
engine coolant and an air conditioning evaporator have of the thermistor may be less than 1O. At extremely
very different normal operating temperature ranges. low temperatures, an electronic module might not be
One disadvantage of thermistors is that because the
Thermistor 5 V-Ref Current limiting resistor
• V-Ref connection
• Ground Reference voltage
regulator
connection
2 terminals Input conditioners Microcomputer Output
drivers
ECT
AMP Microprocessor
Analog MM M
to EE E
MM M
digital OO O
converter RR R
YY Y
Resistance
Courtesy of Navistar, Inc.
Signal return provided through processor
Temperature
The chart indicates resistance of a thermistor decreases as temperature increases.
Output of thermistor is not linear.
Thermistor Engine Coolant Temperature (ECT)
Figure 11-4 Engine coolant temperature (ECT) sensor circuit.
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
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342 Chapter 11
able to accurately determine the difference between an Cold junction with
open circuit with infinite resistance caused by a dis- electronic module and gauge
connected wiring harness connector and the very high
resistance of the thermistor at À258F (À328C). How- Constantan Pure iron wire
ever, the electronic module should be able to accu- wire
rately determine the difference between 328F (08C)
and 348F (18C), which is an important temperature
range for an air conditioning evaporator temperature
sensor.
Resistance Temperature Detectors. Resistance Hot junction Sensing bulb© Cengage Learning 2014
temperature detectors (RTD) are typically two-
terminal temperature measurement sensors constructed Figure 11-6 Type J thermocouple.
of a thin metal wire. Metal has a positive temperature
coefficient (PTC), meaning as the temperature of the thermocouple. Both ends of the two wires are welded
metal increases, the resistance also increases. This is together to form two wire junctions. The junction
illustrated in Figure 11-5. The plotted line points up- where the temperature is being measured is referred to
hill from left to right, indicating a positive slope. as the hot junction, while the reference junction with a
Notice that the plotted line is also straight or linear, known temperature is referred to as the cold junction.
unlike the curved nonlinear plot of the thermistor. This A phenomenon known as the Seebeck effect states that
linear property makes RTDs very accurate over a wide the difference in temperature between the two junc-
temperature range. RTDs are used to measure tem- tions of dissimilar metals produces a low-amplitude
peratures up to about 12008F (6508C), which makes voltage that increases as the difference in temperature
them ideal for diesel exhaust aftertreatment system between the two junctions increases. The voltage pro-
temperature measurements. Many RTDs are made duced by the temperature difference between the two
from platinum, so they may also be known as platinum junctions is used to determine the temperature of the
resistance thermometers (PRT). thermocouple hot junction.
Thermocouples. Thermocouples are temperature The voltage produced by a type J thermocouple is
measurement sensors that are constructed from two only about 30mV (0.030V) when the temperature
dissimilar metals. A thermocouple is formed by join- difference between the hot junction and the cold
ing two wires made of different metals such as iron junction is 11008F (6008C). Therefore, thermocouples
and constantan, as shown in Figure 11-6. This par- require electronic circuitry to amplify the very low
ticular combination of metals is known as a type J amplitude voltage that is produced. The electronic
circuitry must also perform an accurate temperature
350 measurement of the cold junction temperature because
the voltage produced is proportional to the difference
300 in temperature between the two junctions. A thermistor
or special type of diode is typically utilized to measure
250 the cold temperature junction within the electronic
module.
200
Thermocouples are often used to measure diesel
150 exhaust gas temperatures in the exhaust manifold for
an instrument called a pyrometer. Thermocouples are
100 also used to measure the temperatures within the ex-
haust aftertreatment system on some engines.
50
A thermocouple temperature measurement tool is
0 available that permits many high-end DMMs to be
0 100 200 300 400 500 600
Temperature (°C)
Figure 11-5 RTD temperature versus resistance.
Resistance (ohms)
© Cengage Learning 2014
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 343
used to measure temperature. These are very useful These position measurement devices can also be used
tools for troubleshooting problems with temperature to determine the rotational speed of a component, such
sensors. For example, the temperature of the engine as the transmission output shaft speed. The vehicle
coolant indicated by the DMM thermocouple can be speed can then be calculated by applying the rear axle
compared to the temperature indicated by the engine ratio and tire revolutions per mile or per km value to
coolant temperature sensor. However, inaccuracy can the transmission output shaft rotational speed.
occur if the coolant temperature sensor is measuring
the temperature of the coolant in the block and the Potentiometers and Rheostats. Potentiometers and
thermocouple is placed in the expansion tank. The sen- rheostats were described in Chapter 6. These simple
sor being tested and the thermocouple must be placed as resistive devices can be used to measure the angular
close as possible to each other for an accurate temper- position of the accelerator pedal, a fuel level float arm,
ature comparison. or other device. A mechanical linkage is connected to
the movable device and the wiper of the potentiometer
Because a thermocouple operates on the principle or rheostat. In the case of a potentiometer used as an
of dissimilar metals producing a voltage, you should accelerator position sensor, depressing the accelerator
not attempt to repair thermocouple wire. Splice clips, causes the wiper to rotate. The corresponding change
solder, and butt splices used for wire repair will create in voltage provided at the wiper terminal of the po-
a dissimilar metal junction resulting in temperature tentiometer indicates the accelerator position. To pre-
measurement error. Damaged thermocouple wire must vent unintentional acceleration should some wiring
be replaced. Many thermocouple temperature mea- problem cause the accelerator position sensor voltage
surement devices, such as exhaust aftertreatment sen- to increase without the wiper actually moving, many
sors, do not have any electrical connectors between the diesel engines incorporate an idle validation switch
thermocouple and the electronic module. The two or (IVS). The switch closes or opens when the accelerator
three thermocouples for aftertreatment temperature is off-idle to validate that the truck operator is de-
measurement are typically integrated into a remote pressing the accelerator. If a wiring problem such as a
thermocouple electronic module assembly instead of wire-to-wire short causes a voltage to be present at the
connecting the thermocouple directly to the engine ECM accelerator input and the ECM does not detect
ECM. Damaged thermocouple wire may require re- that the idle validation switch indicates an off-idle
placement of the entire thermocouple and electronic condition, the ECM will ignore the accelerator position
thermocouple module assembly. sensor input signal from the potentiometer and limit
engine speed to some value.
If the thermocouple wire insulation becomes
overheated and melts, the two dissimilar conductors Some accelerator position sensors utilize two op-
can make contact with each other to form an inad- positely wired potentiometers to produce two different
vertent second hot junction. This is a common failure voltages. For example, one sensor voltage will increase
with exhaust aftertreatment sensors should the sensor from 1V to 4V when the accelerator is depressed; the
wiring not be correctly routed and clipped. The elec- other sensor voltage will decrease from 4V to 1V when
tronic thermocouple module may not be able to detect the accelerator is depressed. If a wiring problem causes
this failure electrically and will measure the tempera- a voltage to be present at one of the ECM accelerator
ture of this second hot junction instead of the actual position sensor inputs and this voltage does not cor-
hot junction temperature. In the case of exhaust af- respond to the voltage measured by the ECM for the
tertreatment, the difference in temperature between the other sensor, the ECM will ignore the inputs and limit
real hot junction and the second false hot junction can the engine speed to some value.
be several hundred degrees, resulting in problems re-
lated to the aftertreatment system. Details on exhaust Variable Reluctance Sensors. Variable reluctance
aftertreatment are discussed in Chapter 14. sensors are basically miniature AC voltage generators.
Variable reluctance sensors are also known as mag-
Position and Rotational netic sensors or a variety of similar terms. A variable
Speed Sensors reluctance sensor consists of a coil of wire wrapped
around a permanent magnet (Figure 11-7). A low-
Modern trucks have a variety of sensors that are reluctance rotor, known as a target, timing disk, or a
used to detect the position of some mechanical device. variety of similar terms, rotates past the tip of the
These can be as simple as the position of the accel- sensor. The target has raised teeth like a gear. In some
erator (foot throttle) or as complex as the precise cases, the target is also an actual gear. The target is
position of the crankshaft with the engine running.
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
344 Chapter 11
N N
A B
SS © Cengage Learning 2014Target
© Cengage Learning 2014
Target Target
Figure 11-7 Variable reluctance sensor. d
driven by the engine camshaft, crankshaft, transmis- Zero cross
sion output shaft, axle shaft, or other device for which eg
the rotational speed, and in many cases the position, is
to be measured. eout
Reluctance is a property of a material that describes t1 t2 t3
how easily magnetic lines of force can be directed
through the material, as described in Chapters 3 Figure 11-8 Variable reluctance sensor output vol-
and 7. Materials such as iron and steel have very low tage (eg) with target moving from right to left.
reluctance. A change in reluctance near the tip of the
sensor causes the magnetic lines of force created by of the magnetic field as it expands outwardly causes
the permanent magnet in the sensor to be directed into the magnetic lines of force to cut through the coil in
the low-reluctance material as it passes in front of the the opposite direction, compared to when the target
sensor. This change in the shape of the magnetic field tooth was approaching the magnet. This outward ex-
is the basic principle of variable reluctance sensors. pansion of the magnetic field causes a voltage to be
induced in the coil of wire with a polarity opposite of
When a tooth of the target is not close to the per- the voltage induced when the magnetic field was
manent magnet in the sensor, the magnetic lines of moving inwardly, thus producing a negative voltage.
force caused by the permanent magnet surround the Therefore, a complete sine waveform is produced as a
magnet, as shown in Figure 11-7A. When a tooth of tooth and valley of the target pass in front of the
the target rotates so that it nears the tip of the per- sensor, as shown by the eg trace in Figure 11-8.
manent magnet, the magnetic lines of force are re-
directed from their normal paths and are concentrated The frequency of the voltage waveform generated
inwardly by the presence of the tooth on the low- by the variable reluctance sensor indicates the rotational
reluctance target (Figure 11-7B). The magnetic lines speed of the target. Truck antilock brake systems (ABS)
of force moving inwardly cut through the coil wrapped use variable reluctance sensors to measure individual
around the sensor magnet. Cutting magnetic lines of wheel speeds. A rotating tone ring or tone wheel with
force through a coil of wire causes a voltage to be square-cut teeth similar to a gear passes close to the
induced in the coil, as shown by the trace labeled eg in wheel speed sensor to act as a low-reluctance target.
Figure 11-8. In this illustration, the target is moving The tone wheel used in an ABS system may be ma-
from right to left across the tip of the variable reluc- chined or cast as webs in the backside of a brake rotor,
tance sensor, shown at the top of Figure 11-8. or it may be a stamped metal ring attached to the wheel
hub on drum brakes. The tone wheel passes in front of
When the tooth of the target is directly in front of the stationary sensor as the wheel is rotated. The sensor
the magnet contained within the variable reluctance output is an AC voltage with a frequency directly pro-
sensor, the magnetic lines of force have stopped portional to wheel speed. Details on ABS are discussed
moving inwardly and are stationary. This causes the in Chapter 15.
voltage induced in the coil to decrease to zero, as
shown in Figure 11-8. Variable reluctance sensors may also be used as
both speed and position sensors, such as engine cam-
When the low-reluctance target tooth is moving shaft position and crankshaft position. A target with 60
away from the sensor, the magnetic lines of force sur-
rounding the magnet begin to return outward to their
original shape shown in Figure 11-7A. The movement
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 345
or more teeth is attached to the crankshaft and provides The amplitude of the voltage produced by the vari-
the engine ECM with information about engine speed able reluctance sensor must be high enough to be de-
as well as engine crankshaft position. This crankshaft tected by the electronic module that is connected to the
target typically has a single tooth missing, corre- sensor. The amplitude of the voltage produced by vari-
sponding to a specific crankshaft position. The missing able reluctance sensors largely depends on two factors:
tooth on the target causes a gap in the output wave- the air gap and the rotational speed of the target. The air
form, as shown in Figure 11-9. The missing tooth is gap is the distance between the tip of the sensor and the
used as an initial starting point (i.e., cylinder #1 TDC). tooth of the target. The larger the air gap, the lower the
To know if the #1 cylinder is on the compression or voltage produced by the sensor. This occurs because
exhaust stroke, a variable reluctance sensor on the the variable reluctance sensor depends on a distortion
camshaft may also be used in conjunction with the of the magnetic field to generate a voltage. The closer
crankshaft position sensor. The camshaft target may the tooth of the tone wheel is to the tip of the sensor,
have one tooth for each cylinder, plus one extra tooth the greater the distortion of the magnetic field and the
or one larger tooth that corresponds to the missing more lines of force there are to cut through the coil of
tooth on the crankshaft target (see Figure 11-14). wire. Some variable reluctance sensors have adjustable
Alternatively, the camshaft target may be a single air gaps. The air gap adjustment procedure varies, so
raised pin on the face of the camshaft gear indicating a consult the specific OEM’s information for instructions.
reference position. Using the signals from both the
camshaft and crankshaft position sensors, the engine An ABS wheel-speed sensor with too great an air
ECM ‘‘knows’’ the crankshaft position to a high de- gap will cause a problem in the ABS system because
gree of accuracy when the engine is running. Crank- the amplitude of the signal is too low to be accurately
shaft position is vital information for an electronically measured. ABS systems can also detect loose wheel
controlled diesel engine, as will be discussed in bearings or warped or bent tone wheels based on signal
Chapter 14. amplitude. These defects all may cause variation in the
air gap throughout rotation or during cornering, which
Some older electronically controlled diesel engines is detected as variation in the amplitude of the volt-
may only use a camshaft position sensor with a multiple- age produced by the sensor as the wheel is rotated
toothed target to determine crankshaft position. (Figure 11-10). This variation in amplitude is also the
fundamental of AM radio, which uses amplitude
modulation (AM) to carry a signal.
Missing-tooth Tech Tip: Proper wheel-bearing adjustment is
region very important on trucks with ABS because a
loose wheel bearing can cause the ABS sensor
to be pushed away from the tone wheel,
resulting in an inconsistent air gap between
the sensor and tone wheel. Just a small increase
in air gap between the tip of the sensor and the
tone wheel will result in a large decrease in the
voltage produced by the sensor.
© Cengage Learning 2014
© Cengage Learning 2014
Figure 11-9 Variable reluctance crankshaft position The other factor that determines the amplitude of the
voltage produced by a variable reluctance sensor is the
sensor output voltage.
Figure 11-10 Amplitude modulation of wheel speed sensor signal with misadjusted wheel bearings.
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
346 Chapter 11
speed at which the teeth of the tone wheel pass in front The manner in which the Hall effect produces a
of the sensor. The faster the tone wheel passes in front voltage is very different from electromagnetic in-
of the sensor, the greater the amplitude of the voltage duction. Electromagnetic induction requires cutting
produced by the sensor. This occurs because of the magnetic lines of force. The Hall effect generates a
principle of magnetic induction, according to which voltage with no relative motion between the semi-
the more magnetic lines of force that cut through a conductor material and the magnetic lines of force.
conductor per second, the higher the amplitude of the The Hall voltage is generated by the presence of a
voltage induced in the conductor. Peak ABS wheel magnetic field without cutting any magnetic lines of
speed sensor output voltage at 2 mph (3.2 km/h) may force. The Hall voltage can be a DC voltage, provided
only be 1V or so, while at 60 mph (96 km/h) the same the magnetic field strength remains at a constant
sensor output voltage may exceed 35V. For this rea- level.
son, variable reluctance sensors are typically not used
where slow movement must be detected. If the am- The Hall effect can be used to produce a Hall
plitude of the voltage signal produced by the sensor is effect switch or sensor. A Hall effect switch typically
too low, the electronic control module may not be able has three terminals and contains a permanent magnet
to accurately detect the frequency of the signal. This to generate a magnetic field. Two terminals provide a
low speed-sensing limitation of variable reluctance supply voltage and a ground to power the electronics
sensors is not a factor with ABS systems because ABS in the sensor. The third terminal is the sensor output.
is not active when vehicle speed drops below a few The Hall voltage is amplified inside the sensor and is
miles per hour. used to switch a transistor on and off to indicate the
presence of the magnetic field. The example shown
For an engine crankshaft or camshaft position in Figure 11-12 is a Hall effect switch-type camshaft
sensor, too large of an air gap can result in an engine position sensor that uses a switching transistor to
no-start in extreme cold temperatures. The combina- make or break a path to ground. This switching
tion of large air gap and slow cold cranking speed action all takes place with no physical contact be-
results in the amplitude of the signal produced by the tween the Hall effect sensor and the metal target. The
sensor to be too low for the engine ECM to recognize a timing disk is installed to the front of the camshaft.
valid pattern; thus, no fueling occurs. Note in this example that the timing disk has one
‘‘window’’ that is wider than the others are. The
Hall Effect Sensors. Passing a current through a thin signal produced by the Hall effect sensor would be a
layer of semiconductor material causes a difference in square waveform with one longer off pulse when this
potential (voltage) to be developed between the edges wider window passes in front of the Hall effect
of the semiconductor material when exposed to a sensor. The engine ECM uses this wider pulse to
magnetic field. The level of the voltage produced in detect the camshaft position. The ECM can then infer
the semiconductor material is directly proportional to crankshaft position and speed based on the camshaft
the strength of this magnetic field. This principle is position and speed.
known as the Hall effect (Figure 11-11). The voltage
produced due to the Hall effect is referred to as the The width of the magnetic target or tooth that is
Hall voltage. passed near the Hall effect sensor influences the length
of time the signal is generated by the Hall effect sensor
Input + Input + N Input + N
circuit circuit S circuit S
MV MV MV © Cengage Learning 2014
No magnet Increasing magnetism Decreasing magnetism
No Hall effect Increasing Hall voltage Decreasing Hall voltage
Figure 11-11 Hall effect principle.
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 347
Air gap Timing sensor disk
(located on face of
camshaft gear)
Power supply Permanent
for sensor magnet
5 Volt Transducer
reference Signal
from ECM conditioner
Vane Window
CMP
Grd.
Internal Courtesy of Navistar, Inc.
pull up
resistor
ECM
Hall effect sensor (camshaft position sensor)
Figure 11-12 Hall effect switch used to measure camshaft position and engine speed.
Magnet Hall effect Sensor
sensor #6 #1
#5 #2
Hall element
Target
#4 #3
tΔC Sensor output signal
#1 #2
eout © Cengage Learning 2014 #3
© Cengage Learning 2014
Figure 11-13 Hall effect sensor output voltage (eout) Reference
when target is moving from right to left. mark
Figure 11-14 Camshaft target for Hall effect or
variable reluctance sensor.
(Figure 11-13). The bottom trace in Figure 11-13 extra tooth shown in Figure 11-14 creates a reference
shows the sensor output voltage, designated as eout, mark in the sensor output signal, which lets the engine
corresponding to the position of the target as it moves ECM ‘‘know’’ that the next large camshaft target tooth
from right to left across the tip of the Hall effect detected corresponds to cylinder #1.
sensor.
As discussed in previous chapters, the magnetic
Figure 11-14 illustrates a typical camshaft target, field surrounding a current-carrying conductor is di-
which could be used with a Hall effect switch or a rectly proportional to the current flowing through a
variable reluctance camshaft position sensor. The small conductor. Therefore, the Hall effect can be used to
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
348 Chapter 11
determine the amount of current flowing through a DIGITAL MULTIMETER 2,000 ohms
conductor by measuring the magnetic field caused by
the current flow. This is how a clamp-on DC current RECORD MAX MIN
probe operates. The voltage produced by the Hall ef-
fect sensor contained in the clamp-on current probe 2,000 ohms%
corresponds to the strength of the magnetic field HZ
generated by the current flow.
0 1 23 4 5 6 78 90
Pressure Sensors
Strain sensitive pattern MIN MAX HZ
There are a variety of pressure sensors on a modern
truck. A number of pressure sensors are necessary for Tension: mV mA
the diesel exhaust emission control system. These in- Area narrows, V A
clude intake manifold pressure, exhaust manifold resistance increases V A
pressure, and diesel particulate filter differential (delta)
pressure. Pressure sensors may also be used to calcu- A mA A COM V
late flow, as in the case of an exhaust gas recirculation
(EGR) differential pressure sensor, which measures the DIGITAL MULTIMETER 3,000 ohms
pressure drop across a venturi. This is analogous to
measuring the voltage drop across a known value of RECORD MAX MIN
resistance to determine the current flow.
3,000 ohms%
HZ
0 1 23 4 5 6 78 90
MIN MAX HZ
mV mA
V A
V A
A mA A COM V
Potentiometric Pressure Sensors. Potentiometric © Cengage Learning 2014Compression:DIGITAL MULTIMETER 1,000 ohms
pressure sensors are simple potentiometers where the © Cengage Learning 2014
change of length of a Bourdon tube within the sensor Area thickens, RECORD MAX MIN
causes the potentiometer wiper to move corresponding resistance decreases
to the pressure being measured (Figure 11-15). The 1,000 ohms%
result is an output voltage that is proportional to the HZ
pressure being measured. Potentiometric sensors may
be found on some older trucks as sensors (sending 0 1 23 4 5 6 78 90
units) for instrumentation.
MIN MAX HZ
mV mA
V A
V A
A mA A COM V
Strain Gauge Pressure Sensors. A strain gauge is a Figure 11-16 Strain gauge resistance change due to
thin flexible material imprinted with a zig-zag pattern deformation.
of an electrically conductive material, as shown in
Figure 11-16. When subjected to force, the flexible corresponding narrowing or thickening of the conduc-
material stretches resulting in an increase or decrease tive material. The change in length and thickness causes
in the length of the conductive track, as well as a a small change in the resistance of the conductive track.
The strain gauge is attached to a flexible diaphragm
Potentiometric Electrical inside a sensor. Strain gauge type pressure sensors are
element connector constructed so that a reference pressure chamber with a
known pressure is situated on one side of the dia-
Ball bearing Wiper arm phragm, while the other side of the diaphragm is sub-
Helical jected to the pressure being measured. As the pressure
Bourdon being measured increases, the strain gauge deforms with
tube the diaphragm resulting in a change of resistance. A
strain gauge pressure sensor that makes use of the
Pressure change in resistance of a semiconductor material is
fitting known as a piezoresistive sensor.
Figure 11-15 Potentiometric pressure sensor. The change in resistance in a strain gauge pressure
sensor is typically converted to a voltage by an ar-
rangement called a Wheatstone bridge. A Wheatstone
bridge is a diamond-shaped arrangement of three fixed
resistors of known value, plus a variable resistance
identified as RS in Figure 11-17. This is really just a
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 349
–+ Voltage RS R2 Amplifier
Battery regulator –sVeonltsaogre+ Output
circuit
R1 R3
© Cengage Learning 2014
Figure 11-17 Wheatstone bridge used with strain gauge (RS).
series-parallel circuit with a differential voltage mea- value of capacitance is the distance between the two
surement obtained between the two points shown, so plates of a capacitor. In a variable capacitance sensor,
do not let the diamond shape of the resistor arrange- one plate of a capacitor is fixed while the other plate
ment be intimidating. The Wheatstone bridge is con- is attached to a flexible diaphragm as shown in
tained within the sensor. As the value of RS (the strain Figure 11-18. One side of the moveable plate is ex-
gauge) changes when pressure is increased, the voltage posed to a reference pressure, such as a pure vacuum.
measured by the sensor will change accordingly. The The other side of the movable plate is exposed to the
difference in voltage between the two terminals of the sensed pressure. As the sensed pressure increases,
Wheatstone bridge caused by a pressure change may the distance between the movable capacitor plate and
only be a few millivolts. However, a differential am- the fixed capacitor plate decreases resulting in an in-
plifier contained within the sensor amplifies the dif- crease in the capacitance value. If the reference pres-
ference in voltage across the terminals of the sure is a vacuum, then the sensor is an absolute
Wheatstone bridge to a usable level. pressure sensor. This indicates that this sensor can
measure a pressure less than atmospheric pressure.
Strain gauge type pressure sensors are typically
three-terminal sensors. A fixed supply voltage, such as An electronic circuit within the sensor converts the
+5V, a ground circuit, and a sensor output signal in- capacitance change into a corresponding voltage. In
dicating the measured pressure make up these three Figure 11-18, the sensor is supplied with a 5V ref-
terminals. The reference pressure chamber is typically erence voltage and ground. The output voltage at the
a pure vacuum, making the sensor an absolute pressure sensor signal terminal of the sensor is some percentage
sensor. However, the chamber may be vented to at- of the 5V reference voltage. For example, this sensor
mospheric pressure on some sensors, making the sen- may have an output voltage of 0.5V at the signal ter-
sor a gauge pressure type sensor. In the case of a minal when the sensor is measuring atmospheric
differential pressure sensor, such as the sensor used to pressure and 4.5V when measuring 80 psi (550 kPa). If
measure the diesel particulate filter (DPF) differential the pressure being measured were less than atmo-
(delta) pressure, one side of the strain gauge dia- spheric pressure, the output voltage would be less than
phragm is exposed to the DPF inlet pressure, and the 0.5V for this particular sensor. The output voltage of
other side of the diaphragm is exposed to the DPF the sensor is typically linear and increases at a rate
outlet pressure. The sensor output voltage is propor- directly proportionally to the pressure being measured
tional to the differential pressure measured across the (output voltage increases as the pressure being mea-
DPF. This differential pressure can be used by the sured increases).
ECM to determine the amount of soot in the DPF, as
will be explained in Chapter 14. Instead of a DC output voltage, some variable ca-
pacitance sensors may provide a pulse width modula-
Variable Capacitance Pressure Sensors. Variable tion (PWM) output signal where the duty cycle (on-
capacitance pressure sensors, also known as capaci- time) of the PWM signal is proportional to the pressure
tance discharge sensors, are three-terminal sensors that being measured. Electronic circuitry within the sensor
measure pressure through a change in value of an in- generates the variable PWM signal.
ternal capacitor. Recall that capacitance is measured in
units of farads. One of the factors that determine the Piezoelectric Pressure Sensors. Piezoelectric de-
scribes a phenomenon whereby a voltage is produced
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
350 Chapter 11
Plastic Sensed Flexible ceramic
housing pressure diaphragm
Gasket
Movable capacitor plate
(negative) plated on diaphragm
Fixed capacitor plate
(positive) plated on ceramic bed
Adhesive strip
Vent Sign(aglroreutnudrn)
Signal
Sealed reference V. ref. (5V)
pressure
Rigid ceramic bed
Electronic Area in © Cengage Learning 2014
circuit detail
Figure 11-18 Variable capacitance MAP sensor. Electrical
connectors
when certain crystals, like quartz, are subjected to or static pressure, only a change in pressure. Piezo-
mechanical stress. In other words, squeeze a rock and electric pressure sensors currently have minimal usage
produce a voltage. Piezoelectric pressure sensors on modern trucks. However, diesel fuel injectors or
measure a change in pressure by means of a quartz glow plugs someday may contain a piezoelectric
sensing element, as shown in Figure 11-19. Note that pressure sensor to measure the cylinder firing pres-
piezoelectric pressure sensors do not measure a steady sures. This will provide an indication of misfire for
exhaust emissions compliance as well as more precise
Electrical fueling and timing control.
connector
The sensor shown in Figure 11-19 operates on the
Integrated same principles as an accelerometer, a sensor used to
circuit amplifier measure acceleration. A rapid decrease in speed (ve-
Housing locity) is a negative acceleration while an abrupt
change in the direction of travel is a lateral accelera-
Seal ring © Cengage Learning 2014 tion. Accelerometers are used for air bag system crash
detection and advanced ABS systems as will be dis-
Acceration-compensating cussed in Chapter 15.
quartz plate and mass
The piezoelectric effect also works in the reverse;
Quartz sensing element apply a voltage to a quartz crystal and the crystal will
expand. The latest generation of high-pressure com-
Preload sleeve mon rail diesel fuel injectors makes use of this prin-
ciple, which will be discussed in Chapter 14.
Diaphragm
Chemical Composition
Figure 11-19 Piezoelectric pressure sensor. and Property Sensors
There are a variety of other specialized sensors
used on a modern truck. These include sensors that
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 351
determine the composition of the exhaust gas, such as further, the second hand on a mechanical clock is © Cengage Learning 2014
the amount of oxygen and oxides of nitrogen (NOx) never stopped at a particular number on the dial. The
that are present. Other sensors measure the quality of second hand moves in a continuous arc. However, a
the diesel exhaust fluid (DEF) or the engine oil con- digital clock that indicates the time of day to the
dition. The internal operation of these sensors is out- nearest second shows the same value of time for a
side the scope of this book, but additional information period of 1 second, even though time is actually
can be found by researching the Internet links at the continuous and does not stop for 1 second as indi-
end of this chapter. Future sensor technology related to cated by the digital clock.
diesel exhaust emissions control includes ammonia
(NH3) and soot sensors. Diesel exhaust emissions will Analog versus Digital as Related
be covered in Chapter 14. to Music
One simple property sensor is the water in fuel If a microphone was placed in front of a guitar and
(WIF) sensor found at the bottom of the fuel water a single string on the guitar was plucked, the micro-
separator or primary fuel filter housing. This sensor phone would convert the sound waves into an elec-
typically consists of two exposed electrodes with a trical signal. For the most part, the guitar string
parallel-connected resistor. An electrical current is vibrates at one main or fundamental frequency based
passed through the sensor. Diesel fuel and water have on how tight the string is tensioned and the weight
different resistances. If sufficient water is present, the (mass) of the string. An oscilloscope could be used to
difference in resistance is detected and a warning lamp display an amplified voltage waveform representing
is illuminated to alert the operator to drain the fuel the string vibration and the sound that is produced
water separator. (Figure 11-20).
DIGITAL ELECTRONICS If several strings were played at the same time,
such as in a chord, each string would vibrate at a
The term digital is often used in marketing the particular frequency. The microphone would convert
latest device. Something that is digital is typically these combined vibrations into an electrical signal
advertised as being superior to something with older representing the sound produced by the guitar. The
technology that is not digital. The term analog (ana- oscilloscope would display this single waveform as a
logue) is applied to the outdated device. Television complex waveform that appears to have a pattern, but
signals stopped being broadcast in analog format in not like a sine waveform. Playing other chords after
2009 in the United States and were replaced with a the first chord and adding other instruments to form a
digital signal. band would cause the signal produced by the micro-
phone to be a waveform similar to that shown in
One of the simplest comparisons between analog
and digital is the difference between a light switch and Scope
a variable light dimmer control. The light switch can
be in only one of two states: on or off. By comparison, Amp
a light dimmer control can provide a continuous range
of illumination levels, from off to dim to fully Figure 11-20 Plucking a single guitar string produces
illuminated. a fundamental frequency shown on the oscilloscope,
plus harmonic frequencies.
Another comparison between analog and digital is
the measurement and display of time. A conventional
mechanical clock or wristwatch with hands that rotate
continuously is an analog clock. An electronic clock
that displays time with digits, such as with LEDs, is a
digital clock. The hands on an analog clock have a
continuous motion and are never stopped at any
particular number on the dial for any period of time.
The motion of the minute and hour hands may be
slow, but they are never stopped at one specific value
on the dial for any period of time. By comparison, a
digital clock that displays the time of day to the
nearest minute displays the same numerical value for
the current time for a period of 1 minute. Going a step
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
352 Chapter 11
Scope © Cengage Learning 2014
Amp
Figure 11-21 Waveform generated by band playing music.
Figure 11-21. The waveform looks like a random recorder also attempts to record the analog signal from a
waveform without any pattern. However, it does rep- microphone onto magnetic tape.
resent in electrical form the sound that is present at the
microphone. Compact discs (CDs) and MP3 devices have re-
placed records and cassette tapes. The information on a
In the past, the signal generated by the microphone compact disc, such as music, is stored in a numerical
could have been recorded to a vinyl phonograph re- format. This numerical format is referred to as digital.
cord. Groves in the record were cut with bumps that The analog signal recorded from the music shown in
matched the waveform of the signal being recorded. Figure 11-23 could be converted into a digital signal.
After the vinyl record is formed, the record could This analog signal is broken into equal time segments
have been played on a record player (provided any are illustrated by the vertical lines. The amplitude of the
still in existence). A needle on the record player fol- signal is measured or sampled at each time point
lows this groove. The movement of the needle over shown by the dots in Figure 11-23. Sampling indicates
the bumps is transformed back into an electrical sig- that the signal is not being continuously monitored or
nal, which is amplified and used to drive a speaker. recorded. The amplitude of the signal is just being
The speaker changes the amplified electrical signal
back into sound waves. The sound emitted from the 8
speaker is very close to the original signal that was
recorded. 7
The signal produced by the microphone varies con- 6
tinuously and is called an analog signal (Figure 11-22). 5
The phonographic recording device attempts to record
this analog signal onto the vinyl record. A cassette tape 4
5 3
4 2
3
2 1
1 0s 1ms 2ms 3ms
Time
0s 1ms 2ms 3ms
Time Figure 11-23 Analog signal divided into equal time
increments.
Figure 11-22 Analog signal produced by microphone.
Volts
© Cengage Learning 2014
Volts
© Cengage Learning 2014
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 353
7.1
4.8 4.9 8
3.9 4.2 4.1 3.9 © Cengage Learning 20147
3.7 3.5 3.6 3.1
3.2 2.7 Volts6
2.5
2.1 2.0 2.1 © Cengage Learning 20145
0s 1ms 2ms 3ms 4
Time
3
Figure 11-24 Amplitude measured at each time
interval. 2
measured at prescribed time intervals as shown in 1
Figure 11-24. Changes in the analog signal that occur
between the sampling points are not detected. 0s 1ms 2ms 3ms
Time
This boxy-looking waveform could easily be re-
corded by storing the approximate amplitude (height) Figure 11-26 Re-created waveform from numerical
of the signal at each time interval, as shown in the information.
spreadsheet in Figure 11-25. The boxy-looking
waveform could then easily be re-created based on this exactly like the original analog waveform that was
information. The waveform shown in Figure 11-26 created by the musical instruments. Playing this re-
has been re-created from the numerical information. created waveform through a speaker would sound
Note that the re-created waveform does not look something like the original music, but would lack
clarity or fidelity. Some of the information in the
Time (ms) Volts © Cengage Learning 2014 original analog signal has been lost through sampling.
0 2.1 To make the signal less boxy-looking and thus
0.2 3.9 make the music sound much better, the time interval
0.4 3.7 between samples can be decreased such that the human
0.6 7.1 ear cannot easily detect the transitions between time
0.8 3.5 intervals. By reducing the time between samples of the
1 3.2 signal amplitude (more samples per second), the ana-
1.2 4.2 log signal created by the microphone is more accu-
1.4 2.7 rately recorded. The time between samples has been
1.6 4.1 decreased by one-half in Figure 11-27. The waveform
1.8 4.8 re-created from this more frequently sampled numer-
2 4.9 ical data is much closer to the original analog wave-
2.2 3.6 form, as shown in Figure 11-28.
2.4 2
2.6 2.5 Increasing the sampling rate (reducing time be-
2.8 3.9 tween samples) permits a waveform to be re-created
3 3.1 from the stored information that is much closer to the
3.2 2.1 original analog signal. However, this also requires
more data to be stored because there are more time
Figure 11-25 Waveform stored in numerical format. points with the associated signal amplitude to record.
For example, CD audio typically has been sampled at a
rate of 44.1k samples per second.
Old movie cameras using photographic film are
similar to a digital signal. The movie camera attempts
to record motion by taking a series of still pictures in
rapid succession. Motion, such as someone walking,
appears very jerky in old movies. As movie camera
technology progressed, the time between each picture
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
354 Chapter 11
8Volts audio signal. At some point, your ears may not able to
detect the difference between a digitally re-created
7 © Cengage Learning 2014 analog signal such as that produced by a CD player
and a true analog signal such as that produced by
Volts 6 musical instruments at a live concert.
5
4 © Cengage Learning 2014 Digital Numbering System
3 Digital information is not stored or manipulated in
2 the standard base 10 numbering system that you have
used all your life. A base 10 numbering system uses
1 combinations of the numbers 0 to 9 to generate all
numbers. These familiar base 10 numbers of 0 to 9 are
0s 1ms 2ms 3ms known as decimal numbers. There are 10 different
Time numbers: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 in the decimal
numbering system. Instead of decimal numbers, a base
Figure 11-27 Increased sample rate (less time be- 2 or binary numbering system is used when working
tween samples). with digital information. Only the numbers 0 and 1 are
used in a base 2 or binary numbering system. All
8 numbers in the binary system are represented by
7 combinations of a 0 or a 1. For example, the decimal
number 9 is equal to 1001 in binary.
6
5 Many calculators have the ability to convert from
4 base 10 to the binary number system, but converting
from base 10 to base 2 is really not that important for a
3 truck technician. However, those who are interested in
2 computers probably already have some knowledge of
the binary numbering system.
1
The reason that a binary numbering system is used
0s 1ms 2ms 3ms in digital electronics is that there are only two possible
Time digits, 0 and 1, in the binary numbering system. These
two digits are combined to represent any decimal
Figure 11-28 Re-created waveform is now much number. The two possible digits of binary work well in
closer to original analog waveform. digital electronics because a switch also only has two
states, open or closed. A light that is controlled by a
decreased, which improved the quality of the movie. switch can also only have two different states, either
Eventually, motion in a movie appears to be lifelike off or on. There is no in-between state for a light
because your eyes are not capable of detecting the time switch in which the light controlled by the switch is
between frames of the movie. only partially on or partially off.
Each individual frame or picture taken by the Instead of a switch used to control a light, a group
movie camera is like the sampling of a digital signal. of switches wired in parallel and connected to a 1V
Increasing the number of pictures taken per second by battery can be used to represent a binary number. With
a movie camera results in a more lifelike movie. In the the switch closed, a voltmeter would indicate a value
same way, increasing the number of digital samples of 1V across a resistor, as shown in Figure 11-29.
per second (increasing the sample rate) results in a With the switch open, the voltage across the resistor
more accurate representation of the original analog would be 0V. The 1V represents a binary 1 while the
0V represents a binary 0.
The decimal number 9 is represented as 1001 in
binary and could be represented in electrical form by
using four parallel-connected switches, as shown in
Figure 11-30. Closing the first and last switches and
leaving the two middle switches open would cause the
voltage dropped across the resistors to represent the
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 355
DIGITAL MULTIMETER DIGITAL MULTIMETER
MAX MIN
1VRECORD MAX MIN
0VRECORD
0 1 23 4 5 6 78 % 0 1 23 4 5 6 78 %
HZ HZ
90 90
1V MIN MAX HZ Test 1V MIN MAX HZ Test
leads leads
mV mA mV mA
V A V A
V A V A
A mA A COM V © Cengage Learning 2014A mA A COM V
© Cengage Learning 2014
Figure 11-29 Voltage across resistors with 1V source, open and closed switches.
magnetized in one manner represent a binary 1, while
those magnetized in another manner represent a binary 0.
1V Logic Gates
1V 0V 0V 1V Electronics can be divided into two general cate-
gories: analog and digital. Prior to this chapter, ev-
Figure 11-30 Four switches used to represent the erything in this book has dealt with analog systems.
number 9 in binary format. Resistors, transistors, and other components are used
in analog electronics. Ohm’s law, Kirchhoff’s laws,
decimal number 9. This pattern of 1V or 0V could be and other rules are used to predict or explain how an
translated into the number 9 by using four switches. analog electric or electronic circuit will respond.
Larger decimal numbers could be represented by
adding more switches and resistors. In digital electronics, calculating voltage drops,
measuring current, and handling other tasks performed
Each of the digits in a binary number is called a bit. with an analog system are not necessary. Digital
A bit is a single binary character and is either a 1 or a 0. electronics is mostly only concerned with two different
You have probably heard the word bit used in relation voltage levels. Voltages below some value are deemed
to computers. The more bits there are in a binary as a 0 (logic 0), while voltages above some other value
number, the larger the value of the decimal number are deemed as a 1 (logic 1).
that can be represented. Another term often used when
talking about computers is byte. A byte is a group of The focus of digital electronics is logic. Logic is
eight bits. The largest binary number in a single byte is the use of correct or valid reasoning to come to a
11111111, which equates to 255 in ordinary decimal conclusion. A decision based solely on logic would
numbers. The largest decimal number that can be typically not include any emotion. In digital electron-
represented by two bytes (16 bits) is sixteen 1s, which ics, logical decisions are made by devices called gates.
equates to 65,535 in decimal numbers. These gates are physical electronic components that
are composed of transistors and other hardware.
You are probably familiar with the term byte used However, the hardware is not the important thing here.
with computers when referring to information storage The important thing in digital electronics is the deci-
capacity. A CD disk is capable of storing about 682 sion that is made by the hardware (gate).
megabytes of information. This means that 682 million
bytes, each consisting of eight individual bits, can be The terms input and output are used quite fre-
stored on the disk. Each of these bits is either a 1 or a quently when dealing with personal computers. In-
0. Computer hard drives commonly hold several gig- puts to personal computers are devices like the
abytes (billions of bytes) of information. Computer keyboard and mouse. Outputs of personal computers
hard drives use magnetism to record information onto are the display screen and the speakers. Logic gates
the magnetic disk material. Areas of the disk that are have one or more inputs and one output. The inputs
of logic gates are voltages corresponding to logic 1
or logic 0 levels. Because these gates are logical,
they will always make the correct decision. The de-
cision that a logic gate makes is ‘‘what will my
output be, a 1 or a 0?’’
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
356 Chapter 11
In analog electronics, symbols are used to represent Switch A Switch B Out © Cengage Learning 2014
hardware devices such as transistors, resistors, ca-
pacitors, inductors, and other devices. In digital elec- 0 0 0
tronics, symbols are used to represent the logical 0 1 0
decisions that the hardware (gates) will make. 1 0 0
1 1 1
There are several types of logic gates. Each logic
gate is designed to make a specific decision when Figure 11-33 Status of Switch A and Switch B truth
supplied at its inputs with combinations of logic 1 table.
voltage levels and logic 0 voltage levels.
The schematic symbol for an AND gate and the
The first type of gate that will be examined is an truth table for the AND gate are shown in Figure 11-34.
AND gate. To assist in explaining the operation of an This particular AND gate has two inputs and one out-
AND gate, a simple electric circuit will be evaluated. put. The truth table indicates what outcome or decision
The circuit shown in Figure 11-31 consists of two the AND gate will make when each possible combi-
switches wired in series with a battery and a light bulb. nation of logic level (1 or 0) is present at the inputs A
The light bulb is the output, and the two switches are and B of the AND gate. An AND gate could have more
the input. In order to cause the light bulb to illuminate, than two inputs, resulting in a larger truth table.
both switch A and switch B must be closed (on).
Closing just one switch while leaving the other switch Another common logic gate is the OR gate. The
open will result in the light bulb not illuminating. In function of an OR gate is represented by the simple
other words, switch A AND switch B must be closed electric circuit shown in Figure 11-35. This circuit has
for the light bulb to illuminate. Each of the possible two switches that are in parallel. Closing either switch
switch states and the corresponding light bulb output will cause the light to illuminate. Closing both
could be listed in a table, as shown in Figure 11-32. switches will also cause the light to illuminate. It is
necessary to open both switches to cause the light to
Because a switch is a digital device and can only be not be illuminated. Thus, closing either switch A OR
on or off, this table could be modified so that open switch B will cause the light to illuminate. The sche-
switches are identified as 0, while closed switches are matic symbol and truth table for an OR gate with two
identified as 1. Additionally, because the light bulb can inputs are shown in Figure 11-36.
only be on or off, the outcome can be identified as 1
for ON and 0 for OFF, as shown in Figure 11-33. This
type of table is referred to as a truth table.
Switch A Switch B
A B OUT © Cengage Learning 2014
© Cengage Learning 2014 AND Gate 00 0
01 0
10 0
11 1
Figure 11-34 AND gate symbol and truth table.
Figure 11-31 AND gate constructed of two switches
in series.
Switch A
Switch A Switch B Out © Cengage Learning 2014 Switch B © Cengage Learning 2014
Off Off Off
Off On Off
On Off Off
On On On
Figure 11-32 Status of Switch A and Switch B and Figure 11-35 OR gate constructed of two parallel
resulting outcome of light bulb. switches.
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 357
A B OUT © Cengage Learning 2014
© Cengage Learning 2014
A B OUT
OR Gate 00 0 NOR Gate 00 1
01 1 01 0
10 1 10 0
11 1 11 0
Figure 11-36 OR gate symbol and truth table. Figure 11-39 NOR gate and truth table.
Tech Tip: The OR gate symbol looks some- Microprocessors
what like a shovel, which could be used in
mining ore (one way to remember the meaning One of the major breakthroughs in modern tech-
of the shape). nology that occurred during the mid-twentieth century
was the invention of the integrated circuit. An inte-
A NOT gate is another logic gate. In speech, placing grated circuit is a circuit in which the transistors, di-
the word not in front of a word makes the outcome of odes, resistors, and other components are formed
the sentence have the opposite meaning. A NOT gate during the manufacturing process of the device. Inte-
is also known as an inverter. Because there are only grated circuits are commonly known as chips (Fig-
two possible logic states, a NOT gate inverts the logic ure 11-40). Each of the transistors, diodes, and
level at its input. An input of 1 at a NOT gate results in resistors in the integrated circuit is microscopically
an output of 0, and an input of 0 at a NOT gate results small. These components are not just small individual
in an output of 1. The schematic symbol for a NOT or discrete components like diodes and transistors;
gate is shown in Figure 11-37. Note the small circle at instead, the components are formed from layers of
the tip of the arrow in the schematic symbol. silicon made of P-type and N-type material. The logic
gates discussed in the previous section are typically
The NOT gate and the AND gate can be com- made from transistors that are formed in an integrated
bined together to form a NAND gate. The schematic circuit. The manufacturing processes for integrated
symbol and truth table for a NAND gate are shown in circuits have become so refined that integrated circuits
Figure 11-38. The small circle is taken from the circle are now very reliable and relatively low cost. The
in the NOT gate. Note that the output of the truth table small size of the components formed in an integrated
for a NAND gate is opposite that of an AND gate. circuit also reduces the electric power requirements of
the device.
The NOT gate and the OR gate can be combined
together to form a NOR gate. The output of the truth Modern computers contain a device called a pro-
table for a NOR gate is the opposite of an OR gate cessor. The processor is the main component in the
(Figure 11-39). computer. The processor is only capable of working
with binary numbers (1s and 0s). The term micro-
Out NOT Gate A Out © Cengage Learning 2014 processor is often used as a generic reference to a
digital processor. The microprocessor is not capable of
A 01 ‘‘thinking’’ or ‘‘knowing,’’ but these terms are often
used to describe what is occurring inside the micro-
10 processor. The microprocessor just processes infor-
mation based on its programming instructions. Any
Figure 11-37 NOT gate and truth table. decisions that the microprocessor makes are the out-
comes of how the microprocessor is programmed or
A B OUT © Cengage Learning 2014 instructed to respond. These program instructions use
the status of the microprocessor inputs in the decision-
NAND Gate 00 1 making process.
01 1
10 1 The microprocessor is an integrated circuit and
11 0 may contain millions of transistors. Microprocessors
are complex devices that are beyond the scope of this
Figure 11-38 NAND gate and truth table. book. However, it is not necessary to fully understand
how a microprocessor operates to troubleshoot a sys-
tem that is microprocessor controlled. Most modern
trucks contain a variety of electronic modules that are
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
358 Chapter 11
P Type © Cengage Learning 2014
N Type
Poly
Contact
Metal
Figure 11-40 Integrated circuit.
microprocessor based. The function of some of these Non-volatile memory is memory that is retained
modules will be covered in later chapters. through a disconnection of electric power to the elec-
tronic module. Non-volatile memory is used to retain
Memory the microprocessor’s programing instructions. A per-
sonal computer contains a hard drive on which the
You have probably heard the term memory used operating system and program files are stored. Dis-
frequently in reference to personal computers. Mem- connecting the power to the personal computer has no
ory is basically just a place where digital information effect on the data stored on the hard drive because it is
is stored. Memory is binary, meaning that a memory stored magnetically. Truck electronic modules do not
location is either a 1 or a 0. Many types of memory are have a hard drive to store the microprocessor’s pro-
integrated into truck electronic modules. These types gram instructions. Instead, the program instructions are
of memory can be broken down into two main cate- stored in electronic non-volatile memory. This is
gories: volatile memory and non-volatile memory. similar to the type of memory used in some MP3
players and most other consumer electronic devices.
Volatile memory is memory that is lost or reset
when power is taken away from the electronic module. Most non-volatile memory can be erased and re-
The random access memory (RAM) of a personal written with new information. Non-volatile memory
computer is volatile memory. If the power has ever used in truck electronic modules that can be rewritten
gone out while you were working or playing a game on is typically electrically erasable, programmable, read
a personal computer, you probably lost a portion of only memory (EEPROM). Some types of EEPROM
your work or had to start your game over again. Your are designed to be erased and rewritten many times.
work or play is being written to RAM until you save This type of EEPROM is used in truck electronic
the information to the hard drive. Volatile memory modules to store information that changes frequently.
such as RAM is used by the microprocessor to tem- For example, the engine ECM may maintain several
porarily store information in a way that is similar to pieces of information such as total engine hours and
writing on a chalkboard with a piece of chalk. Dis- total accumulated vehicle mileage (odometer). When
connecting the power to the electronic module causes the key is switched off, the engine ECM may update
the information stored in RAM to be lost in a way that this information stored in EEPROM. This is like sav-
is similar to erasing the chalkboard. ing a file that you have been working on to the hard
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 359
drive of a personal computer before shutting off the Like a human brain, microprocessors also have
computer. inputs and outputs. Microprocessors evaluate the
information taken from their inputs and use this
Flash memory is a type of EEPROM that is used information to make decisions based on the micro-
to retain the major portion of module programming processor’s programming. The decisions made by the
instructions. Flash memory differs from standard microprocessor cause the microprocessor to control the
EEPROM in that large chunks or sections of flash state of its outputs accordingly.
memory must be erased at the same time, unlike
standard EEPROM that can be erased and rewritten in Microprocessor Inputs and Outputs. All inputs to a
small sections or bytes. The type of flash memory used modern microprocessor are digital (1 or 0). All outputs
in electronic modules typically has a limited number of from the microprocessor are also digital. However, an
write cycles so it is not used to store information that electronic module that contains a microprocessor can
changes frequently. The terms flash or reflash are of- have both analog inputs and analog outputs, as will be
ten used to describe the operation of reprogramming explained later in this section.
the flash memory section on an electronic module,
such as the engine ECM. OEMs often find problems Most of the sensors addressed earlier in this chapter
with the original programing instructions, such as an provide a variable signal corresponding to the tem-
engine performance problem under specific or unique perature, pressure, or other quantity being measured.
operating conditions. To resolve these problems, a Because microprocessors can only accept digital in-
revised version of software is released by the OEM and formation, the analog signals provided by sensors must
the ECM is reprogrammed or flashed with the revised be converted to the digital (binary) equivalent. An
software. integrated circuit called an analog to digital (A/D)
converter is used to convert the analog voltage mea-
All types of memory store information in a digital sured at the input of the electronic module into a
format. This information storage is accomplished by digital signal. The analog voltage at an electronic
storing an electric charge in a floating gate of a very module input at a particular instant in time is measured
small field effect transistor (FET), similar to storing a by the A/D converter and assigned a binary (digital)
charge in a capacitor. If a charge is present in the value, as illustrated in Figure 11-41.
floating gate, the FET cannot be switched on, indi-
cating one bit with logic state 0. If a charge is not In another example, the battery voltage level is a
present in the floating gate, the FET can be switched necessary piece of information for the decision making
on, indicating one bit with logic state 1. The charge of many electronic modules, such as the engine ECM.
can be stored in the floating gate almost indefinitely, An analog value of 12V measured at the battery
even when the power to the device is interrupted. To voltage input of the electronic module must first be
erase the memory, a voltage is applied that causes the converted to the digital or binary equivalent of 12. The
electric charge stored in the floating gates to be dis- binary number for 12 is 1100. The A/D converter
charged. The theory of operation of some types of would change the 12V measured at the input to a bi-
flash memory are explained by the Heisenberg un- nary value of 1100. The microprocessor can work with
certainty principle and quantum tunneling; therefore, this binary number and ‘‘know’’ through its pro-
the physics of flash memory is definitely outside the gramming that a binary value of 1100 means that there
scope of this book. is 12V present at the electronic module battery voltage
input. Increasing the voltage at the input to 13V causes
Inputs and Outputs the A/D converter to supply the microprocessor with
1101, the binary equivalent of 13. However, if the
The five human senses act as inputs to the human battery voltage were 12.4V, it would be necessary to
version of a processor known as the brain. This human round down to 12V because this simple conversion
processor was pre-programmed with some specific from analog to digital is only capable of 1V steps.
instructions for survival such as breathing. Other pro-
gramming was added or modified by life experiences. Because it is necessary for the microprocessor to
Touching a hot stove causes the touch sensors (via resolve the truck’s battery voltage level more accu-
electrical impulses) to provide information to the rately than 1V steps, the A/D converter could be de-
human’s processor. The processor responds by com- signed to provide a single bit for each 1/10 of a volt
manding the muscles (also via electrical impulses) to present at the input. Therefore, a value of 12.1V mea-
take the hand off the stove, NOW! This command to sured at the A/D converter input is equal to 121 bits
the muscles is an output of the human processor. when each bit is designed to be worth 1/10 of a volt.
This 121 would equate to 1111001 in binary. Note that
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
360 Chapter 11
5 volts Analog signal is sampled
0 volts at precise intervals to
determine the voltage at
that instant (in this case it
is 5 volts)
Voltage 0 – 2 volts 2 – 4 volts 4 – 5 volts 3 Voltage is assigned
from a value
sensor
Assigned 1 23 The assigned value is
value 01
translated into a binary
Binary 10 11 1 1 code
code
On The numbers in the © Cengage Learning 2014
binary code (1s and 0s)
Off are represented by
1 2 3 4 5 6 a digital code
(Time in microseconds) 1 = On
0 = Off
Figure 11-41 Analog to digital conversion.
it is necessary to have more bits in order to represent from the A/D converter (Figure 11-42). Only two
12.1V in binary than 12V. A principle of digital wires or conductive paths between the A/D converter
electronics is that in order to measure something and the microprocessor are necessary for this serial
more accurately, more bits are necessary. This is method (one path being the common ground).
called resolution or volts per bit. An actual A/D
converter may be described as an 8-bit device, mean- If you own a personal computer, you probably
ing there are 11111111 binary or 255 possible steps know the clock speed of the processor used in the
into which the analog voltage being converted could computer. Clock speeds in the gigahertz region are
be divided. Typically, the analog input voltage is common in PCs. The clock in a processor generates a
scaled or reduced by voltage divider circuits within the continuous pulse or square wave. This clock pulse is
electronic module before being supplied to the A/D used to cause the magic of digital processing to occur.
converter. This scaling limits the maximum voltage to The clock speed refers to the frequency of this pulse.
the A/D converter to some value, such as 5V. The A/D The higher the frequency of the clock pulse, the faster
converter uses this maximum voltage as a reference
voltage. Therefore, an 8-bit A/D converter would have 8 micro
a resolution of 19.6mV per bit for a 5V reference seconds
voltage.
1 micro
The other factor besides sample rate that impacts sec
the quality of digitally recorded music is the resolution 1V
of the amplitude of the original analog signal being
converted. For example, CD audio typically has 16-bit 0V
resolution of the signal amplitude or 65,536 possible
voltage steps. 01 1 1 10 0 1
The output of the 1s and 0s from the A/D converter 12.1V Digital Microprocessor © Cengage Learning 2014
can be serial or parallel. Serial means the A/D con- input A/D output 01 1 1 10 0 1
verter will produce one bit after the other. For example,
the A/D converter may output each bit for a period of 1 Figure 11-42 Serial processing of a digital signal.
msec per bit in a very slow system (by today’s stan-
dards). This means that it would take 8 msec for the A/
D converter to produce the value of an 8-bit binary
number. The microprocessor could place these 8-bits
in a memory storage location as they were received
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 361
Processor Digital Reference To output
signal voltage devices
regulator Diagnostics
Small Input Analog Microprocessor Output
analog conditioners to digital drivers
signal converter Microprocessor
Input Amp
sensor
Analog Analog Memory
Input signal to digital Memory
sensor converter Memory
© Cengage Learning 2014
Digital signal
Figure 11-43 Analog signal is conditioned, converted to digital, and processed by the microprocessor.
the processor is capable of processing digital infor- the circuit is simplified to its most important compo-
mation and performing instructions. nents to illustrate the principles.
An overview of A/D converters, microprocessors, If a 1kO resistor were connected to the positive
and memory is shown in Figure 11-43. Analog signals terminal of a 12V battery at one end and left discon-
supplied by sensors are amplified and conditioned if nected or floating at the other end, a voltmeter would
necessary. Signal conditioning refers to the manipu- indicate 12V between the end of the resistor that is
lation of the signal so that it is properly prepared for floating, as shown in Figure 11-44. The reason for this
the next stage of the process. The scaling of the analog is that the resistor and the voltmeter form a series
input voltage so that it is always less than 5V at the voltage divider circuit. The resistor has 1kO of resis-
actual input of the A/D converter described previously tance; the voltmeter has approximately 10MO of in-
is an example of signal conditioning. Filtering or ternal resistance (see the Extra for Experts section on
smoothing of the analog voltage using a capacitor is Additional DMM Information in Chapter 2 for an
also a part of signal conditioning. After the analog
signal is conditioned, the analog signal is converted to 12V DIGITAL MULTIMETER 1KΩ
a serial digital signal. This digital signal is an input to
the microprocessor. The microprocessor places this MAX MIN
digital information into RAM memory for temporary 12VRECORD
use. The microprocessor then accesses its program- %
ming instructions from flash memory to determine
what should be done next, based on the information HZ
obtained from the sensor.
0 1 23 4 5 6 78 90
Electronic Module Input Circuitry. Electronic mod-
ules used in a truck electrical system such as the en- MIN MAX HZ Internal
gine ECM depend on input devices like sensors. An 10MΩ
electronic module input device such as a sensor is mV mA
connected to an input terminal of the electronic V A
module. Some of the internal circuitry used for signal V A
conditioning in a typical electronic module will be
examined in this section. This circuitry is not as A mA A COM V © Cengage Learning 2014
complicated as you might have imagined especially if
Figure 11-44 Measuring voltage between floating
1k ohm resistor connected to B+ and ground with
DMM voltmeter with 10M ohm internal meter
resistance. Almost all voltage is dropped across the
meter’s internal resistance.
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
362 Chapter 11
explanation). The laws of series circuits indicate that 12V
nearly all of the voltage is dropped across the 10MO of
internal resistance inside the voltmeter. The voltmeter (Inside of electronic module)
is merely displaying the voltage that is dropped across
its 10MO of internal resistance, which is effectively 1KΩ
12V. This is an important concept. Use Ohm’s law if
necessary to calculate the voltage dropped across the Digital © Cengage Learning 2014
10MO of internal voltmeter resistance and the voltage
dropped across the 1kO resistor. You will find that Input A/D output
nearly all of the voltage is dropped across the internal terminal Microprocessor
resistance of the voltmeter, thus the DMM reading of
approximately 12V. Figure 11-46 An A/D converter measures voltage
like a DMM voltmeter; nearly all of the 12V is
If the 1kO resistor were instead connected to ground dropped across the very high internal resistance of
at one end and left floating at the other end, a voltmeter the A/D converter. The A/D converter measures
would indicate 0V across the resistor (Figure 11-45). approximately 12V and converts this to the digital
This is because there is zero voltage being dropped equivalent.
across the resistor. The voltmeter is in parallel with the
resistor so the voltmeter indicates that 0V is dropped terminal is effectively floating. The internal resistance
across the resistor. This may seem obvious, but the of the A/D converter is very high, like a DMM volt-
concept is important. meter. If the top of the resistor shown in Figure 11-46
is connected to +12V, then the A/D converter would
The circuitry contained inside an electronic module measure about +12V referenced to ground. This is very
that is connected to the input terminals of the module similar to the circuit shown in Figure 11-44 except the
can be simplified to a resistor and a voltmeter. This high internal DMM voltmeter resistance is the internal
resistor may be connected to a positive voltage inside resistance of the A/D converter. Compared to the 1kO
the module like the example shown in Figure 11-44 resistor, the internal resistance of the A/D converter is
for some types of inputs. Alternatively, this resistor 10 Mega O or more. Therefore, nearly all the voltage,
may be connected to ground inside the module like the 12V in this case, will be dropped across the high in-
example shown in Figure 11-45 for other types of ternal resistance of the A/D converter, causing the A/D
inputs. The A/D converter is like the DMM voltmeter converter to read a value of 12V for the voltage value at
shown in the previous examples. The A/D converter is this input. A millivolt or so is also dropped across the
connected to the module input terminal, as shown in 1kO resistor, but this is a negligible amount in this case.
Figure 11-46. This terminal is where the vehicle
wiring harness would provide connection to the sensor The 1kO resistor shown in Figure 11-46 is called a
or switch that is acting as an input device to the pull-up resistor. The resistor pulls the input terminal
electronic module. up to some voltage level when nothing external to the
module is connected to the input terminal. In this
The A/D converter measures the voltage between example, the input is ‘‘pulled-up’’ or elevated to battery
the input terminal and ground. In the example shown voltage. With nothing external to the electronic module
in Figure 11-46, nothing external to the electronic connected to the input terminal, this particular input is
module is connected to the input terminal so the input pulled-up to 12V through the 1kO resistor inside of the
module. This concept will be discussed in more detail
DIGITAL MULTIMETER later in this chapter.
MAX MIN Adding External Devices to the Input. If a variable
0VRECORD resistor-type sensor that provides a path to ground
0 1 23 4 5 6 78 % were connected to the input terminal of the electronic
module, the input terminal would no longer be floating
HZ and a series voltage-divider circuit would be formed.
90 The 1kO pull-up resistor inside the module and the
variable resistor outside the module are connected in
MIN MAX HZ Internal 1KΩ series, as shown in Figure 11-47. This connection will
10MΩ cause current to flow from the +12V source, through
mV mA the pull-up resistor and through the sensor resistance to
12V V A
V A
A mA A COM V © Cengage Learning 2014
Figure 11-45 Measuring voltage across floating 1kO
resistor connected to ground.
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 363
6V 12V
dropped
on pull-up (Inside of electronic module)
resistor 1KΩ
Digital
A/D output Microprocessor
1KΩ 6V A/D converter © Cengage Learning 2014
dropped measures voltage
on sensor dropped on sensor
Figure 11-47 Connecting a variable resistor to the input terminal.
ground. If the sensor also has a resistance of 1kO, the The A/D converter would measure the 6V that is
pull-up resistor and the sensor would each have 6V dropped across the sensor, convert this analog voltage
dropped across them as the 12V supplied by the battery to a digital value, and provide the microprocessor with
divides equally. The A/D converter is measuring the the binary equivalent of 6V. The microprocessor now
voltage between the input terminal and ground. ‘‘knows’’ that the voltage at this input terminal is 6V.
Therefore, the A/D converter is connected in parallel
with the sensor. This means that the A/D converter is In reality, the internal resistance of the A/D con-
actually measuring the voltage that is dropped across verter is in parallel with the 1kO external variable
the variable-resistor type sensor because parallel- resistor that is connected to the input. However, the
connected devices must always have the same voltage internal resistance of the A/D converter is so high
dropped across them, per the rules of parallel circuits. compared to the 1kO external variable resistor that the
A/D converter’s internal resistance can be ignored.
Do not let the fact that one of the resistors is inside This is a very important concept to understand. Placing
of an electronic module and the other resistor, the a very high resistance in parallel with one of two much
variable resistance of the sensor, is outside of smaller resistors that are wired in series results in al-
the module cause confusion. With Ohm’s law and the most no difference in the voltage level that is dropped
laws of series circuits, it does not really matter where across either of the original series resistors.
the series-connected resistors are located. The fact that
the pull-up resistor and the sensor resistance each have To prove this concept, verify that the voltage dis-
6V dropped across them is just applied Ohm’s law played on the DMM voltmeter shown in Figure 11-48
along with the rules of series circuits. is correct (ignoring the internal resistance of the
DMM). The first step is to calculate the equivalent
12V DIGITAL MULTIMETER 1KΩ
10MΩ 1KΩ
RECORD MAX MIN
6.000V% HZ
0 1 23 4 5 6 78 90
MIN MAX HZ © Cengage Learning 2014
mV mA
V A
V A
A mA A COM V
Figure 11-48 Series-parallel circuit with 1kO connected in parallel with 10MO, connected in series with 1kO.
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
364 Chapter 11
resistance for the two parallel resistors, which is 1kO at the module input terminal to become 8V, as shown
in parallel with 10MO. This results in a value of in Figure 11-49. This is because the fixed-value pull-
999.9O for the equivalent resistance. This 999.9O is in up resistor is 1kO and the variable-resistor sensor is
series with the 1kO resistor, resulting in a total circuit 2kO and these two resistors are connected in series
resistance of 1999.9O. Ohm’s law indicates that a total (ignoring the high internal resistance of the A/D con-
current of 0.0060003A (6.0003mA) will flow through verter). The A/D converter measures this 8V at the
the circuit. Multiply this current by the equivalent re- input terminal, converts the analog signal to a digital
sistance for the two parallel-connected resistors to find signal of equivalent value, and supplies the micro-
the voltage dropped across the parallel resistors. There- processor with this information. The microprocessor
fore, the voltage dropped across the parallel combination now ‘‘knows’’ that the voltage at this input terminal
of the 1kO resistor and the 10MO resistors is 5.9997V. is 8V.
However, this is essentially 6V and most DMMs would
indicate 6.000V when configured to display the highest If the variable-resistance type sensor, was a
possible precision. thermistor that is measuring engine coolant tempera-
ture, the microprocessor could be programmed to
Tech Tip: Knowing the level of precision or know that 6V at this input means that coolant tem-
accuracy of measurements required to diagnose perature is 1208F (498C), while 8V at this input means
a problem comes with experience. Expecting that coolant temperature is 608F (168C).
too much precision in a voltage measurement
during troubleshooting may result in needless The 1kO pull-up resistor in Figure 11-49 is used to
replacement of components. Conversely, not form a series voltage divider network with the resis-
being accurate or precise enough in your tance of the sensor connected to the input terminal in
measurements when necessary may cause you this example. The pull-up resistor is said to cause the
to misdiagnose the problem. For example, there input to be pulled-up to +12V. With nothing external
is no general rule on when a difference of to the electronic module connected to the module’s
100mV is important and when 100mV can be input terminal (Figure 11-46), the voltage measured at
ignored. However, most OEMs’ troubleshoot- the floating input terminal referenced to ground is
ing information typically provides a range of +12V. An electronic module input terminal that is
acceptable readings, such as a voltage mea- internally connected to a pull-up resistor is inclined to
surement, which should be observed. be pulled-up to some positive voltage level until
something outside the module, such as the resistance
Modifying the variable-resistance sensor so that its of a sensor providing a path to ground, causes the
resistance is now 2kO will cause the voltage measured voltage measured at the input to decrease. The pull-up
resistor inside the module is necessary because it forms
a series voltage divider circuit with the device that is
connected outside of the module to the input terminal
such as a sensor. The pull-up resistor also limits the
4V 12V
dropped
on pull-up (Inside of electronic module)
resistor
1KΩ Digital
A/D output
Microprocessor
2KΩ
8V A/D converter measures © Cengage Learning 2014
dropped voltage dropped on
on sensor sensor (8V)
Figure 11-49 Sensor changes to 2kO, and voltage measured by A/D converter increases to 8V as voltage divides
between pull-up resistor and sensor resistance.
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 365
Input (Inside of electronic module) © Cengage Learning 2014
terminal
Digital
A/D output Microprocessor
10KΩ
Pull-down
resistor
Figure 11-50 Pulled-down input.
amount of current that will flow if the input terminal is Figure 11-51. This would cause a voltage drop of 2V
grounded and permits diagnosis of wiring failures, across the pull-down resistor. The A/D converter that
such as an open or shorted-to-ground circuit. This will is connected in parallel with the pull-down resistor
be discussed in detail later in this chapter. would convert this voltage into the digital equivalent
of 2V for use by the microprocessor. The micro-
The internal input circuitry of an electronic module processor would be programmed to ‘‘know’’ that a
can also be constructed such that a resistor is con- voltage of 2V at this input means that air pressure is
nected between the input terminal and ground inside 50 psi. The programming instructions of the micro-
the module. This type of input is referred to as being processor, which converts an input voltage into a
pulled-down (Figure 11-50). The resistor inside the pressure, temperature, or other physical measurement,
module is called a pull-down resistor. The resistor is called a transfer function. A transfer function is a
pulls the input circuit down to ground level voltage or math equation used to describe the relationship be-
0V with nothing outside the electronic module such as tween the physical variable the sensor is measuring,
a sensor connected to the input terminal. An input that such as pressure, and its output voltage. For example,
is pulled-down is designed to be connected to a device if an air pressure sensor provides an output voltage of
outside of the module that provides a positive voltage 2V at 50 psi (345 kPa) and an output voltage of 3.5V at
to the module’s input terminal. The pull-down resistor 100 psi (690 kPa), the transfer function for the sensor
creates a series voltage divider circuit with a device in units of psi would be:
that is connected to the input terminal outside of the
module such as a sensor that supplies a positive output Voltage ¼ 0:03 Â Pressure þ 0:5
voltage signal. or
For example, an air pressure sensor that provides an Pressure ¼ 33:333 Â ðVoltage À 0:5Þ
output signal voltage of 2V when measuring a pressure
of 50 psi (345 kPa) could be connected to a pulled- Therefore, a voltage of 4.1V measured at the electronic
down input terminal of a module as shown in module input would indicate the pressure is 120 psi
5V reference A/D converter
voltage measures 2V
(Inside of electronic module)
Pressure 2V output Digital
sensor A/D output Microprocessor
10KΩ © Cengage Learning 2014
2V dropped
on pull-down
resistor
Figure 11-51 Air pressure sensor supplying 2V to pulled-down input.
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
366 Chapter 11
(827 kPa). The transfer function for this sensor is connected to a switch because a switch also only has
plotted in the graph shown in Figure 11-52. This two different states: closed or open. Voltages present
particular sensor has a linear output, as indicated by at a digital input below some value are considered as
the straight line shown in the graph. If you have taken logic 0 state while voltages above some value are
algebra, you may recognize the y = mx + b format of considered logic 1 state. For example, a voltage below
this sample transfer function. For nonlinear sensors 1V measured at a module digital input terminal is typ-
such as thermistors, the transfer function would be ically considered logic 0 state while voltages above
more complex and the plotted line would be curved, about 3V are considered logic 1 state. Voltages between
not straight. 1V and 3V might not be ‘‘guaranteed’’ to always be
evaluated as either logic 1 or logic 0 states, so the circuit
Pull-down resistors ensure that the voltage mea- is designed to stay away from voltages in this unknown
surement at an electronic module will be 0V when the region. Digital inputs do not require an A/D converter
input terminal is floating. Pull-down resistors also because the input is already digital (high or low, 1 or 0).
limit the amount of current that will flow if the elec-
tronic module input terminal is connected to a positive In reality, there are additional signal conditioning
voltage source. components to filter (smooth) the input voltage and
additional voltage divider circuits inside the module to
Digital and Analog Electronic Module Inputs. There further reduce the voltage supplied to the micro-
are four basic types of electronic module inputs. Inputs processor’s digital input terminal. However, these de-
can be classified as being pulled-up or pulled-down. vices are not so important for a basic understanding of
The circuit shown in Figure 11-46 is a pulled-up input. electronic modules.
The circuit shown in Figure 11-50 is a pulled-down
input. Electronic module digital inputs, like analog inputs,
may also be classified as being pulled-down or pulled-
Electronic module inputs can also be classified as up depending on the device that is connected to the
being analog type or digital type. The input circuit input. For example, an electronic module digital input
examples shown in Figure 11-47 and Figure 11-51 are terminal that is connected to ignition voltage (+12V)
analog inputs. The analog input voltage must be con- that is used to determine if the key switch were in the
verted to the digital equivalent binary 1s and 0s by an ignition position or in the off position would be pulled-
A/D converter before being supplied to the micro- down. This is because when the key switch is in the off
processor. For example, the A/D converter would (open switch) position, the ignition voltage is 0V. The
transform the 2.0V analog value in Figure 11-51 to digital input would indicate logic 0 state with the key
10100 binary, given a resolution of 0.1V per bit. switch in the off position because of the connection to
ground through the pull-down resistor (Figure 11-53,
Electronic module digital inputs are inputs that are top). With the key switch in the ignition (switch
capable of only recognizing two different states or two closed) position, the voltage at the input would be
different ranges of voltage. Digital inputs are typically ignition voltage or about 12V (Figure 11-53, bottom).
The digital input would indicate logic 1 state, indi-
5 cating that the key switch is in the ignition position.
The microprocessor would use the status of this digital
4 input in the decision-making processes involving the
state of the key switch. A pulled-down digital input
y = 0.03x + 0.5 may also be referred to as an active-high input.
3
Electronic module pulled-up digital inputs would
2 typically be connected to switches or other devices that
provide a path to ground when closed, such as the park
1 brake switch shown in Figure 11-54. With the park
brake switch open, the digital input would be deter-
0 mined as being logic 1 state because of the voltage
0 25 50 75 100 125 150 measured at the microprocessor input due to the pull-
Pressure (psi) up resistor’s connection to a 12V source. With the
switch closed, the digital input would be determined as
Figure 11-52 Transfer function for pressure sensor. being logic 0 state because of the path to ground
Volts provided by the closed switch. The amount of current
that flows in this path to ground is minimal because of
© Cengage Learning 2014
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 367
12V (Inside of electronic module)
Ignition Microprocessor
switch interprets input
open
as logic 0
Microprocessor
10KΩ
Pull-down
resistor pulls
input down to 0V
12V (Inside of electronic module)
Ignition Microprocessor
switch interprets input
closed
as logic 1
Microprocessor
10KΩ
12V dropped © Cengage Learning 2014
across pull-down
resistor
Figure 11-53 Pulled-down digital input measuring key off and key in ignition position.
the series resistance provided by the 10kO pull-up The output voltage of the accelerator position sensor is
resistor. The switch and the pull-up resistor become used to determine how much power the driver is re-
series voltage divider circuits. The switch has near questing from the engine. Using a digital input for the
infinite resistance when open and near zero resistance accelerator would not be a good idea because the en-
when closed. The pull-up resistor causes the voltage at gine would respond with low idle-only fuel or maxi-
the input terminal to be nearly the supply voltage value mum fueling, with nothing in between. This would
(12V in this case) or nearly 0V depending on the make the truck very difficult to drive. Therefore, an
switch being opened or closed. A pulled-up digital accelerator-position sensor would be connected to
input may also be referred to as an active-low input. an ECM analog input instead of an ECM digital input.
An analog input would permit the ECM, to determine
Analog Input Examples. Analog inputs are capable the accelerator position based on the voltage measured
of determining a specific voltage level at the input at the analog input as shown in Figure 11-55. This
within a predefined range of voltages. Analog inputs input is pulled-down because the output of the accel-
are typically connected to a device that has a range of erator position sensor (APS) is a variable positive
resistance or devices that supply a variable voltage. An voltage. The microprocessor in the engine ECM could
example of a device that would be connected to an be programmed to ‘‘know’’ that a voltage of 1V means
analog input is an electronically controlled diesel en- low idle, while a voltage of 4V means the accelerator
gine accelerator-position sensor. This sensor is often a is pressed to the floor.
potentiometer and is used to measure the amount the
truck operator is depressing the accelerator pedal. Note the 5V reference voltage shown in Figure 11-55.
Modern electronically controlled diesel engines are In order for the ECM to accurately determine acceler-
‘‘drive by wire’’ with respect to the accelerator. There ator position, the voltage supplied to the accelerator
is no cable between the accelerator and the fuel sys- position sensor must be precisely 5V. The accelerator
tem, which controls how much fuel is to be injected. position sensor shown in Figure 11-55 is a potentiom-
eter. Supplying the potentiometer with 6V instead of 5V
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
368 Chapter 11
12V
(Inside of electronic module) Microprocessor
10KΩ interprets input
Microprocessor as logic 1
Park brake
switch open
12V measured
at input
12V
(Inside of electronic module) Microprocessor
10KΩ interprets input
Microprocessor as logic 0
Park brake © Cengage Learning 2014
switch closed
0V measured
at input
Figure 11-54 Pulled-up digital input with switch open (upper) and switch closed (lower).
(Inside of electronic module)
5V Digital © Cengage Learning 2014
Ref.
APS A/D output
100 Microprocessor
KΩ
Figure 11-55 Accelerator position sensor connected to pulled-down analog input.
will cause the voltage measured at the ECM input ter- voltage (typically +5V) which is vital for accurate
minal to be higher than it would be if the sensor were sensor measurements.
supplied with 5V as a reference voltage. This will cause
the ECM to ‘‘think’’ that the accelerator is partially Tech Tip: Wiring harness problems with three-
depressed, even though the truck operator does not have wire sensor reference voltage and ground
his foot on the accelerator. Conversely, if the reference (return) circuits can result in sensor measurement
were 4V instead of 5V, the ECM would not detect the errors. High levels of circuit resistance caused by
accelerator being fully depressed when the truck oper- terminal corrosion can result in misdiagnosis and
ator is requesting maximum engine power. An elec- unnecessary component replacement.
tronic voltage regulator within the electronic module is
responsible for supplying a precise sensor reference
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 369
Engine coolant 5V (Inside of electronic module) © Cengage Learning 2014
temperature Ref.
10KΩ Digital
sensor A/D output Microprocessor
Figure 11-56 Engine coolant temperature sensor connected to pulled-up analog input.
An electronic module analog input could be designed either off or on, like a switch. The programming in-
to be pulled-up if the external device connected to the structions in the microprocessor cause the micropro-
input terminal, such as a sensor, supplied a variable cessor to determine when to switch on and when to
resistance path to ground. An example is a thermistor switch off each driver, based on the status of the mi-
used to measure coolant temperature, such as the cir- croprocessor’s inputs. One of several digital outputs of
cuit shown in Figure 11-56. As the resistance of the the microprocessor is used to act as the control signal
thermistor decreases due to an increase in coolant for the driver, as shown in Figure 11-57. This driver
temperature, the voltage measured at the input termi- shown is a low side driver. A digital 0 provided by
nal of the module would also decrease accordingly. microprocessor output #1 causes the low side driver
The microprocessor in the engine ECM can then apply not to provide a ground for the relay coil, similar to
the sensor transfer function information that is stored an open switch that could be used to control the relay.
in the ECM program memory to determine the coolant A digital value of 1 supplied by microprocessor
temperature based on the voltage measured at its output #1 causes the low side driver to switch on and
coolant temperature input terminal. sink a path to ground for the relay coil, similar to a
closed switch. The decision to switch the low side
To summarize, there are four basic types of elec- driver on or off is based on the microprocessor’s
tronic module inputs: programming instructions. In this example, the relay
for the on-off air fan clutch is being controlled by a
n Pulled-up digital input low side driver. The microprocessor’s programming
n Pulled-down digital input instructions are designed to provide an output of 0 not
n Pulled-up analog input to switch on the fan and a value of 1 to switch on the
n Pulled-down analog input fan. The logic statement in Figure 11-57 illustrates
the microprocessor programming instructions for this
Although a circuit diagram may not indicate what type output.
of electronic module input the external circuit is
connected to, you should be able to figure this out Electronic Module Analog Outputs. Microproces-
based on the type of device present (i.e., a switch or a sors provide digital values as outputs. Many times, a
sensor) and what is being supplied to the electronic digital value of 1 or 0 is fine if what is being controlled
module by the device (i.e., a positive voltage or a path will be either on or off, such as a relay coil or a light.
to ground). Having this knowledge can be very useful Otherwise, an integrated-circuit device called a digital
in troubleshooting, as will be demonstrated in later to analog (D/A) converter is used to transform a digital
chapters. value into an analog value. Analog outputs would be
used by a CD player to produce the audio signal that
Electronic Module Digital Outputs. In Chapter 6, drives the speakers to produce music.
the concept of low side drivers and high side drivers
was introduced. These drivers are transistors being A digital output can also be switched off and on
used as an electronic switch. Low side drivers ‘‘sink’’ rapidly by the microprocessor using PWM so that it
or provide a path to ground, meaning conventional has the appearance of being an analog output. For
current is flowing from the load into the electronic example, instrument panel illumination dimming can
module. High side drivers ‘‘source’’ or provide a be controlled by switching a high side driver off and
positive voltage, meaning conventional current is on several times per second using PWM to control the
flowing out of the electronic module to the load. The lamp intensity.
control of the drivers is digital, meaning the driver is
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370 Chapter 11
Microprocessor programming
IF input A = logic 0 OR IF input B < 1V
THEN
Output 1 = 1
ELSE 12V
Output 1 = 0
86 30
12V
Fan 10KΩ
override
switch
Input A 85 87 87A
Engine coolant 5V Low Air fan
Ref. Output 1 side clutch
solenoid
temperature driver
sensor 1KΩ Digital © Cengage Learning 2014
output
A/D
Input B
Figure 11-57 Microprocessor controlling one of its outputs based on its input values and programming
instructions.
MULTIPLEXING voltage divider circuit. The microprocessor would re-
spond to this voltage level at this input by causing
The term multiplexing refers to methods used to some specific action to occur, based on its program-
combine more than one channel of information into a ming instructions. Opening the switch connected to the
common signal path. Multiplexing is not a new idea, 1kO resistor and closing the switch connected to the
having been used in the communications industry for 2kO resistor will cause the voltage level at the input to
many years. There are many different types of multi- increase from 6V to 8V. The microprocessor could be
plexing. The most common forms of multiplexing used programmed to cause some other action to occur due
in truck electrical systems are analog multiplexing and to the change in voltage measured at the input.
time-division multiplexing.
The single analog input is being used for four dif-
Analog Multiplexing ferent purposes in the example shown in Figure 11-58.
The microprocessor could cause four different actions
An example of analog multiplexing in truck to occur based on which switch has been closed. These
electrical systems is the use of a group of switches actions could be something as simple as switching on
wired in parallel that are connected to a single analog four different lights or something more complicated.
input of an electronic module. The example shown in One use of analog multiplexing in modern trucks is the
Figure 11-58 illustrates a pulled-up analog input that four steering-wheel mounted cruise control switches.
is connected to a group of parallel-connected switches. The rotation of the steering wheel requires the use of a
Each switch has a different value of series resistance clockspring to provide electrical continuity as the
and each switch corresponds to a different type of steering wheel is rotated. By reducing the number of
action. Therefore, closing only the switch connected to wires necessary to connect to four different cruise
the 1kO resistor in Figure 11-58 will cause the voltage control switches (on, off, set, resume) from five to two
measured at the analog input to change from 12V to (including the ground), the number of circuits in the
6V as the pull-up resistor in the electronic module and clockspring can be reduced. This also reduces the
1kO resistor in series with the switch form a series number of wires in the wiring harness.
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
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Sensors, Digital Electronics, and Multiplexing 371
12V
(Inside of electronic module)
1KΩ Digital
A/D output
Microprocessor
1K 2K 3K 4K © Cengage Learning 2014
Figure 11-58 Analog multiplexing.
Time-Division Multiplexing wires is being time-shared by the sensors and the
microprocessor inputs.
Time-division multiplexing used in modern trucks
can be described as the time-sharing of a pair of wires. Rotary switches are not actually used in time-
Like a vacation home time-share, several different division multiplexing but are just being used to present
electronic devices use the pair of wires to transmit the concept of time-division multiplexing.
their information at different times.
Serial Data Communications. The serial and USB
The simplest example of time-division multi- ports found on a PC are a form of serial data com-
plexing is the use of a pair of rotary switches that are munication. The 1s and 0s are transmitted one after the
rotated to the same position at the same time (syn- other to communicate digital data between the PC and
chronized). The example in Figure 11-59 permits the some other device plugged into a USB port.
output of four different variable resistance sensors to
use one pair of wires to connect to four different an- The use of digital serial data communications is
alog inputs of an electronic module. Both of the rotary common in modern truck electrical systems to permit
switches in this example are moved to position A. the various electronic modules on the truck to com-
The microprocessor takes a reading on input A and the municate with each other. Serial data communication
switches are then both moved to position B. The is a form of time-division multiplexing. A pair of wires
microprocessor then takes a reading on input B and the is typically used as the communication means. This
switches are moved to position C, then position D, and pair of wires is often referred to as a data link. Even
back to position A, and so forth. If the rotary switches though this terminology is not technically correct, the
are both rotated very quickly in unison with each other, term data link will be used in the remainder of this text
then it almost appears that all four sensor values are for a serial communications network because the term
being measured almost continuously by the micro- has widespread usage in the industry to describe the
processor through the use of two wires. The pair of pair of wires used to communicate information in a
A Inputs © Cengage Learning 2014
B A
C B
D C
D
Reference ground
Figure 11-59 Time division multiplexing concept. Rotary switches with four sensors connected to four analog
outputs. Switches are simultaneously placed in the same position for a brief period of time, permitting two
wires to carry the information of four different inputs.
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
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372 Chapter 11
digital format. The various modules on the truck take SAE J1587/J1708 Standard
turns using this common pair of wires connected to
each module. Each module will transmit (place) 1s and For many years, SAE J1587 was used as the
0s on this common data link while the other modules message content standard or protocol used in truck
connected to the pair of wires receive or ‘‘listen’’ to serial data communications. The J1587 messages have
this digital data. The 1s can be a voltage value mea- a bit transmission rate of 9600 bits per second (bps).
sured across the two wires that is above some level,
while the 0s can be a voltage value measured across SAE J1708 is the standard that defines the hard-
the two wires that is below some level. The digital data ware, including the physical data link wiring used with
is transmitted and received serially or one bit after the the J1587 protocol. Together, the J1708 and J1587
other. standards are often referred to as the ATA data link.
The J1708-defined data link consists of a twisted pair
Only one module at a time may transmit informa- of copper wires covered with a standard wire insula-
tion on the data link, but all modules can receive this tion material. The wires are twisted together to reduce
information at the same time. This is like being in a the effects of electromagnetic interference (EMI). EMI
meeting with several people present. Only one person will be discussed in more detail in later chapters. The
at a time can speak or no one will be able to under- two data link wires are typically identified as + and –
stand what is being said. The digital data is several bits by most OEMs. However, neither wire is directly
in length and includes information such as which connected to a positive or negative voltage source.
module is sending the information, what type of in-
formation will be transmitted, and the actual infor- An interim message protocol standard that also uses
mation such as the engine oil pressure. the SAE J1708 hardware is the SAE J1922 protocol. The
J1922 protocol is similar to the J1587 protocol.
The engine ECM may be connected to the auto-
matic transmission ECU and the ABS ECU on a truck SAE J1587 Message Structure. Messages transmitted
through a serial communications data link. The engine over J1587/J1708 first indicate the major system or
ECM has direct access to a great deal of information device that is sending the information, called the
such as engine oil pressure and coolant temperature message identification (MID). SAE has assigned an
obtained through its own hardwired sensors. This industry-wide three-digit MID for each type of major
information can be useful to other modules in their system that is connected to the J1587/J1708 data link.
decision-making processes. For example, the informa- For example, the engine ECM is MID 128 and the
tion that the engine ECM receives from the accelerator- transmission ECU is MID 130.
position sensor is a necessary piece of information for
the automatic transmission controller’s decisions about Next, a parameter identifier (PID) is transmitted.
when to make the next shift. Instead of hardwiring the A PID is an SAE assigned number indicating the
accelerator-position sensor to both the engine and the specific information, such as PID 110 indicating en-
automatic transmission control module, the engine gine coolant temperature. After the PID, the actual
ECM can share the accelerator position information data such as the engine coolant temperature is trans-
with all the modules on the truck using a data link. mitted in a coded format. After the PID and its data,
additional PIDs and their associated data could be
The information being transmitted on the data link transmitted within the same message, up to maximum
may not be useful for all modules. Data that is not message length of 21 bytes. Any compatible electronic
needed by a module will be ignored. This is like sitting device throughout the industry that is connected to the
in a meeting (or classroom) where information that J1587/J1708 data link is capable of decoding the MID,
you do not care about is being presented. You will PID, and the actual data.
probably not listen very attentively until the topic of
the meeting shifts to something that pertains to you Following the MID and PIDs, a checksum value is
personally or the classroom instructor indicates that transmitted. The checksum is used for error checking
the material will definitely be on the test. to ensure that all devices have received the data cor-
rectly. If a device indicates an error, the information is
The digital information that is transmitted onto the retransmitted.
data link in modern trucks is defined by a Society of
Automotive Engineers (SAE) standard so that each SAE J1939 Standard
electronic module, regardless of who manufactured the
device, is able to understand the information being The J1587/J1708 serial communications standard,
transmitted on the data link. The standard is like a along with J1922, has mostly been replaced by SAE
common language that is understood by all modules. J1939. The bit transmission rate of J1939 is 250k or
500k bps compared to the 9600 bps rate of J1587/J1708.
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 373
This indicates that many more messages can be between the two conductors is not actually 75O. This
transmitted and received in the same amount of time resistance value is describing what is known as the
with a J1939 data link system compared to J1587/ characteristic impedance of the cable.
J1708. The difference in data transmission rates be-
tween J1939 and J1587/J1708 is similar to the differ- Impedance is like resistance but also includes the
ence between the fastest available cable modem and effect that alternating current has on inductors and
dial-up Internet service. capacitors. Inductors and capacitors react differently to
alternating current than they do to direct current due to
J1939 Specification Cable. The relatively low data the energy storage capabilities of inductors and ca-
transmission rate of the J1587/J1708 data link does not pacitors. An ohmmeter cannot measure this impedance
require much special attention other than twisting the because an ohmmeter only supplies a known value of
two conductors together. However, the high data DC voltage and measures the amount of DC current
transmission rate of the J1939 data link requires some passing through the unknown resistance, then calcu-
special consideration to provide reliable communica- lates the resistance using Ohm’s law. The resistance
tions. Unlike J1587/J1708, both the protocol and the between the two conductors of the coaxial cable is
hardware, such as the physical data link cable, are infinitely high because the plastic insulation material is
covered under sections of the same J1939 standard. an insulator. However, the coaxial cable has some
Two different versions of cable are defined by SAE specific value of impedance. Impedance, like resis-
J1939. Both versions of cable use a twisted pair of tance, is also measured in ohms. This is the reason that
copper wires but with a special plastic insulation. The coaxial cable has a 75O rating, even though the re-
special plastic insulation maintains a specific value of sistance between the two conductors is near infinity.
capacitance between the two wires of the data link. The characteristic impedance for the transmission
The two copper wires are like the two plates of a ca- media (cable) becomes more important as the fre-
pacitor. The plastic insulation material is the dielectric quency of the signal increases.
between the plates of this capacitor. Twisting of the
wires together limits electromagnetic interference and Do not worry if you do not understand the concept
susceptibility, but it also maintains a specific distance of characteristic impedance. Just realize that you cannot
between the two data link wires. Recall that the dis- connect your ohmmeter across the conductors of coaxial
tance between the plates of a capacitor is one of the cable rated at 75O and measure a resistance of 75O. It
factors that determine the value of capacitance. may be helpful to think of impedance as being ‘‘AC
resistance’’ and the resistance that you can measure
Together, the insulation material and the distance with a DMM ohmmeter as being ‘‘DC resistance.’’
between the two conductors cause the capacitance
between the two wires of the data link to be some The cable used in a J1939 data link has a 120O
specific value. This is important for reliable high- characteristic impedance. This 120O characteristic
speed data communications. An example of high- impedance cannot be measured with an ohmmeter in
frequency wiring with which you may be familiar is the same way that the 75O characteristic impedance of
satellite or cable television coaxial cable. This coaxial coaxial cable cannot be measured using an ohmmeter.
cable has a center copper conductor surrounded by a
special insulation material. A wire mesh surrounds this Two different J1939 standards define the data link
insulation material. The center copper conductor is one cable. The simplest type of cable is defined by SAE
conductor; the wire mesh is the other conductor. The standard J1939/15. This standard describes a twisted
insulation material surrounding the center conductor is pair of insulated wires with a special insulation ma-
the dielectric between the two plates of the capacitor terial. The twisted pair is then covered with a thin
(the two conductors). This causes a specific amount of plastic cable jacket. The jacket of cable that meets the
capacitance between the center conductor and the wire requirements of SAE J1939/15 standard is marked
mesh. Again, a specific value of this capacitance is with lettering indicating SAE J1939/15.
very important for good TV reception.
The other standard is SAE J1939/11. This standard
If you have ever tried to substitute plain insulated also describes a twisted pair of wires. However, this
wire for this special coaxial cable, you know that the standard includes a metal foil covering over the
quality of the picture suffers. The reasons for this are twisted pair called a shield (Figure 11-60). A third
beyond the scope of this book. However, you may noninsulated copper wire contacting the shield acts as
have noticed that the coaxial cable is referred to as a drain wire. The drain wire is connected to ground at
75O cable. The resistance of the wire or the resistance one point in the network through a capacitor, typically
within one of the electronic modules. The whole as-
sembly is also covered with a plastic jacket. The
purpose of the shielding is to provide immunity to
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
374 Chapter 11
© Cengage Learning 2014
Figure 11-60 J1939/11 shielded cable.
electromagnetic interference. Simply put, the shielding Network Topology. Topology refers to the manner
prevents stray electric fields, such as those generated in which a computer network is physically laid out.
by a CB radio, from causing information on the data The J1939 network is defined as a bus topology. The
link to be corrupted. The jacket of cable that meets the term bus is used to describe a single communications
standards of SAE J1939/11 is marked with lettering line shared by several devices. In the case of SAE
indicating SAE J1939/11. J1939, a single data link cable (twisted pair of wires) is
the shared communications line that routes throughout
Some OEMs use combinations of J1939/11 and the vehicle as shown in Figure 11-61. This single
J1939/15 on the same truck. The data link inside the cable is known as the backbone. Each module is
cab may be J1939/15 while the data link outside the connected to the backbone by a section of cable less
cab is J1939/11 shielded cable. than 1 m in length called a stub. The stubs are spliced
into the backbone near each electronic module
SAE J1939 cable is specified to have yellow in- throughout the vehicle.
sulation on one wire and green insulation on the other
wire. The yellow wire is designated CAN+ or CAN Terminating Resistors. You have probably had
HIGH while the green wire is designated CAN– or trouble understanding an announcer at an outdoor
CAN LOW. CAN stands for controller area network, sporting event because of multiple loudspeakers at
which is a type of communications network as ex- various distances from you causing the sound to arrive
plained later in this section. at your ears at different times. For similar reasons, the
J1939 data link requires a 120O terminating resistor
Tech Tip: Because of the high rate of data at each end of the backbone, or the use of special
communications transmitted over the J1939 electronic terminating bias circuits. Terminating bias
data link, specific OEM instructions should be circuits currently have limited usage in on-road truck
followed when repairs are performed on the J1939 networks. Therefore, only terminating resistors
data link wiring. Also, never substitute ordin- will be addressed in this text. The purpose of the ter-
ary wire for J1939 data link cable because the minating resistor is to prevent signal reflections from
plastic wire insulation material is not the same interfering with the data communications. Reflections
thickness or dielectric as J1939 specified cable can be thought of as being like echoes. The terminating
and could result in communication problems. resistors are not the same as the 120O characteristic
Engine ABS Trans
ECM ECU ECU
Stub Stub Stub
Stub Backbone Stub © Cengage Learning 2014
Terminating Body Instrument Terminating
resistor ECU cluster resistor
Figure 11-61 J1939 is classified as bus topology.
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Sensors, Digital Electronics, and Multiplexing 375
impedance of the data link but are real, ordinary re- Terminating resistors have a second function be-
sistors that can be measured using an ohmmeter. The sides reducing signal reflections. Terminating resistors
resistors are connected across the CAN+ and CAN– also provide an electrical load across CAN+ and
circuits at the two ends of the J1939 data link back- CAN–, similar to a pull-down resistor as described in
bone. These resistors absorb signal energy instead of the previous section. The top oscilloscope trace shown
permitting the signal energy to bounce back into the in Figure 11-62 indicates that the voltage difference
data link like an echo and corrupt the signal. between CAN+ and CAN– slowly decays to 0V with
no terminating resistors present. Compare this to the
One important thing to know about the terminat- lower oscilloscope trace in Figure 11-62 with both
ing resistors is that typically no communication be- terminating resistors present where the signal quickly
tween electronic devices can occur over a J1939 drops to 0V. The slow signal decay to 0V due to the
network without at least one of these terminating missing terminating resistors results in devices con-
resistors being present across the data link, as illus- nected to the bus misinterpreting the signal.
trated in Figure 11-62. This illustration shows the
voltage measured between CAN+ and CAN– using an Controller Area Network (CAN). A controller area
oscilloscope. Time is shown on the horizontal axis network (CAN) describes a type of communications
and the voltage difference between CAN+ and CAN– network in which there is no master controller. No
is shown on the vertical axis. With no terminating single electronic module is responsible for controlling
resistor connected across the data link, the signal the network and there are no network controllers as
reflections result in no bus communications being there are in most computer networks. Instead, each
possible. If just one terminating resistor is present, electronic module in a CAN contains a communica-
communication is possible, although there may be tions chip called a CAN transceiver that is designed
intermittent J1939 communications problems due to specifically to communicate with CAN transceivers in
signal reflections.
V diff collapses very slowly
1>
0
1<
Both J1939 terminating resistors missing
V diff collapses quickly to 0V © Cengage Learning 2014
1>
0
1<
Both J1939 terminating resistors present
Figure 11-62 J1939 oscilloscope patterns with terminating resistors missing (upper) and present (lower).
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
376 Chapter 11
other electronic modules that are connected to the Start of frame
same communications network. None of these modules Arbitration field
is in charge of the network. Control field
Data field
The CAN transceivers located in the various CRC field
electronic modules on a truck—such as the engine Ack field
ECM, the automatic transmission ECU, and the ABS End of frame
ECU—all take turns using the J1939 data link to
communicate specific information. This information Data frame © Cengage Learning 2014
includes vehicle speed, engine speed, coolant tem-
perature, oil pressure, and many other pieces of in- Interframe Interframe
formation. When one module has finished using the space space
data link to transmit a message, another module takes
over and uses the same data link to transmit its Figure 11-63 J1939 message format.
message. In a modern truck even with a large amount
of data link ‘‘traffic,’’ the J1939 data link is sitting For example, the engine ECM is assigned a source
idle with no messages being communicated more than address of 0 by the SAE J1939 specification while the
50 percent of the time. The transfer of information automatic transmission ECU is assigned a source
over the J1939 data link occurs so fast that it appears address of 3. Lower source address numbers indicate a
that all of the modules are transmitting on the data higher priority of the devices; therefore, the engine
link at the same time. ECM is assigned a source address of 0.
What is actually happening in a CAN network is After the arbitration field, a control field indicating
that each module is patiently waiting a turn to begin the length of the message is transmitted. After the
transmitting its information. The high rate of data control field, the actual data is transmitted. Typically,
communications on J1939 means that the time that a 64 bits of each message are the actual data contained in
module must wait for another module to finish com- parameter packages known as a suspect parameter
municating is very brief (microseconds). However, a number (SPN). An SPN is a number assigned by SAE
method of determining which message is the most to each parameter or measurable factor, such as SPN
important if two or more modules begin to commu- 110, engine coolant temperature, or SPN 175, engine
nicate at the same time (known as arbitration) is a part oil temperature. Several related SPNs are grouped to-
of the J1939 specification. If two controllers begin gether as a PGN. Therefore, a J1939 message for PGN
transmitting their messages at the exact same time, 65262 contains SPN 110 and SPN 175, along with four
the message with higher importance is communicated other SPNs related to engine temperature. A J1939
first and the controller with a less important message SPN is similar to a J1587 PID. Whenever possible,
will stop transmitting and will wait for another turn. both utilize the same number for a common parameter,
These communication rules for determining message such as engine coolant temperature being represented
importance have been programmed into each CAN by PID 110 in a J1587 message and SPN 110 in a
transceiver so no module needs to be in charge of the J1939 message.
network.
Lastly, J1939 messages contain a method of ver-
SAE J1939 Message Structure. A J1939 message ifying that all electronic modules connected to the
contains many pieces of information, as shown in J1939 data link have received the same message
Figure 11-63. The arbitration field at the beginning of called the cyclic redundancy check (CRC). If any
the message contains the priority, which is used to module on the J1939 data link indicates that an error
determine which message is more important if two or has occurred in receiving the message, the message is
more controllers begin communicating at the same retransmitted.
time. The arbitration field may also contain the pa-
rameter group number (PGN). The PGN indicates Altogether, a typical J1939 message contains 128 bits.
what type of data the message contains, such as engine The time for one electronic control module connected
speed related information (PGN 61444) or engine to a 500k bps CAN bus to broadcast a typical 128-bit
temperature related information (PGN 65262). Lower message, such as a message containing PGN 65262
PGNs indicate a higher message priority. The source engine temperature information, is 256 msec. This
address for the type of module that is sending the means that more than 3900 standard J1939 messages
information also comprises the arbitration field. per second could theoretically be transmitted and
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 377
received by any electronic module that is connected to Other Types of Digital
a 500k bps CAN bus. This large number of messages Communication
per second makes it appear that several messages are
being sent at the same time, although each electronic In addition to the J1939 and J1708 data links,
module is just borrowing or time-sharing the two wires other more primitive forms of digital communication
for 256 millionths of a second. are commonly found in truck electrical systems. Up
to this point, PWM has only been discussed as a
The actual 1s and 0s observed on a J1939 CAN means to control the average current by switching a
data link are determined by the difference in voltage DC voltage off and on to provide lamp dimming or
between the CAN+ and CAN– circuits. Figure 11-64 alternator voltage regulation. PWM is also a form of
illustrates what a two-channel (two-input) oscillo- digital communication. For example, on some en-
scope with one channel connected between CAN+ gines the ECM controls the variable geometry tur-
and ground and a second channel connected be- bocharger vane position by modification of the duty
tween CAN– and ground might indicate. The ver- cycle of a single-wire PWM control signal supplied to
tical axis in Figure 11-64 indicates the voltage the turbocharger actuator. This control signal is ref-
referenced to ground while the horizontal axis in- erenced to ground. A duty cycle of 10 percent of this
dicates time. If the difference in voltage between control signal could indicate that the ECM is com-
CAN+ and CAN– is approximately 0V, then the bit manding the turbocharger actuator to drive the vane
is logic 1 (recessive). If the difference in voltage position to 10 percent of the maximum position. The
between CAN+ and CAN– is approximately 2V, PWM signal is only a means of communication in this
then the bit is logic 0 (dominant). This may seem case and is not a variable voltage supply for the de-
backward, but all the devices connected to the J1939 vice. Variable geometry turbochargers are discussed
data link know the rules. The binary value shown in in Chapter 14.
Figure 11-64 would be interpreted as 101. If this
were a J1939 data link operating at 500k bps, then The universal asynchronous receiver/transmitter
the time for one bit would be 2 msec. Therefore, (UART) is another type of simple digital communi-
only 6 msec of activity is shown across the hori- cation. Asynchronous transmission refers to the
zontal axis in Figure 11-64. transmission of data without the use of a digital pulse
train called a clock signal between the two devices.
The high rate of data communication possible over Instead, start bits are used at the beginning of each
the J1939 data link has had a significant impact on digital message packet, which synchronizes the two
modern truck electrical systems. The use of multi- devices. UART typically only requires a single wire
plexing has extended into several areas of the modern with voltage levels being referenced to ground.
truck, as will be shown in the chapters that follow.
Additionally, other types of communications networks LIN (local interconnect network) is a serial com-
utilized on modern trucks will be introduced. munication protocol with a maximum transmission
5.0
4.5
4.0
Dominant
3.5
CAN +
Bus voltage
3.0 V difference Recessive
© Cengage Learning 2014Recessive
2.5
2.0
CAN –
1.5
1.0
0.5
0
Figure 11-64 J1939 CAN+ and CAN– voltage levels referenced to ground.
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378 Chapter 11
rate of 20k bps. Unlike a CAN network, one device oscilloscope to view activity on the J1939 data link.
is the master device and initiates all communication Note how the signal quickly rises and falls, unlike the
while the other devices merely respond when signal shown in Figure 11-62 where the terminating
commanded. A LIN network may be used as a resistors were missing. Oscilloscopes can be useful
means for the engine ECM, the master device, to tools in diagnosing electrical problems with the J1939
receive information from remote sensor modules. data link. More information on troubleshooting elec-
Typically, only one wire is necessary for LIN com- trical problems with the J1939 data link such as shorts
munications, with voltage levels being referenced to to ground or open circuits will be discussed in
ground. Chapters 14 and 15.
SENT (single edge nibble transmission) is a low- Although it might be possible to interpret each bit
cost one-way serial communication protocol used to of a message displayed on an oscilloscope connected
permit sensors and other devices to send information to across the CAN+ and CAN– terminals to determine
an ECM or ECU. The term nibble describes a group of what information a module is communicating, this
four binary bits, or one-half of a byte. would be very difficult to perform and would be
very time consuming. Therefore, the tool of choice
Troubleshooting the Multiplexed when troubleshooting much of the electrical system
Truck on modern trucks is a PC and a special communi-
cation adapter device designed to permit the com-
In Figure 11-65, a two-channel oscilloscope trace puter to be connected to the truck’s data links as
of a J1939 message is shown. One oscilloscope shown in Figure 11-66. The computer is connected
channel is connected between CAN+ and ground, and to the truck’s J1939 or J1587/J1708 data link via a
the other oscilloscope channel is connected between standardized diagnostic connector on the truck. This
CAN– and ground. The acknowledge bits shown in permits the computer to interface or communicate
Figure 11-65 indicate an end of the message. The with the various electronic modules on the truck. A
amplitude of the acknowledge bit is greater than the 6-pin diagnostic connector shown in Figure 11-67 is
other dominant bits in the message because all con- used on older trucks that only have a J1587/J1708
trollers connected to the bus are transmitting this bit at data link. Modern trucks with J1939 have a 9-pin
the same time indicating that they have all received the diagnostic connector as shown in Figure 11-68. The color
message. Figure 11-65 illustrates a typical waveform of this 9-pin diagnostic connector is specified as gray
pattern you would expect to see using a two-channel or black for 250k bps J1939 networks. A green-colored
5.0
Acknowledge
4.0 bits
3.0
2.0
1.0 © Cengage Learning 2014
V-DC 50 100 150 200 250 300 350
0 μs
Figure 11-65 Dual-channel oscilloscope trace of J1939 messages.
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Sensors, Digital Electronics, and Multiplexing 379
Personal computer (PC) BF 6-Pin J1587/1708 connector
Adapter cable
CAE A = Data bus, dominant high (+)
D B = Data bus, dominant low (–)
C = Battery positive
D = Dummy
E = Ground
F = Dummy
© Cengage Learning 2014
© Cengage Learning 2014
Figure 11-67 6-Pin J1587/J1708 diagnostic connector
pin assignments.
Adapter cable 9-pin diagnostic connector may be used by some
OEMs to identify 500k bps J1939 networks. This
Adapter cable identification of the J1939 transmission rate is neces-
sary because attempting to communicate at the wrong
Cable adapter transmission rate will cause all CAN communications
to cease.
Communication adapter
Special software packages provided by the en-
Figure 11-66 Communications adapter provides gine, automatic transmission, and ABS system
interface between the vehicle J1939 CAN bus and PC. manufacturers provide troubleshooting assistance for
these complex systems. Collectively, the PC, com-
munications adaptor and the software packages may
be referred to as an electronic service tool (EST).
Some examples of ESTs will be introduced in later
chapters.
D
CE
F Front view
of connector
A G
B Cavity Label
J A Battery ground
H B +12V DC
C J1939 data link (+)
D J1939 data link (–) © Cengage Learning 2014
E J1939 shield
F J1587 data link (+)
G J1587 data link (–)
H Plug
J Plug
Figure 11-68 9-Pin diagnostic connector pin assignments.
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380 Chapter 11
Summary
n A modern truck may have several different types of used to convert real-world analog signals into the
sensors used to measure temperature, pressure, digital equivalent for use by the microprocessor.
speed, and position.
n The two main categories of memory are volatile and
n A variable reluctance sensor is a small version of an non-volatile. Volatile memory is lost when electric
AC generator. These types of sensors are often used power is disconnected from the device while non-
to measure a rotational speed such as an ABS wheel volatile memory is retained thorough power
speed sensor. disconnection.
n The Hall effect describes the generation of voltage n The internal circuitry in an electronic module input
in the presence of a magnetic field. The voltage that is typically has a pull-up resistor or pull-down resistor.
generated is proportional to the magnetic field strength.
Unlike electromagnetic induction, no magnetic lines of n Electronic module inputs can be digital or analog
force have to be cut to generate this voltage. types of inputs. These digital or analog inputs can
be pulled-up or pulled-down.
n An analog signal is a signal that can be any level
within a range. A digital signal can only be one of n Multiplexing refers to methods used to combine
two different values. more than one channel of information into a com-
mon signal path. Common forms of multiplexing
n Logic gates are hardware devices used to make a used on modern trucks include analog multiplexing
decision. There are several types of logic gates. A and time-division multiplexing.
truth table describes the decision made by the logic
gate to all the possible inputs. n Serial data communication is a form of time-division
multiplexing. An example of serial data communi-
n A microprocessor is an integrated circuit that con- cation is the SAE J1939 data link.
tains thousands of logic gates. The microprocessor
only accepts digital information and provides results n A controller area network (CAN) is a communica-
in digital form. An analog to digital converter is tions network without a master control device.
Suggested Internet Searches
Try these web sites for more information on CAN, SAE J1939, sensors, and electronics:
http://www.vector.com
http://www.sae.org
http://www.ngk-detroit.com
http://www.sensata.com
http://www.wema.com
http://www.conti-online.com
Review Questions C. The frequency of the AC voltage generated by a
variable reluctance sensor increases as the speed of
1. Which of the following is a true statement? the target passing in front of the sensor increases.
A. A variable reluctance sensor is a Hall D. The amplitude of the AC voltage generated by a
effect device. variable reluctance sensor decreases as the speed of
the target passing in front of the sensor increases.
B. Air gap refers to the space between the
center electrode and ground electrode in
a diesel engine spark plug used in the
combustion chamber to ignite the diesel
fuel.
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Sensors, Digital Electronics, and Multiplexing 381
2. How would a thermistor with an NTC respond to an increase in temperature?
A. The voltage produced by the sensor C. The resistance of the sensor would decrease.
would increase. D. The resistance of the sensor would increase.
B. The voltage produced by the sensor
would decrease.
3. A potentiometer would probably be used to measure which of the following?
A. Engine coolant temperature C. Vehicle speed
B. Pre-2007 diesel engine intake throttle D. Accelerator position
position
4. Which of the following would be considered digital?
A. Mechanical clock C. Phonograph record
B. Sound of a single guitar string being D. Wall switch used to control an electric light
plucked
5. The odometer information in an electronic engine ECM would probably be stored in which type of memory?
A. Non-volatile memory C. Flash memory
B. RAM D. Floppy disk memory
6. An electronic module input terminal that is internally pulled-down and is connected to a switch that is open
would probably detect approximately how much voltage at that input terminal?
A. Near battery positive voltage level C. Depends if switch is normally open or normally
B. About 0V closed
D. A negative voltage level
7. What is the difference between a digital input and an analog input?
A. A digital input is always pulled-up and C. Digital inputs are used with microprocessors; analog
an analog input is always pulled-down. inputs are used with transistors.
B. A digital input is typically connected to D. Both types of inputs are the same.
an on-off type of switch and an analog
input is typically connected to
something for which a range of voltages
must be measured, such as a sensor.
8. Which module is in charge of a CAN used on a modern truck?
A. Jefe module C. Whichever module is larger
B. Router module D. No single module is typically in charge of a controller
area network
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382 Chapter 11
9. Which best describes data communications with a J1939 specification CAN?
A. Modules are assigned a priority. When C. The engine ECM controls the network and decides
a module with a higher priority has which modules it wants to permit to communicate.
information to communicate, all message
traffic on the data link stops in the D. All modules are simultaneously communicating
middle of a message so that the highest- information.
priority module can communicate.
B. Any electronic module can
communicate when the data link is not
being used by another module.
10. Analog multiplexing describes which of the following?
A. Use of a single digital input to measure C. Modulating a high-frequency signal onto a pair of
several different analog voltages wires
B. The use of several analog inputs to D. The use of an analog input connected to several
measure several different digital devices parallel-connected switches, each of which has a
at the same time different value of series resistance
11. Where are the terminating resistors typically located on a J1587/J1708 serial communications network?
A. At the extreme ends of the network C. In the hub or router
B. In the exact center of the network D. External terminating resistors are not typically used on
J1587/J1708 networks
12. Which conductor has green wire insulation in a J1939/11 data link cable?
A. CAN neutral C. CAN ground
B. CAN + or CAN H D. CAN – or CAN L
13. A two-input AND gate is most like which of the following?
A. Two switches wired in parallel C. Two switches wired in a series-parallel configuration
B. Two switches wired in series D. An SPST switch
14. A two-input OR gate is most like which of the following?
A. Two normally open switches wired in C. Two DPST switches
series D. Two switches wired in a series-parallel configuration
B. Two switches wired in parallel
15. The output of a variable reluctance sensor is which of the following?
A. Variable amplitude AC voltage C. DC voltage
B. Variable frequency AC voltage D. Both A and B
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
CHAPTER
12 Instrumentation
Learning Objectives
After studying this chapter, you should be able to:
n List the various types of electromechanical gauges and explain the operation of each type.
n Describe the four types of magnetic gauges.
n Explain the purpose of a bucking coil in a three-coil gauge.
n Modify the vehicle speed pulses-per-mile setting.
n Troubleshoot a problem with conventional instrumentation.
n Explain the concept of multiplexed instrumentation using J1587/J1708 or J1939 data link.
n Describe the operation of a stepper motor used in an instrument panel cluster.
n Diagnose multiplexed instrumentation using OEM information.
Key Terms instrument voltage regulator pyrometer
(IVR) sender
bimetallic gauge sending unit
breakout box ISO symbol stepper motor
breakout T
bucking coil liquid crystal display (LCD)
DIP switch
instrument panel cluster (IPC) magnetic gauge
programmable parameter
INTRODUCTION terms for their instrument panel cluster, as will be dis-
cussed in Chapter 13.
Trucks have a variety of gauges, indicator lamps,
and audible alarms to keep the truck operator informed In the past, many gauges found on a truck were
of the status of various truck systems. Collectively, mechanical or electromechanical-type gauges. Most
these devices are referred to as instrumentation. Most trucks now use multiplexed electronic instrumentation.
OEMs place a group or cluster of these gauges and
lamps into a package called an instrument panel This chapter is divided into two main sections. The
cluster (IPC). However, OEMs may utilize proprietary first section discusses conventional non-multiplexed in-
strumentation. The second section discusses multiplexed
instrumentation and stepper-motor-driven gauges.
383
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
384 Chapter 12
CONVENTIONAL needle responds to the rate of the cable rotation to
INSTRUMENTATION indicate vehicle speed.
Conventional instrumentation describes non- Mechanical speedometers have several drawbacks.
multiplexed IPCs in which each electrically controlled These devices are often inaccurate. They also require a
gauge and indicator lamp is individually hardwired to gearing change if rear-axle ratio or tire sizes are
a sensor or switch. These conventional IPCs may also changed. Mechanical speedometers may also make it
contain mechanical gauges such as air pressure gauges difficult to remove and install the IPC because of the
that require plumbing between the gauge and the cable connection. The cable must also be lubricated
pressure source. periodically to minimize needle pulsation.
The gauges found in a conventional IPC are typi- Because diesel engines do not have an ignition sys-
cally analog gauges (Figure 12-1). The indicator tem like that found on a gasoline engine, the tachometer
needle in the gauge sweeps through a circular arc. This on older mechanical diesel engines was typically a
type of gauge is described as analog because the mechanical cable-driven gauge similar to a speedometer.
movement of the gauge is continuous, like the move- The tachometer cable was typically driven from the fuel
ment of the hands of a clock. injection pump.
The two main categories of analog gauges are Electromechanical Gauges
mechanical and electromechanical.
Electromechanical gauges are electrically operated
Mechanical Gauges gauges that cause a mechanical movement of the
gauge needle or pointer. Regardless of whether the
Mechanical gauges were utilized extensively in the gauge is used to indicate fuel level, coolant tempera-
past and may still be found on some modern trucks. ture, or oil pressure, the electromechanical gauge is
The conventional Bourdon tube-type air pressure really an indication of the level of electric current
gauge is an example of a mechanical gauge. The air flowing through the gauge’s coils. Most electrome-
pressure gauges in the instrument panel display chanical gauges use a sensing device known as a
the pressure in the primary and secondary air systems. sender or sending unit to transform some physical
The air pressure indicator may be contained within one quantity such as fuel level into a variable resistance.
gauge housing on some trucks and have two separate The variable resistance caused by the sending unit is
needles, similar to the two hands of a clock, that are connected in series with the corresponding gauge’s
typically colored green (primary) and orange (sec- coils. The resistance of the sending unit controls the
ondary). Air lines must be routed from the air tanks to current flow through the gauge’s coils, causing the
the pressure gauges. Some trucks may also have an air analog gauge needle to indicate the corresponding oil
brake application gauge that indicates the pressure pressure, coolant temperature, fuel level, or other
being supplied to the primary brake chambers. quantity. There are two main types of electrome-
chanical gauges: bimetallic and magnetic.
Speedometers in trucks of the past were typically
cable-driven gauges. A gear in the transmission or Bimetallic Gauges. Bimetallic gauges make use of a
transfer case causes the cable to rotate at a rate cor- bimetallic strip, like that found in circuit breakers,
responding to the vehicle speed. The speedometer
© Cengage Learning 2014
Figure 12-1 Conventional instrument panel cluster.
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Instrumentation 385
viscous fan drives, and other temperature-dependent Bimetal Heating
devices. A bimetallic strip is made from two thin strips strip coil
of dissimilar metals such as brass and steel that are
bonded together (Figure 12-2). In this example, the E F
two strips of metal are both the same length at 708F
(218C). When the bimetallic strip is heated to 1408F © Cengage Learning 2014
(608C), both the steel and the brass will expand.
However, the amount of expansion that will occur for a Figure 12-3 Bimetallic gauge.
given temperature is not the same for all metals. Brass
will expand much more than steel when heated. When into gauge needle movement. Thus, the current flow
heated, both of the individual strips of dissimilar metal through the heating coil causes the bimetallic strip to
forming the bimetallic strip must become longer due to heat, which causes the gauge needle to indicate the
the expansion. The bimetallic strip is bent into an arc amount of current flowing through the heating coil.
or section of a circle because of the greater length of
the brass strip compared to the steel strip. The strip The sending unit for bimetallic gauges is typically a
will bend in such a way that the brass material is on the variable resistance device. The example shown in
outside of the bend to permit the brass strip to be Figure 12-4 is a fuel level gauge circuit. The fuel level
longer than the steel strip. sending unit shown is a rheostat. A float in the fuel
tank is mechanically connected to the rheostat wiper.
Both metals also will not contract (shrink) the same This causes the rheostat resistance to change in ac-
amount when cooled. When the strip is cooled to 08F cordance with fuel level (Figure 12-5). In this exam-
(À188C), the brass will contract more than the steel, ple, the resistance of the rheostat decreases as the tank
causing the strip to deflect in the opposite direction. is filled. This decrease in resistance causes an increase
in current passing through the heating coil. The in-
The amount of curvature of the bimetallic strip that crease in current through the heating coil causes the
occurs is directly proportional to the temperature of the bimetallic strip to deflect, resulting in movement of
bimetallic strip. This permits the strip deflection to act the gauge needle toward the full indication on the
as an indicator of strip temperature. To take advantage gauge face.
of the properties of a bimetallic strip, a coil of wire is
wrapped around the bimetallic strip as shown in the A bimetallic gauge depends on the current through
fuel gauge example in Figure 12-3. The heat generated the heating element surrounding the bimetallic strip to
by the current flow through the heating coil causes accurately display the quantity being measured.
the bimetallic strip or spring to bend into an arc. The Therefore, changes in the system voltage such as en-
more current that flows through the heating coil, the gine running (14V) and engine not running (12.6V)
more heat that is generated in the heating coil, and would cause a large amount of inaccuracy in bimetallic
the more the bimetallic strip bends. The gauge is de- gauges if the gauge heating coils were supplied with
signed to transform the bending of the bimetallic strip battery voltage. Ohm’s law indicates that more current
will flow through the heating coil when the engine
70°F 140°F 0°F © Cengage Learning 2014 is running than when the engine is not running because
of the increase in system voltage with the engine
Figure 12-2 Bimetallic strip made of brass and steel. running. This change in system voltage will result in
gauge inaccuracy. To prevent fluctuating electrical
system voltage from causing gauge inaccuracy, an
instrument voltage regulator (IVR) is used to pro-
vide a constant voltage source to the gauge heating
coils. In the past, IVRs used rapidly opening and
closing contacts to generate a pulse width modulated
(PWM) type of voltage that equated to an average
voltage of about 5V DC (Figure 12-6). Modern IVRs
are electronic devices with no moving components,
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
386 Chapter 12
Resistance Bimetallic arm Heating
Pointer coil
Battery Float © Cengage Learning 2014
Figure 12-4 Fuel level sensor circuit.
Instrument
voltage
regulator
Variable resistor
Moveable © Cengage Learning 2014
float
Figure 12-5 Fuel level sensor or sending unit.
Bimetal Regulated Instrument
output voltage
arm Heating voltage regulator
Voltage coil Contacts
supply
input
terminal
Instrument Input side Output side © Cengage Learning 2014
voltage 12V DC (approx.)
regulator 5V average
(12V pulsating)
Radio
choke
Figure 12-6 Instrument voltage regulator (IVR).
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Instrumentation 387
which supply a constant voltage to the gauge heating D C
coils. Permanent
Pointer magnet
Because bimetallic gauges make use of a heating
element, rapid changes in the quantity being measured— Armature © Cengage Learning 2014
such as fuel level in the fuel tank on a bumpy road—
do not cause rapid movement of the gauge needle. Conductor
Bimetallic gauges may also take a minute or more
when cold to accurately indicate fuel level or other Pivot
quantity.
Figure 12-7 D’Arsonval gauge movement.
Magnetic Gauges. Magnetic gauges use the princi-
ples of electromagnetism to produce gauge needle rotate accordingly. Reversing the direction of current
movement. Conventional magnetic gauges have coils flow through the conductor will cause the needle to
that are directly connected to the sensor or voltage deflect in the opposite direction. Ammeters are often
source being measured. Modern magnetic gauges may d’Arsonval-type movements because current flow
use an electronic module in the IPC to control the through the conductor in the opposite direction will
voltage supplied to the gauge coils. The sensor acts as cause the gauge needle to rotate in the opposite
an input to the electronic module with these electron- direction.
ically controlled magnetic gauges. The module then
controls the current through the gauge coils. These Ammeters are zero-center gauges with a D or – sign
electronically controlled magnetic gauges will be ad- to indicate the batteries are discharging and a C or + sign
dressed later in this section. to indicate the batteries are charging (Figure 12-8). The
ammeter shunt is located in the charging system circuit
There are four main types of magnetic gauges: such that cranking-motor current is not measured by the
d’Arsonval, three coil, two coil, and air core. ammeter. When there is zero current flow into or out of
the batteries, the ammeter will indicate zero or will be
D’ARSONVAL GAUGES centered. When current is flowing from the batteries,
A d’Arsonval-type gauge makes use of a permanent the ammeter will indicate a negative value of current or
discharging. When current is flowing into the batteries,
horseshoe magnet that surrounds a moveable electro- the ammeter will indicate a positive value of current or
magnet or armature attached to the gauge needle charging.
(Figure 12-7). Current flow through the armature
winding causes the gauge needle to move due to in-
teraction between the magnetic field caused by the
permanent magnet and the magnetic field caused by
the electromagnet. Increasing the current flow through
the electromagnet causes the gauge needle to deflect or
ALT Ammeter conditions ALT
DC DC
ALT
DC
Discharging High charge rate Normal
Battery is discharged. Battery is partially charged Battery is charged © Cengage Learning 2014
AC generator is not charging and AC generator is and AC generator is
or is not maintaining vehicle’s recharging it. supplying the vehicle’s
electric needs. electrical needs.
Figure 12-8 Ammeter needle movement indicating charging or discharging batteries.
Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
388 Chapter 12
12V Empty Full
Low N N High
Bucking S
S
S
N
Empty
Full © Cengage Learning 2014
Figure 12-9 Three-coil gauge.
THREE-COIL GAUGES that the needle will maintain. Note that the low reading
Three-coil gauges make use of three coils, as the coil and the bucking coil are wound in opposite di-
rections. The two coils are also placed next to each
name implies. The three coils form three electro- other so that current flow through the bucking coil
magnets (Figure 12-9). These three coils are called the causes a magnetic field that is opposite to or ‘‘bucks’’
high reading coil, the low reading coil, and a bucking the magnetic field formed by the low reading coil. This
coil. The gauge needle has a permanent or electro- causes these two opposing magnetic fields to attempt
magnet attached to it. A three-coil gauge is designed so to cancel each other out.
that the magnet on the needle is more attracted to ei-
ther the high reading coil or the combination of low The three coils inside the gauge and the sending
reading coil and bucking coil magnetic fields. The unit form a series-parallel circuit (Figure 12-10). Ig-
resistance of the sending unit determines which re- nition voltage (+12V) is supplied to the low reading
sulting magnetic field is stronger and thus the position coil. The fixed resistor in parallel with the low reading
Fuel gauge
3 Low reading Bucking High reading
coil coil coil 1
Ignition
switch Fixed EF
resistor
2
Sending © Cengage Learning 2014
unit
Figure 12-10 Sending unit resistance is low, indicating an empty fuel tank.
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
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