120 Auto Engine Performance and Driveability
Many circuits can travel on the same bus sorted by
voltage, frequency, and the type of signal they use. This net-
work is referred to as multiplexing. Multiplexing is the use of
a single wire or pathway to transmit two or more messages
at the same time. The same wire can be used to transmit any
number of messages if the complete circuit for each mes-
sage is separate. Many operations can be carried on at the
same time over the same wire, without interfering with each
other. The simplest type of multiplexing is a battery ground
cable, which carries the electrical impulses of the ignition,
headlight, and radio circuits while keeping each separate.
Data busses are made of copper wire. Copper is used
for minimal electrical resistance. In the future, fiber optics
may be used to create the vehicle data bus. Since fiber
optics have the ability to simultaneously transmit multiple
signals with almost no resistance, information can be trans-
mitted, received, and processed at a much faster rate than
over copper wire.
Data Bus Class
The speed at which a data bus carries information defines
its class. At present, there are three classes of busses:
p
Class A busses carry information at a maximum speed
of 10 kilobits per second.
p
Class B busses carry information at speeds of 10 to
125 kilobits per second.
p
Class C busses carry information at up to one million
kilobits per second.
In the future, Class D busses, capable of carrying over
one million kilobits per second, may be developed. The bus
itself does not determine speed, since it is simply a wire.
Bus speed, and therefore class, is determined by the ECMs
that are connected to the bus. A Class C bus may sometimes
carry information at Class A or B speeds, depending on the
speeds of the attached ECMs. However, a bus cannot carry
information at a speed higher than its maximum.
Controller Area Network (CAN)
Vehicle manufacturers are now using a more sophis-
ticated bus called a controller area network (CAN). The
CAN consists of a bus connecting various ECMs. On a CAN
system, however, each ECM has a unique address, or code.
Therefore, every signal from an ECM has an identification
code telling all other ECMs from where the information origi-
nates. The circuits inside of each ECM read the code before
processing the signal. If the information is not intended for
the ECM, it is ignored. This keeps information from entering
the wrong ECM. In some cases, the main computer deter-
mines where the information should be sent.
In addition to the identification code, information from
the sending ECM also includes a priority code. If two ECMs
send a signal to a third ECM, the signal with the highest-
priority code is processed first. Sometimes an ECM may
interrupt a low-priority message to receive a higher-priority
message. The ECM resumes processing the lower priority
message once the higher-priority message has been pro-
cessed. Determining priority may be the job of the main
computer. This type of ECM programming is sometimes
called dataflow or multithread programming.
Figure 7-4. Varying the length of the pulse width allows the ECM to control the operation of many devices. A—Narrow pulse width.
B—Wide pulse width. Note the difference in the length between the pulses. (Fluke)
A B