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format, refer to SAE J1708 document Standard For Data Communications Interface and
to SAE J1587 document, Standard For Data Interchange.
Electronic Control Units (sometimes called ECUs) are computerized units that control
a particular vehicle subsystem. For example, an Engine ECU helps maximize engine
efficiency, a Transmission ECU helps control transmission operation, and an ABS
(Anti-lock Brake System) ECU maximizes braking performance. An ECU designed
for one system cannot be interchanged with an ECU designed for another.
ECUs are connected to the J1708 data bus. They send and receive data to each other
and to other devices on the bus. Each ECU is programmed to respond to and to gen-
erate specific data.
The SMC is connected to the J1708 data bus and responds to data from the Engine
ECU. It uses road speed data from the Engine ECU to calculate its own pointer infor-
mation. It also responds to other data on the J1708 data bus and can display it on the
Message Center. Examples are engine hours and diagnostic messages. The SMC has
no calibration switches and requires no programming. See Figure 3-5.
The SMC calculates and stores its own odometer, trip odometer, clock and alarm
clock information independent of the Interface Module, and has a built-in audible
alarm. The SMC automatically displays warning messages when an alarm condition
is detected.
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