Page 351 - Understanding Automotive Electronics

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10 DIAGNOSTICS

that the use of these two diagnostic instruments will be phased out within a
few years.
ELECTRONIC CONTROL SYSTEM DIAGNOSTICS
Each microprocessor-based electronic subsystem has the capability of
performing some limited self-diagnosis. A subsystem can, for example, detect a
loss of signal from a sensor or detect an open circuit in an actuator circuit as
well as other failures. As long as the subsystem computer is still functioning it
can store fault codes for detected failures. Such diagnosis within a given
subsystem is known as on-board diagnosis.
However, a higher level of diagnosis than the on-board diagnosis is
typically done with an external computer-based system that is available in a
service shop. Data stored in memory in an on-board subsystem is useful for
completing diagnosis of any problem with the associated subsystem. Such
diagnosis is known as off-board diagnosis and is usually conducted with a
special-purpose computer.
In order for fault code data to be available to the off-board diagnosis
computer, a communication link is required between the off-board equipment
and the particular subsystem on board the vehicle. Such a communication
system is typically in the form of a serial digital data link. A serial data link
transmits digital data in a binary time sequence along a pair of wires (one of
which is normally ground). Before discussing the details of on-board and off-
board diagnosis, it is perhaps worthwhile to briefly discuss automotive digital
communications.
Intermodule communications within an automobile itself and between
each on-board subsystem and an off-board system are analogous to
communications between computers. That is, they take the form of a local
area network (LAN).
There are several important issues involved in any LAN, including the
communication protocol, the bit rate, access to the network, and
synchronization between the various subsystems. In a LAN, each subsystem
connected to the serial data link is called a node. Normally one of the nodes
serves as a master that has the capability of controlling access to the network.
The other nodes are called slaves and respond to software-controlled commands
issued by the master.
One of the possible problems that can occur in a LAN is the simultaneous
transmission of messages by two separate nodes (called collision). Resolution of
potential collisions is controlled by the master using a software capability
known as arbitration.
An example of an automotive digital data system is the Controller Area
Network (CAN) that was developed by the Robert Bosch Company in
Germany. CAN is a serial asynchronous communication protocol that connects
electronic control modules’ sensors and actuators in automobiles. Among its
many functions and services is a digital data link.

338 UNDERSTANDING AUTOMOTIVE ELECTRONICS