Page 339 - Understanding Automotive Electronics

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9 AUTOMOTIVE INSTRUMENTATION

control. The driver can, for example, select to display the entertainment system
operation. This display mode permits the driver to select radio, tape, or CD,
and to tune the radio to the desired station and set the volume. In vehicle
diagnostic mode, the CRT can be configured to display the parameters
required by the mechanic for performing a diagnosis of any on-board
electronic system.
In Figure 9.26, several electronic systems are connected by the digital
data link. Tying systems together this way has great potential performance
benefits for the vehicle. Each automotive subsystem has its own primary
variables, which are obtained through measurements via sensors. A primary
variable in one subsystem might be a secondary variable in another system. It
might not be cost-effective to provide a sensor for a secondary variable to
achieve the best possible performance in a stand-alone subsystem. However, if
measurement data can be shared via the digital data link, then the secondary
measurement is potentially available for use in optimizing performance.
Furthermore, redundant sensors for measuring primary variables can be
eliminated by an integrated electronics system for the vehicle. For example,
wheel speed measurements are primary variables for ABS systems and are also
useful in electronic transmission control.
The various subsystems in Figure 9.26 have all been identified in other
sections of this book and will not be discussed further here, except for the
system manager. This subsystem is responsible for coordinating data transfer
and regulating the use of the data bus so that no two systems are transmitting
simultaneously.
Essentially, the digital data link provides a sophisticated
communication system between various subsystems. Among the issues of
importance for such a communication system are the physical protocol and
message format. It is highly advantageous to have a standard protocol for all
automobiles. The Society of Automotive Engineers (SAE) is working to
develop a standard protocol for the high-speed digital data link. This link
operates at a data rate of 1 megabit/sec and can be implemented with wire or
optical fiber. Any of a number of bit-encoding schemes are useful for
message formats, the details of which are unimportant for the present
discussion.
Some form of network arbitration is required for determining priority of
the use of the link whenever there is conflict between subsystems for its use.
This feature is typically handled by the system manager.
The basic message structure is derived assuming that the majority of data
on the link is regularly sent. This means that the content of each message is
known (only the actual data varies).
The potential for incorporating the CRT as an automotive display will be
greatly enhanced if the solid-state CRT becomes available at sufficiently low
cost. It can, potentially, lead to the so-called glass cockpit described next. Such a

326 UNDERSTANDING AUTOMOTIVE ELECTRONICS