Multiplex Communications Network

Aboard this vehicle are several electronic components which can be controlled by an off-board tester to provide valuable self-diagnostic information. These components are part of an electrical system network, designed to control powertrain, chassis and body subsystems. The overall electronic network system consists of two microprocessor-based modules which interface through a common communication link. This communication link consists of a twisted wire pair (Data Bus Plus and Data Bus Minus) routed throughout the vehicle, Multiplex communications network signals are carried over an unshielded twisted pair. No special cabling is needed for the multiplex communication network. The wire pair is arranged in a "bus" configuration or party line, and is routed throughout the vehicle. Modules in the system connect to this bus so that the bus serves as a common tie point to each module.

Fault Tolerance
The multiplex communication network system will remain operational even with the severing of one of the bus wires. Communications will also continue if one of the bus wires is shorted to ground or battery positive voltage (B+) or a termination resistor is lost. This faulted condition is detected and reported to the host by the module's network bus interface circuits. In addition, one or more failed modules will not inhibit communications on the remainder of the network.

Purpose
This section contains information necessary for diagnosis of communication link conditions which may affect normal functional operation of the vehicle. Customer symptom may be directly related to these conditions. In order to effectively diagnose the communication link between all modules, the diagnostic procedures outlined should be followed with each step performed in the order presented.