Circuit Description

The communication among control modules is performed through the High speed GMLAN serial data circuits and the Low speed GMLAN serial data circuit. The modules that need real time communication are attached to the High speed GMLAN network. The body control module (BCM) is the gateway between the high and low speed networks. Communication is necessary between the high and low speed serial data modules for the vehicle to start. Refer to Body Control System Description and Operation  for more information about the gateway.

Signal supervision is the process of determining whether an expected signal is being received or not. Some messages are sent on a periodic basis and are interpreted as a heartbeat of a device. If such a signal is lost, the signal supervision part of the software will set a no communication DTC (U code) against the missing device. This code is mapped on the Tech 2 screen as a code against the physical device. A lost communication DTC typically is set in modules other than the module with a communication failure.