Circuit/System Description

The serial data is transmitted on two twisted wires that allow speeds up to 500 Kb/s. The twisted pair is terminated with two 120 ohms resistors, one is internal to the engine control module (ECM) and the other can be a separate resistor in a connector assembly or in another control module. The resistors are used as the load for the Powertrain High Speed GMLAN buss during normal vehicle operation. The Powertrain High Speed GMLAN is a differential bus. The Powertrain High Speed GMLAN serial data bus (+) and Powertrain High Speed GMLAN serial data (-) are driven to opposite extremes from a rest or idle level of approximately 2.5 V. Driving the lines to their extremes, adds one volt to the Powertrain High Speed GMLAN serial data bus (+) circuit and subtracts one volt from the Powertrain High Speed GMLAN serial data bus (-) circuit. If serial data is lost, control modules will set a no communication code against the non-communicating control module. Note that a loss of serial data DTC does not represent a failure of the module that set it.