GMLAN Circuit Description

The data link connector (DLC) allows a scan tool to communicate with the GMLAN serial data circuit. On this vehicle, only GMLAN high speed is used. That means that the serial data is transmitted on 2 wires at an average of 500 Kbps. The high speed dual wire GMLAN is a differential bus. That means that two bus lines, GMLAN high and GMLAN low are driven to opposite extremes from a rest or idle level. The idle level which is approximately 2.5 volts is considered a recessive transmitted data and is interpreted as a logic 1. Driving the lines to their extremes means adding 1 volt to GMLAN high wire and subtracting 1 volt from GMLAN low wire. This dominant state is interpreted as a logic 0. GMLAN network management supports selective start up and is based on virtual networks. A virtual network is a collection of signals started in response to a vehicle event. The starting of a virtual network signifies that a particular aspect of the vehicles functionality has been requested. A virtual network is supported by virtual devices which represents a collection of signals owned by a single physical device. So, any physical device can have one or more virtual devices. The signal supervision is the process of determining whether an expected signal is being received or not. Failsofting is the ability to substitute a signal with a default value or a default algorithm, in the absence of a valid signal. Some messages are also interpreted as a, heartbeat, of a virtual device. If such a signal is lost, the application will set a no communication code against the respective virtual device. This code is mapped on the Tech 2 screen as a code against the physical device. Note that a loss of serial data DTC does not normally represent a failure of the module that set it.