Information Bus: Description and Operation

Data Link Communications Description and Operation

Circuit Description

The communication among control modules is performed through the Class 2 serial data communication circuit and the GMLAN high speed serial data communication circuit. The control modules that need real time communication are attached to the high speed GMLAN network. The data link connector (DLC) allows a scan tool to connect into the serial data networks and communicate with the control modules on the Class 2 or high speed GMLAN serial data lines. In order for the scan tool to communicate with the control modules on the high speed GMLAN network, a CANdi module is needed. The CANdi module behaves as an interface between the scan tool and the high speed GMLAN network.

On Diesel Engine equipped vehicles, a Serial Data Gateway Module is used as a gateway between networks. The purpose of the gateway is to translate serial data messages between the GMLAN high speed bus and the GMLAN low speed bus. The gateway will interact with each network according to that network's transmission protocol. Refer to Body Control System Description and Operation (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. In the absence of a valid signal, the device receiving the information may substitute a default value or a default algorithm in place of the missing signal. The no communication DTC is mapped on the scan tool screen as a code against the physical device.

A lost communication DTC typically is set in control modules other than the control module with a communication failure.

Class 2 Serial Data Line

Class 2 serial data is transmitted on a single wire at an average of 10.4 kbps. Class 2 uses a variable pulse width modulation to carry data and depending on the message it may operate faster or slower. The bus will float at a nominal 7 volts during normal operation. Each control module can pull this lower during the transmission. The bus is not at battery positive voltage or ground potential during normal operation. When the ignition switch is in RUN, each control module communicating on the Class 2 serial data line sends a state of health (SOH) message every 2 seconds to ensure that the control module is operating properly. When a control module stops communicating on the Class 2 serial data line, for example if the control module loses power or ground, the SOH message it normally sends on the data line every 2 seconds disappears. Other control modules on the Class 2 serial data line, which expect to receive that SOH message, detect its absence. Those control modules in turn set a lost communication DTC associated with the loss of SOH for the non-communicating control module. The DTC is unique to the control module which is not communicating. Note that a loss of serial data DTC does not normally represent a failure of the module that set it. The Class 2 serial data line on this vehicle is a star configuration.

GMLAN High Speed Circuit Description

GMLAN high speed data is transmitted on two twisted wires that allow speed up to 500 Kb/s. The twisted pair is terminated with 120 ohm resistors. Depending on content one is after the Fuel Pump Flow Control Module and the other one is either internal to the engine control module (ECM) or it is connected near the data link connector (DLC) end of the link. The resistors are used to reduce noise on the High Speed GMLAN bus during normal vehicle operation.

The high speed GMLAN is a differential bus. The high speed GMLAN serial data bus (+) and high speed GMLAN serial data (-) are driven to opposite extremes from a rest or idle level. The idle level, which is approximately 2.5 V, is considered recessive transmitted data and is interpreted as a logic 1. Driving the lines to their extremes, adds 1 volt to the high speed GMLAN serial data bus (+) and subtracts 1 V from the high speed GMLAN serial data bus (-) wire. This dominant state is interpreted as a logic 0. If a communication signal is lost, the application will set a no communication code against the respective control module. This code is mapped on the scan tool screen as a code against the physical device.

Note: A loss of serial data DTC does not represent a failure of the control module that the code is set in.

Data Link Connector (DLC)

The data link connector (DLC) is a standardized 16-cavity connector. Connector design and location is dictated by an industry wide standard, and is required to provide the following:

* Scan tool power battery positive voltage at terminal 16
* Scan tool power ground at terminal 4
* Class 2 serial data at terminal 2
* Common signal ground at terminal 5
* High speed GMLAN serial data bus (+) at terminal 6
* High speed GMLAN serial data bus (-) at terminal 14

Serial Data Reference

The scan tool communicates over the various busses on the vehicle. When a scan tool is installed on a vehicle, the scan tool will try to communicate with every module that could be optioned into the vehicle. If an option is not installed on the vehicle, the scan tool will display No Communication for that options control module. In order to avert misdiagnoses of No Communication with a specific module, refer to Data Link References (Initial Inspection and Diagnostic Overview) for a list of modules, the busses they communicate with, and the RPO codes for a specific module.