The communication among control modules is performed through the high speed GMLAN serial data bus, the low speed GMLAN serial data bus, and with RPO HP2 the high speed hybrid GMLAN serial data bus. The modules that need real time communication are attached to the high speed GMLAN busses. There are several modules that act as the gateway module or translator for communication between the various serial data busses. With RPO HP2 the serial data gateway module (SDGM) is the gateway and on all others the body control module (BCM). All high speed modules are activated by a communication enable or wakeup circuit from another module as determined by the power mode master (PMM).

Refer to Data Communication Schematics to determine the circuit and module that activates specific high speed modules.

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.

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 COMM for that options specific control module.

In order to avert misdiagnoses of No Communication with a specific module, refer to Data Link References for a list of modules, the busses they communicate with, and the RPO codes for a specific module.

The body control module (BCM) in this vehicle functions as a gateway or translator. The purpose of the gateway is to translate serial data messages between the high speed GMLAN bus and the low speed GMLAN bus for communication between the various modules. The gateway will interact with each network according to that network's transmission protocol.

One example of this necessary communication is the communication between the engine control module (ECM)/powertrain control module (PCM) which is high speed serial data and Theft Deterrent Module which is low speed serial data. If these modules can not exchange information, the vehicle will not start.

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

The SDGM in this vehicle functions as a gateway or translator. The purpose of the gateway is to translate serial data messages between the high speed GMLAN serial data bus, the low speed GMLAN serial data bus, and the high speed hybrid GMLAN serial data bus for communication between the various modules. The gateway will interact with each network according to that network's transmission protocol.

The data link connector (DLC) allows a scan tool to communicate with the high speed GMLAN serial data modules. The serial data is transmitted on 2 twisted wires that allow speed up to 500 Kb/s. The twisted pair is terminated with two 120-ohm resistors, one is internal to the engine control module (ECM) and the other is after the electronic brake control module (EBCM), or if equipped, the suspension control module. 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 volts, 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 volt from the high speed GMLAN serial data bus (-) wire. 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 Tech 2 screen as a code against the physical device. Note: a loss of serial data DTC does not represent a failure of the module that the code is set in.

The high speed GMLAN serial data allows communication between the body control module (BCM), ECM, transmission control module (TCM), vehicle communication interface module (VCIM), 4WD control module, EBCM, and the suspension control module depending on RPO.

The high speed hybrid GMLAN serial data modules do not communicate directly with the scan tool. The SDGM communicates on all serial data busses and relays the high speed hybrid GMLAN serial data information to the scan tool. The high speed hybrid GMLAN serial data bus operates the same as the high speed GMLAN bus with the exception of the need of the SDGM for communication between the modules and a scan tool.

The data link connector (DLC) allows a scan tool to communicate with the low speed GMLAN serial data circuit. The serial data is transmitted over a single wire to the appropriate control modules. Under normal vehicle operating conditions, the speed of the bus is 33.33 Kb/s. This protocol produces a simple pulse train sent out over the low speed GMLAN serial data bus. When a module pulls the bus high, 5 volts, this creates a logic state of 0 on the bus. When the bus is pulled low, 0 volts, it is translated as a logic state of 1. To wake the control modules connected to the low speed GMLAN serial data bus, a wake up signal is sent out over the bus. Modules connected to the low speed GMLAN bus can be part of a virtual network as described in the other GMLAN circuit descriptions above. The modules on the low speed GMLAN serial data bus are connected to the bus using several splice or "star" connectors separating groups of modules. The following lists state the splices and modules connected to the low speed serial data circuits:

I/P Splice

Body Splice

The data link connector (DLC) is a standardized 16-cavity connector. The DLC low speed serial data circuit is connected directly to the instrument panel (I/P) splice then to all other splices or modules. Connector design and terminal location is dictated by an industry wide standard, and is required to provide the following:

With RPO HP2 the DLC also connects the high speed hybrid GMLAN bus to the scan tool for programming purposes. The terminal and circuits are as follows: