G8 V6-3.6L (2008)
Information Bus: Description and Operation
Data Link Communications Description and Operation
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. Refer to Body Control System Description and Operation (See: Body and Frame/Body Control
Systems/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 scan tool 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.
GMLAN High Speed Circuit Description
The data link connector (DLC) allows a scan tool to communicate with the high speed GMLAN serial data circuit. The serial data is transmitted on two
twisted wires that allow speed up to 500 Kb/s. The twisted pair is terminated with two 120 ohms resistors, the location of these resistors is listed in the
Master Electrical Component List (See: Locations/Component Locations) . 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 one volt to the high speed
GMLAN serial data bus (+) and subtracts one 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 scan tool 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 BCM, engine control module (ECM), transmission control module (TCM),
steering angle sensor (SAS), electronic brake control module (EBCM) and the OnStar(R) vehicle communication interface module (VCIM)
GMLAN Low Speed Circuit Description
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 buss is 33.33 Kb/s. This protocol produces a
simple pulse train sent out over the GMLAN low speed serial data bus. When a module pulls the buss high, 5 volts, this creates a logic state of 0 on the
buss. When the buss is pulled low, 0 volts, it is translated as a logic state of 1. To wake the control modules connected to the GMLAN low speed serial
data buss, a high voltage wake up pulse is sent out over the buss, the voltage level of the pulse is +10 volts. Modules connected to the GMLAN low
speed buss can be part of a virtual network as described in GMLAN High Speed Circuit Description above. The modules on the GMLAN low speed
serial data buss are connected to the buss using several splice or "star" connectors separating groups of modules. Refer to Data Communication
Schematics (See: Diagrams/Electrical Diagrams) for specific schematics information.
The following lists state the splices and modules connected to the low speed serial data circuits:
I/P Splice
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BCM
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Secondary information centre (SIC)
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Instrument panel cluster (IPC)
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Inflatable Restraint Sensing and Diagnostic Module (SDM)
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Theft deterrent module (TDM)
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Infotainment unit - integrated radio chassis (IRC)
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Heating ventilation and cooling (HVAC) control module
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Remote function actuator (RFA)
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Active occupant sensing (AOS) module
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Heated seat module
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OnStar(R) vehicle communication interface module (VCIM)
Data Link Connector (DLC)
The data link connector (DLC) is a standardized 16-cavity connector. The DLC low speed serial data circuit is connected directly to the I/P splice then to
all other splices or modules. Connector design and location is dictated by an industry wide standard, and is required to provide the following:
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Pin 1 GMLAN low speed communications terminal
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Pin 4 Scan tool power ground terminal
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Pin 5 Common signal ground terminal
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Pin 6 High speed GMLAN serial data bus (+) terminal