STS RWD V8-4.6L (2007)
Information Bus: Description and Operation
Data Link Communications Description and Operation
Circuit Description
The communication among modules is performed through the low speed and high speed GMLAN serial data circuits. The modules that need real time
communication are attached to the high speed GMLAN network. The instrument panel module (IPM) is the gateway between the networks. The purpose
of the gateway is to transfer information from one network to another. The gateway will interact with each network according to that network's
transmission model. Refer to Body Control System Description and Operation for more information about the gateway.
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's 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.
High Speed GMLAN 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 2
twisted wires that allow speed up to 500 Kbps. The twisted pair is terminated with two 120 ohms resistors, one is internal to the engine control module
(ECM) and the other is internal to the instrument panel module (IPM). 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 volts is considered a recessive transmitted data
and is interpreted as a logic 1. Driving the lines to their extremes adds 1 volt to high speed GMLAN serial data bus (+) and subtracts 1 volt from high
speed GMLAN serial data bus (-) wire. This dominant state is interpreted as a logic 0.
Two different outputs are used by IPM to activate the high speed GMLAN bus:
* Accessory wake up, active in all power modes when IPM is awake except Crank and in a remote start request
* High speed communication enable, active in all power modes when IPM is awake and in a remote start request
The accessory wake up signal wakes up the ECM and the transmission control module (TCM) only. This output is disabled in Crank power mode for
internal fail enable of the ECM and TCM. The high speed communication enable signal wakes up all the other modules on the high speed GMLAN bus.
Low Speed GMLAN 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. The transmission speed for GMLAN low speed is up to 83.33 Kbps. This high speed is used for
programming purposes only. Under normal vehicle operating conditions the speed of the bus is 33.33 Kbps. This protocol produces a simple pulse train
sent out over the low speed GMLAN serial data bus. To wake up the control modules connected to the low speed GMLAN serial data bus, a wake-up
pulse of 10 volts is sent out over the bus.
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
* Common signal ground at terminal 5
* Low speed GMLAN serial data bus at terminal 1
* High speed GMLAN serial data bus (+) at terminal 6
* High speed GMLAN serial data bus (-) at terminal 14
High Speed GMLAN Serial Data Line
The GMLAN serial data communications circuit on this vehicle is in a linear topology. The following modules are connected to the link, in order from
data link connector (DLC) to the end of the linear configuration:
* The instrument panel module (IPM)
* The electronic suspension control (ESC) module, with F85
* The communication interface module (OnStar), with UE1