Corvette V8-6.2L (2008)
Information Bus: Description and Operation
Data Link Communications Description and Operation
Circuit Description
The communication among modules is performed through the class 2 and high speed GMLAN serial data circuits. 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 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 (See: Body and Frame/Body Control Systems/Description and Operation) for more
information about the gateway.
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:
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Scan tool power battery positive voltage at terminal 16
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Scan tool power ground at terminal 4
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Common signal ground at terminal 5
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Class 2 signal at terminal 2
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High speed GMLAN serial data bus (+) at terminal 6
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High speed GMLAN serial data bus (-) at terminal 14
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 ohm resistors, one is internal to the engine control module
(ECM) and the other is internal to the body control module (BCM). 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. 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 vehicle 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.
The interaction between high speed GMLAN and class 2 is assured by BCM, which is the gateway. Any message from the class 2 modules to the high
speed GMLAN modules is translated by BCM without carrying the original transmitter ID. Therefore the GMLAN modules consider the BCM as being
the originator of all class 2 messages that they receive. 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 DLC to the end of the linear configuration:
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The body control module (BCM)
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The communication interface module (OnStar(R))
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The electronic suspension control (ESC) module
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The transmission control module (TCM)
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The electronic brake control module (EBCM)
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The engine control module (ECM)
Class 2 Circuit Description
The data link connector (DLC) allows a scan tool to communicate with the class 2 serial data circuit. Class 2 serial data is transmitted on a single wire at
an average of 10.4 Kbps. The bus is active at 7 volts nominal and inactive at ground potential. When the ignition switch is in RUN, each module
communicating on the class 2 serial data line sends a state of health (SOH) message every 2 seconds to ensure that the module is operating properly.
When a module stops communicating on the class 2 serial data line, for example if the module loses power or ground, the SOH message it normally
sends on the data line every 2 seconds disappears. Other modules on the class 2 serial data line, which expect to receive that SOH message, detect its
absence; those modules in turn set an internal DTC associated with the loss of SOH of the non-communicating module. The DTC is unique to the module
which is not communicating, for example, when the body control module (BCM) SOH message disappears, several modules set DTC U1064. Note that a
loss of serial data DTC does not normally represent a failure of the module that set it.
The BCM emulates the GMLAN modules on the class 2, embedding the original transmitter ID in GMLAN messages to class 2. Therefore the class 2
