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Data Link Communications



DATA LINK COMMUNICATIONS DESCRIPTION AND OPERATION

CIRCUIT DESCRIPTION
The serial data communication among the control modules is performed through multiple serial data communication links. The class 2 serial data line is always present. The high speed GMLAN or controller area network (CAN) may be present for different vehicle options. The data link connector (DLC) allows a scan tool to communicate with the modules on class 2 or high speed GMLAN serial data lines. In order for the scan tool to communicate with the modules on high speed GMLAN link, a CANdi module is needed. The CANdi module behaves as an interface between the scan tool and the high speed GMLAN data link. The CAN is used for functional communication only, among the modules connected to it.

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
- Class 2 signal at terminal 2
- High speed GMLAN serial data bus (+) at terminal 6
- High speed GMLAN serial data bus (-) at terminal 14

CLASS 2 SERIAL DATA LINE
Class 2 serial data is transmitted on a single wire at an average of 10.4 kbps. This value is an average, 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 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 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 inflatable restraint sensing and diagnostic module (SDM) SOH message disappears, several modules set DTC U1088. 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. The powertrain control module (PCM) has an additional class 2 serial data circuit to body control module (BCM). If one of the class 2 serial data circuits to the PCM opens, communication will not be interrupted. The following modules communicate on the class 2 serial data line:
- The audio amplifier (AMP), w/UQ7 and Y91
- The body control module (BCM)
- The communication interface module (OnStar(R)), w/UE1
- The digital radio receiver (DRR), w/U2K
- The driver door module (DDM)
- The electronic brake control module (EBCM)
- The generator battery control module (GBCM)
- The HVAC control module
- The hybrid control module (HCM), w/HP2
- The inflatable restraint sensing and diagnostic module (SDM)
- The instrument panel cluster (IPC)
- The memory seat module (MSM), w/AN3
- The passenger door module (PDM)
- The powertrain/engine control module (PCM/ECM)
- The radio
- The rear seat audio (RSA) controller, w/UK6
- The rear seat entertainment (RSE) assembly w/U42
- The rear wheel steering control module (RWSCM), w/NYS
- The remote playback device - CD player (CDX), w/U1S
- The transfer case shift control module (TCSCM), w/NP1/NP8
- The transmission control module (TCM), w/M74

The class 2 serial data line allows a scan tool to communicate with these modules for testing purposes, checking for DTCs, and to activate/enable/disable functions. These class 2 serial data circuits are bussed together via 2 splice packs:
- SP205-Located behind the instrument panel (I/P) near the headlamp switch connector.
- SP207-Located in the I/P harness.

Refer to Computer/Integrating Systems Component Views.

HIGH SPEED GMLAN SERIAL DATA LINE (PHT)
The data link connector (DLC) allows a scan tool to communicate with the high speed GMLAN serial data circuit through the CANdi module. The serial data is transmitted on two 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 energy storage control module (ESCM). 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 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 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 powertrain control module (PCM) and hybrid control module (HCM) are functionally communicating on both class 2 and high speed GMLAN data links. However the diagnostic communication of the above modules with the scan tool is performed on class 2 data link only. All DTCs set by the high speed GMLAN modules, including HCM, are reported by the PCM on class 2. Data displaying, output controls and programming of the control modules connected to high speed GMLAN only, is performed through high speed GMLAN link.

The GMLAN serial data communications circuit is in a linear topology. The following modules are connected to the link, in order from DLC to the end of the linear configuration:
- The energy storage control module (ESCM)
- The starter/generator control module (SGCM)
- The electro-hydraulic power steering (EHPS)
- The hybrid control module (HCM)
- The powertrain control module (PCM)
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