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Data Link Communications Description and Operation

The communication among control modules is performed through the high speed GMLAN serial data circuit, CAN serial data circuit, and the Keyword 2000 serial data circuit. The modules that need real time communication are attached to the high speed GMLAN network or the CAN network. Some control modules use GMLAN/CAN network for communication between themselves while they use separate Keyword 2000 serial data circuit only for communication with a scan tool. Refer to for a list of modules and the buses they communicate with.

Note: A loss of serial data DTC does not represent a failure of the module that the code is set in.

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 120 Ω resistors. 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 V, 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. 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 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.

The controller area network (CAN) serial data line does not communicate with the scan tool via the data link connector (DLC). The CAN serial data line is high speed serial data bus used to communicate information between the engine control module (ECM) and the transmission control module (TCM). Typical data-transmission speeds must be high enough to ensure that the required real-time response is maintained.

The addressing scheme employed with CAN assigns a label to every message, with each message receiving a unique identifier. The identifier classifies the content of the message. Each module processes only those messages whose identifiers are stored in the module acceptance list. This is CAN form of message filtering.

The identifier labels both the data content and the priority of the message being sent. Each module can begin transmitting its most important data as soon as the bus is unoccupied. When more than one module starts to transmit simultaneously, the message with the highest priority is assigned first access. A module responds to failure to gain access by automatically switching to receive mode, the module then repeats the transmission attempt as soon as the bus is free again.

The Keyword protocols utilise a single wire bi-directional data line between the module and the scan tool. The message structure is a request and response arrangement. Keyword 2000 is used for scan tool diagnostics only. The modules do not exchange data on these systems.

The data link connector (DLC) is a standardised 16-cavity connector. Connector design and location is dictated by an industry wide standard, and is required to provide the following:

The scan tool communicates over the various buses 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 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 buses they communicate with, and the RPO codes for a specific module.