The serial data communication among the control modules is performed through multiple serial data communication links depending on vehicle RPO applications. The class 2 serial data line is always present. It is also possible for there to be a high speed GMLAN serial data line and a J1939 serial data circuit. 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 the 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. There is a controller area network (CAN) connector to allow communication between a scan tool and modules on the J1939 serial data circuit.

All modules that use high speed GMLAN serial data also use class 2 serial data. Refer to Data Link References to identify which modules are class 2 only modules, or combination class 2, J1939, and high speed GMLAN modules.

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:

The controller area network (CAN) connector allows communication between the scan tool and the J1939 serial data circuits. The function of the terminals is listed below:

Class 2 serial data is transmitted on a single wire at an average of 10.4 kbps. 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 a lost communication DTC associated with the loss of SOH for the non-communicating module. The DTC is unique to the module which is not communicating. 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 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 of the resistors is connected near the data link connector (DLC) end of the link and the other is internal to the ECM/PCM end of the link. The DLC terminating resistor is not internal to a module. 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. 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.

Modules connected to the high speed GMLAN serial data circuits are the transmission control module (TCM), glow plug control module (GPCM), and the engine control module/powertrain control module (ECM/PCM). The high speed GMLAN control modules functionally communicate on both class 2 and high speed GMLAN data links.

The J1939 Serial Data Line is a serial data circuit that operates similarly to the high speed GMLAN serial data line in that the J1939 utilizes two twisted wires and has terminating resistors. However, instead of being connected to the DLC, the J1939 serial data line is connected to a Controller Area Network (CAN) connector. The CAN connector allows communication between the scan tool and the J1939 serial data circuits.

The following modules are connected to the J1939 serial data circuit: transmission control module (TCM), glow plug controller (GPC), electronic brake control module (EBCM), data recorded module (DRM), serial data gateway module (SDG), electric shift selector, and engine control module/powertrain control module (ECM/PCM).

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