Data Link Communications

DATA LINK COMMUNICATIONS DESCRIPTION AND OPERATION

CIRCUIT DESCRIPTION
The data link connector (DLC) allows a scan tool to communicate with modules on the class 2 serial data line. The serial data line is the means by which the microprocessor-controlled modules in the vehicle communicate with each other. Once the scan tool is connected to the class 2 serial data line through the DLC, the scan tool can be used to monitor each module for diagnostic purposes and to check for diagnostic trouble codes (DTCs). 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 7.0 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.

DATA LINK CONNECTOR (DLC)
The data link connector (DLC) is a standardized 16-cavity connector located below the instrument panel and close to the steering column. Connector design and location is dictated by an industry wide standard, and it 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 serial data signal at terminal 2

CLASS 2 SERIAL DATA LINE
The class 2 serial data line on this vehicle is a star configuration. The following modules are attached to SP 201 and communicate on the class 2 serial data line:
- The body control module (BCM)
- The driver information center (DIC), w/Trip
- The electronic brake control module (EBCM)
- The inflatable restraint sensing and diagnostic module (SDM)
- The instrument panel cluster (IPC)
- The memory seat module (MSM) - driver, w/D44
- The powertrain control module (PCM)
- The radio
- The transfer case shift control module (TCSCM), w/NP8

DIAGNOSTIC ENABLE CIRCUIT W/NP1
The transfer case shift control module is equipped with the ability to store diagnostic information, which is useful to a technician in the event of module, component or wiring failures. This information can be retrieved from the transfer case shift control module (TCSCM) by way of flash codes, which are displayed on the 3 transfer case shift control switch buttons. These buttons are located on the instrument panel to the right of the steering wheel. For more information, refer to Diagnostic Trouble Code (DTC) Displaying in Transfer Case - NVG 233-NP1.