Regulations and agreements among the automobile manufacturers have resulted in the data link connector (DLC). This connector must be used for scan tool and programming access to all communications systems used for those purposes on the vehicle. The connector must have:

1. a 16 pin connector configuration to which all scan tools connect
2. scan tool battery power always available at pin 16
3. scan tool ground always available at pin 4
4. a clean signal ground always available at pin 5.

The rest of the pins are available for serial data communications to the vehicle systems.

The serial data circuits are the means by which the microprocessor controlled modules in the vehicle communicate with each other and the scan tool. There are four different types of protocols on this vehicle.

General Motors utilizes the Class 2 communications system for most vehicle system controllers that must exchange data and commands. Class 2 data is pulse width modulated. The pulse width modulation of Class 2 data allows better utilization of the data line.

The messages carried on Class 2 data streams are also prioritized. This means if two devices try to communicate on the data line at the same time, only the higher priority message will continue. The device with the lower priority message must wait.

The modules on the Class 2 data line are:

The Controller Area Network (CAN) serial data line is high speed serial data bus used to communicate information between the Electronic Brake Control Module (EBCM), 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 (such as engine speed). Each module processes only those messages whose identifiers are stored in the module's acceptance list. This is CAN's 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 utilize a single wire bi-directional data line between the modules and the scan tool. The message structure is a request and response arrangement that has some similarities to the UART system. Both Keyword 82 and Keyword 2000 are used for scan tool diagnostics only. The modules do not exchange data on these systems.

The modules on the Keyword 82 data line are:

The modules on the Keyword 2000 data line are: