The serial data circuit is 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.

U.S Federal regulations require that all automobile manufacturers establish a common communications system. General Motors utilizes the Class 2 communications system.

Class 2 serial data is transferred at a rate of 10,400 bits per second, and the voltage is toggled between zero and 7 volts. When communication is not occurring on the data line the voltage is zero or ground and is switched to 7 volts when active.

Class 2 data is also pulse width modulated. Each bit of information can have one of two lengths: long or short. 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 Controlled Area Network (CAN) serial data line is high speed serial data bus used to communicate information between the Electronic Brake and Traction Control Module (EBTCM), the Engine Control Module (ECM) and the Transmission Control Module (TCM). Typical data-transmission speeds range from about 125 kilobits per second to 1megabits per second, and 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. With this type of communication, a voltage level of less than 20 percent of B+ is considered a logic 0. A voltage level greater than 80 percent of B+ is considered a logic 1.

The message structure is a request and response arrangement that has some similarities to the UART system.

The modules on the Keyword 82 data line are:

The control modules on the Keyword 2000 data line are: