Class II Serial Data

U.S. Federal regulations require that all of the automobile manufacturers establish a common communications system. General Motors utilizes the Class II communications system. Each bit of information can have one of two lengths: long or short. This allows the vehicle wiring to be reduced by the transmission and the reception of the multiple signals over a single wire. The messages that are carried on Class II data streams are also prioritized. In other words, if two messages attempt to establish any communications on the data line at the same time, only the message with higher priority will continue. The device with the lower priority message must wait. The most significant result of this regulation is that the regulation provides the scan tool manufacturers with the capability of accessing data from any make or model vehicle sold in the United States.

The data displayed on the Tech 1 scan tool, Tech 2 scan tool or other scan tools will appear the same, with some exceptions. Some scan tools will only be able to display certain vehicle parameters as values that are a coded representation of the true or actual value. For more information on this system of coding, refer to Computers and Control Systems / Description and Operation for the decimal/binary/hexadecimal conversions. On GM automobiles, the Tech 1 and Tech 2 display the actual values for the vehicle parameters. It will not be necessary to perform any conversions from the coded values the to actual values.