Class II Serial Data

Class II Serial Data
U.S. Federal regulations require that all 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 vehicle wiring to be reduced by the transmission and reception of multiple signals over a single wire. The messages carried on Class II data streams are also prioritized. In other words, if two messages attempt to establish 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 it provides 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, Tech 2 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 Decimal/Hexadecimal/Binary Conversion Tables under this heading. On GM automobiles, the Tech 1 and Tech 2 display the actual values for vehicle parameters. It will not be necessary to perform any conversions from coded values to actual values.