C 2500 Truck 2WD V8-5.7L VIN R (1997)
Information Bus: Description and Operation
Serial Data Communications
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.
