Firebird V6-3.8L VIN K (1995)
Information Bus: Description and Operation
UART Serial Data
Beginning in 1995 two methods of data transmission will be used. One method involves a Universally Asynchronous Receiving/Transmitting
(UART) protocol. UART is an interfacing device that allows the on board computer to send and receive serial data. "Serial data" refers to
information which is transferred in a linear fashion - over a single line, one bit at a time. A "data bus" describes the electronic pathway through
which serial data travels. The UART receives data in a serial format, converts the data to parallel format, and places them on the data bus (which is
recognizable to the on board computer). The UART also accepts parallel data from the data bus, converts the data to serial format, and transmits
them to the Tech 1 or other scan tool. This method has been the common strategy for establishing a communication link between the on board
control module and the off board monitor/scanner since 1981.
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 tool 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 section on decimal/binary/hexadecimal conversions. 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.
