Serial Data

SERIAL DATA COMMUNICATION TYPES
Beginning in 1995 two methods of data transmission will he used. One method involves a Universally Asynchronous Receiving/Transmitting (UART) protocol. UART is an interfacing device that allows the on hoard 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 hoard 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 hoard control module and the off hoard monitor/scanner since 1981.

CLASS II COMMUNICATIONS
U.S. Federal regulations now require that all automobile manufacturers establish a common, or CLASS II communications system. Class II data is pulse width modulated. Each bit of information can have one of two lengths: long or short. This would allow 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 will provide all scan tool manufacturers with the capability of accessing data from any make or model vehicle sold here in America.

The data displayed on the Tech 1 or other scan tool will appear the same, but with some exceptions. Some scan tools will only be able to display certain vehicle parameters as values that are 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 will always display the actual values for vehicle parameters. It will not be necessary to perform any conversions from coded values to actual values.