General System Description

Decimal/Hexadecimal/Binary Conversion Table:

SERIAL DATA COMMUNICATIONS

UART Serial Data
Two methods of data transmission are 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 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 decimal/binary/hexadecimal conversions table. 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.

Decimal/Binary/Hexadecimal Conversions
Beginning in 1996, Federal Regulations require that all auto manufacturers selling vehicles in the United States provide scan tool manufacturers with software information to display vehicle operating parameters. All scan tool manufacturers will display a variety of vehicle information which will aid in repairing the vehicle. Some scan tools will display encoded messages which will aid in determining the nature of the concern. This method of encoding involves the use of a two additional numbering systems: Binary and Hexadecimal.

The binary number system has a base of two numbers. Each digit is either a 0 or a 1. A binary number is an eight digit number and is read from right to left. Each digit has a position number with the farthest right being the 0 position and the farthest left being the 7 position.

The 0 position, when displayed by a 1, indicates 1 in decimal. Each position to the left is double the previous position and added to any other position values marked as a 1. Refer to the Decimal/Hexadecimal/Binary conversion table when using scan tools that display encoded messages.

The hexadecimal system is composed of 16 different alpha numeric characters. The alpha numeric characters used are numbers 0 through 9 and letters A through F. Refer to the Decimal/Hexadecimal/Binary conversion table when using scan tools that display encoded messages.

The hexadecimal system is the most natural and common approach for scan tool manufacturers to display data represented by binary numbers and digital code.

COMMON OBD II TERMS

Diagnostic
When used as a noun, the word diagnostic refers to any on-board test run by the vehicle's Powertrain Control Module. A diagnostic is simply a test run on a system or component to determine if the system or component is operating according to specification. The following list defines the major vehicle on-board diagnostics. Depending on emission requirements in the area of vehicle sale, certain diagnostics listed below may not apply.

^ Misfire
^ Oxygen sensors
^ Oxygen sensor heaters
^ EGR
^ Catalyst monitoring

Enable Criteria
The term "enable criteria" is engineering language for the conditions necessary for a given diagnostic test to run. Each diagnostic has a specific list of conditions which must be met before the diagnostic will run. "Enable criteria" is another way of saying "conditions required".

The enable criteria varies with each diagnostic, and typically includes, but is not limited to the following items:

^ engine speed
^ vehicle speed
^ ECT
^ MAP
^ barometric pressure
^ IAT
^ TP
^ high canister purge
^ fuel trim
^ TCC enabled
^ A/C on

Trip
Technically, a trip is a key on-run-key off cycle in which all the enable criteria for a given diagnostic are met, allowing the diagnostic to run. Unfortunately, this concept is not quite that simple. A trip is official when all the enable criteria for a given diagnostic are met. But because the enable criteria vary from one diagnostic to another, the definition of trip varies as well. Some diagnostics are run when the vehicle is at operating temperature, some when the vehicle first starts up; some require that the vehicle be cruising at a steady highway speed, some run only when the vehicle is at idle; some diagnostics function with the TCC disabled. Some run only immediately following a cold engine start-up.

A trip then, is defined as a key on-run-key off cycle in which the vehicle was operated in such a way as to satisfy the enable criteria for a given diagnostic, and this diagnostic will consider this cycle to be one trip. However, another diagnostic with a different set of enable criteria (which were not met) during this driving event, would not consider it a trip. No trip will occur for that particular diagnostic until the vehicle is driven in a way as to meet all the enable criteria.