Information Bus: Description and Operation

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.


ISO (9141-2) Serial Data
Regulations require that all automobile manufacturers establish a common communications system. The Metro utilizes the International Organization for Standardization (ISO [9141-2]) communications system. It specifies the requirements for setting up the interchanged digital information between the on-board emission-related Control Modules of road vehicles and the OBD II scan tool as specified in J1978. This communication is established to facilitate compliance with California Code of Regulation. 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.

Decimal/Binary/Hexadecimal Conversions
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.

Binary
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.

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