Push-pull serial bus coupled to a plurality of devices each having

Abstract

A high speed serial bus is disclosed have particular application for use in passing messages in a multiple processor computer system. The serial bus includes a three-wire serial link having lines identified as 'SDA', 'SDB' and 'ground'. The ground line provides a common reference for all devices coupled to the serial bus. A message controller is coupled to each agent for transmitting and receiving serial data along the bus. Both lines of the serial bus as well as the ground are coupled to a bus state detector in the message controller which provides three basic signal outputs. The bus state detector determines whether or not the bus is in use, a collision has occurred between messages, and decodes data received on the bus. Data which is transmitted along the serial bus is driven on lines SDA and SDB 180 degrees out of phase relative to each other. The message controller encodes messages to be transmitted using, in the present embodiment, well known Manchester encoding techniques. A bus idle state occurs when all transmitters are off allowing both lines SDA and SDB be pulled high by pull-up resistors. Valid data states may occur any time a single transmitter is transmitting. When two or more transmitters begin transmitting a collision state exists. The message controller recognizes collisions and provides a back-off algorithm.