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Abstract

A transmission collision avoidance scheme for a distributed network data processing communication system including a half-duplex communication board coupled to the system bus. In addition to a serial data bus and a clock bus, the system bus is configured to include a busy status line that is monitored by each device connected in the system. When a device desires to send a message, it initially proceeds to busy out the busy status line, by causing a busy flag to be placed on the line. It also checks the busy status line in order to ascertain whether a busy flag was already set, namely, whether another device has already requested service. If the bus is busy, the device cannot transmit until the bus becomes free and the requesting device resets the bus busy flag for a retry interval. Assuming that the bus is free, the transmitting device proceeds to transmit. When the device for whom the message is intended receives the message, it sends an acknowledgement message back to the transmitting device, signifying a successful transmission. Because of circuit operation and propagation delays, there exists the possibility of more than one device setting a busy flag and checking the status of the bus without being aware that another device is simultaneously requesting use of the bus. In this event, a transmission collision will occur, so that the intended receivers see no sensible transmission and, consequently, return no acknowledgement messages to the collision-participating transmitters. When a transmitter fails to receive an acknowledgement message, it assumes that a collision has occurred, and proceeds to repeat the above procedure for transmitting its message pursuant to a prescribed priority scheme, the higher priority device being assured of transmitting its message without a collision on the bus with a message from the lower priority device.

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First Claim