초록 ▼

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 monitor

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.

대표청구항 ▼

In a digital data communication network wherein a plurality of processor devices are indirectly coupled to a communication bus over which messages between respective devices are transmitted, a system for preventing simultaneous transmission of messages by plural ones of said devices and thereby avoi

In a digital data communication network wherein a plurality of processor devices are indirectly coupled to a communication bus over which messages between respective devices are transmitted, a system for preventing simultaneous transmission of messages by plural ones of said devices and thereby avoiding a transmission collision on said bus comprising: first means, coupled with a respective device and responsive to a request for a transmission from its processor, for causing a bus seizure representative signal to be placed on said bus and to thereby be supplied to each of the devices coupled to the bus; second means, coupled to said bus and coupled to and responsive to the operation of said first means, for examining said bus for the presence of a bus seizure representative signal that has been placed on the bus by another device; and third means, coupled to said second means, said bus and said device, for causing said device to transmit a message over said bus in response to said second means finding that no bus seizure signal has been placed on the bus by another device, but otherwise inhibiting said device from proceeding with the transmission of a message.