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Method and apparatus for local area communication networks
| Details |
Inventors: Ulug, Mehmet E.;
Assignee: General Electric Company (Schenectady, NY)
Primary Examiner: Olms; Douglas W.
Assistant Examiner: Scutch, III; Frank M.
Attorney, Agent or Firm: Burgujian; Richard V., Davis, Jr.; James C., Webb, II; Paul R.
A bus accessing method and apparatus for the practice thereof in a communication network is disclosed. The network comprises a transmit bus, a receive bus, a plurality of BIUs connected between the transmit and receive buses and a head-end for directing signals leaving the transmit bus onto the receive bus. Each information packet has two control bytes respectively following the start frame delimiter and preceding the end frame delimiter. Each BIU includes means for comparing the bits it receives with the bits as transmitted thereby, a bit mismatch indicating that a downstream BIU has been preempted. During packet transmission, upon detecting a bit mismatch, the transmitting BIU sets one or both control bytes to "1010" so that receiving BIUs in the network are apprised of the preemption. Subsequently, preempted BIUs transmit their packets in accordance with a predetermined protocol, each packet so transmitted having its control bytes set to "1010". In the case where there are no preempted BIUs, each BIU transmits a packet with the control bytes set to "0101", thereby apprising receiving BIUs that there are no preempted BIUs. Each free mode BIU with a packet to transmit commences transmission substantially immediately following the last transmitted packet if there are no preempted BIUs in the system and the buses are idle. |
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DETAILED DESCRIPTION In accordance with the present invention, there is provided a bus accessing method and a communication network for practicing the method.
The communication network comprises a transmit bus, a receive bus, means for directing signals leaving the transmit bus onto the receive bus and a plurality of BIUs coupled between the transmit and receive buses.
Each BIU is coupled to a user device for which it transmits and receives information packets on the transmit and receive buses, respectively.
Each BIU includes means for detecting signals originating from an upstream direction away from the directing means on the transmit bus, means for detecting signals on the receive bus and means for comparing bits received with the bits of an information packet as transmitted thereby.
The comparing means provides a collision signal upon detecting a bit mismatch, the mismatch indicating that a downstream BIU has been preempted and entered a preempted mode.
Each information packet includes a control portion which when set to a predetermined value, indicates to BIUs receiving the packet on the receive bus that at least one BIU in the system is in the preempted mode.
Each BIU includes means for setting the control portion to the predetermined value, indicating a preemption, upon its respective comparing means providing the collision signal.
Each BIU in the preempted mode gains access to the transmit bus in accordance with the predetermined P-BID protocol and transmits its information packet with the control portion set to the predetermined value indicating the preemption.
Each BIU receiving a packet on the receive bus, upon determining that the control portion is set, is apprised there is at least one preempted BIU in the system which will gain bus access to transmit its packet, in accordance with the P-BID protocol.
In the case where there are no preempted BIUs in the system, each BIU transmitting a packet does so with the control portion not set.
Each BIU receiving such a packet is apprised that there are no preempted BIUs in the system
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I/O Systems 
