Digital Equipment Corporation

David Thompson

William D Strecker

Richard J Casabona

John E Buzynski

An arbitration technique for controlling access to a bit-serial bus by multiple nodes in a data processing network. Upon detection of no carrier on the bus (56), a node desiring access to the bus waits a predetermined number of quiet slots (60, 64), each slot being a predetermined interval. If that period elapses without another node's carrier being detected (64), the node desiring access is permitted to transmit (64, 68). For each node, two such delay-interval possibilities are provided, one high slot count (and, hence, low priority) and one low slot count (and, hence, high priority). The delay-interval selection for a node is switched from time to time on a round-robin basis so that all nodes get equal average priority. The high value of the delay interval is N+M+1 slots, where N is the node number and M is the maximum number of nodes allowed on the bus; the low value is N+1 slots. Initially, each node uses the former value. Upon unsuccessful contention for the bus, the delay-interval selection used next by the node depends on the number, LW, of the node which last won access to the bus. Upon detecting a carrier while waiting for access to the bus (i.e., losing arbitration to a higher-priority node), the node which is waiting for the bus compares its node number N to the number LW of the node which started transmitting (58). If LW was less than N, the node waiting for access uses a new waiting time of N+1 slots the next time the delay interval begins (62A); if LW was greater than N, the new delay interval value is N+M+1 slots (62B).

Dual-count, round-robin distributed arbitration technique for serial

A bus structure for use in a computer network requiring high availability and reliability of communications. Multiple bus paths (2A, 2B) are provided. When a transmission is to be made, under most circumstances the path is selected at random, with all paths being equally probable. Thus, failure of a path is detected quickly. Each host device in the network connects to the bus paths through an interface, or port (1). The task of path selection is carried out by the ports, independently of the host devices. The ports also detect path failures and automatically switch over to an alternate good path upon detection of such a failure, all without host involvement. Virtual circuit communications between hosts are transparent to path selection and switching, so the only indication to a host device of a path failure is a decrease in throughput. Most of the signal processing apparatus of each port (10, 20A, 20B) is shared by the paths, only one path being supported at any given time. Thus, the addition of a second bus path involves only minimal cost.

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David Thompson