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

Abstract

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).