CIRCUITRY FOR ALLOCATING ACCESS TO A DEMAND-SHARED BUS

摘要

CIRCUITRY FOR ALLOCATING ACCESS TO A DEMAND-SHARED BUS Circuitry is disclosed for allocating requests for demand-shared bus access among a plurality of service requesting ports. During bus contention time, each requesting port synchronously and sequentially applies the digits of its assigned unique priority code to the bus beginning with the most significant digit. After the application of all digits, only the requesting port having the highest code remains in contention and it seizes the bus. Increased flexibility in port preference is provided by the use of a plurality of status flip-flops in each port for generating dynamic port parameter bits. The generated parameter bits are normally applied to the bus as the most significant bits of a dynamic port priority code during contention time. However, the selective application of a mask signal to a mask conductor during contention time causes each requesting port to prevent any parameter bits from being applied to the bus as long as the mask signal remains. This returns control of the port preference to any unmasked parameter bits and to the assigned port priority codes.