PRECEDENCE CONTROL SYSTEM

PURPOSE:To prevent simultaneous requests to shared resources by constituting precedence control circuits of individual devices of a distributed request control system with the same hardware to vary priority levels of individual devices. CONSTITUTION:The turn-on output of an AND gate 23 is applied to a terminal J of a JK-type FF24 to turn on it, and an occupation permission signal ACK is generated to occupy a common bus. Simultaneously, a NOT circuit 25 is turned off, and a line BUSY is set to the low level, and all AND gates 23 of devices other than a device 21-3 which occupies a shared resource are turned off to inhibit generation of the occupation permission signal ACK in individual devices. Further, a request signal REQS3 turns off AND gates 23 of lower devices by a request line REQ4 to inhibit double the generation of the occupation permission signal ACK in lower devices. Consequently, the device 21-3 monitors only request signals of request lines REQ1-REQ3 from devices, which higher priority levels are given to by a precedence control register 30-3, independently of presence/ absence of the request signal of a request line REQ5 to get the bus occupation right. Thus, priority levels of individual devices are set flexibly by a program.