DATA PROCESSING SYSTEM

1,243,619. Data processing. INTERNATIONAL BUSINESS MACHINES CORP. 14 May, 1970 [27 June, 1969], No. 23367/70. Heading G4A. A data processing system comprises a CPU connected (e.g. by a ring bus) to a plurality of ancillary units (e.g. I/O control units or other CPU's), each of which includes means to generate a unit selected signal when it correctly or falsely responds to an address signal received from the CPU, there being means to generate a warning signal if two or more of the ancillary units generate unit selected signals in response to the same address signal. A unit selected signal is generated if an address bus portion of the ring bus has the address of the unit on it and at least one bit on a data bus portion of the ring bus is 1. The unit selected signal is ORed on to a line of the ring bus, and if the incoming portion of this line at any unit producing a unit selected signal already has such a signal on it, a warning signal is ORed on to another line of the ring bus. The CPU responds to the warning signal causing each unit producing a unit selected signal to identify itself to the CPU by a signal on a respective line.