COMMUNICATION MULTIPLEXER USING A RANDOM ACCESS MEMORY FOR STORING AN ACKNOWLEDGE RESPONSE TO AN INPUT/OUTPUT COMMAND FROM A CENTRAL PROCESSOR

A data processing system includes a central processing subsystem, a main memory subsystem, and a number of peripheral subsystems including a communication subsystem all coupled in common to a system bus. Subsystems communicate with each other during asynchronously generated information bus transfer cycles. Each one of the subsystems receives information by providing any one of three signal responses including a positive acknowledge signal indicating an immediate response, a negative acknowledge signal indicating that the unit will most likely be busy for an extended period of time, and a quasi-negative response indicating that the unit will probably be ready during the next asynchronously generated bus transfer cycle. In order to expediate the response in the case of the transfer over the system bus of an input/output order from the central processing subsystem to the communication subsystem, apparatus in the communication system stores a positive acknowledge or a negative acknowledge signal for each communication channel. The signal is set to indicate a negative acknowledge when the previous input/output order filled the last communication control block. The signal is set to indicate a positive acknowledge when the previous input/output order emptied the communication control block.