COMMUNICATION MULTIPLEXER

A data processing system includes a central processing unit, a main memory, and a communication subsystem servicing a number of communication lines. The communication subsystem includes a free running timer 32, a line microprocessor 56 for communicating with the communication lines 17 and a shared memory 44, and an I/O microprocessor 36 for communicating with the shared memory 44 and the central processing unit and main memory (via 16). The line microprocessor 56, desiring the communicate with a specified communication line after a predetermined time delay, loads a first mailbox in shared memory 44 with a binary number indicative of the predetermined time delay. The I/O microprocessor 56 adds the output of the free running timer 32 to the binary number, stores the result in a location in a random access memory 40, and periodically compares the result against the free running timer output. The I/O microprocessor loads a second mailbox (in 44) with a control character when the results of the comarison indicate that the predetermined time delay is accomplished. The line microprocessor responds to the information in the second mailbox to communicate with the specified communication line.