Abstract: A microprocessor-based system serves as a message routing switch for communication between a number of different devices. Any device attached to the switch can communicate with any other attached device through a number of serial and parallel Input/Output ports. Communication between devices attached to the switch is in one of two formats. In a first method of communication, an attached device can be directly associated with another attached device in which case any communication from one is immediately transmitted to the other. In a second method of communication, an attached device communicates with the switch by means of fixed format packets, each of which contains a specification of its destination and error checking capabilities. This second method of communication permits examination of the validity of messages transmitted to or from the switch and enables message retransmission in the case of error.