Abstract: A data processing system includes a data processing unit and a number of communication units for coupling a corresponding number of terminal devices for transmission and reception of signals over a number of communication lines. Apparatus checks the operation of the communication units and includes logic circuits which couple to the receiver and transmitter circuits of each unit, circuits for transmitting checking signals to the receiver circuits of the unit and circuits for receiving signals transmitted by the transmitter circuits of the units. Additionally, each of the logic circuits couples to the data processing unit and are arranged to be enabled in succession by the data processing unit for transmission and reception of the checking signals and response signals respectively while still allowing the units not being checked to operate normally.