PURPOSE: To simplify the system constitution by applying timing control to a control loop formed by 2-core cables between terminal equipments so as to use the control information for both the transmission and reception.

CONSTITUTION: Terminal boards 1a, 1b of terminal equipments A, B are connected by a 2-core cable 2 to form a control loop for transmission/reception. In sending the control information from the terminal equipment A to the equipment B, after the unreceived control information from the terminal equipment B is confirmed by a photocoupler 4a and the terminal set A turns on a line current drive IC 3a. Thus, a loop current flowing from the power supply voltage Va is given to a photocoupler voltage generating resistor 5b via a photocoupler 4b and the voltage drop across the resistor 5b is detected by a detection IC 6b. Thus, a 2-core cable is enough for the terminal connection and the system is constituted simply with low cost.