PURPOSE: To achieve bypassing easily as a backup function for a system, by providing a bypass function for a remote terminal in a looped network by adding only a timer circuit which responds to a clock pulse and a relay.

CONSTITUTION: Comparators OP1 and OP2 compare received signals from transmission lines with reference levels V1 and V2 to detect data and clock pulses individually. A resistance R5 and a capacitor C1 constitute a timer circuit, which inputs the clock pulse via a driver DR1 and applies its output to the base of a transistor TR3. This TR3 turns on to drive a relay RY, whose normally closed contact SW is opened, but when the clock pulse is not received normally and when terminal electric power is not supplied, the contact SW is closed to form a bias circuit.