PURPOSE: To prevent loop-back by discriminating a data bit pattern.

CONSTITUTION: When a data comes through a line A, a mark state detecting circuit 65 disables a receiver 62 and a driver 64 and sends data received by a receiver 61 to a driver 63, and the driver 63 drives it to a line B. Though the signal driven to the line B is propagated to the receiver 62, the receiver 62 is not operated because it is in the disabled state. Consequently, a mark state detecting circuit 66 judges that data does not come from the line B, and the circuit 66 transmits continuously an enable signal to the receiver 61 and the driver 63.