PURPOSE: To cope with a fault with proper changeover of a switch when a fault occurs by transmitting cyclically and simultaneously the same data to the 1st and 2nd transmission lines and monitoring the output of the switch that switches data of both transmission lines.

CONSTITUTION: Transmission controllers 11∼15 are connected in a loop form via data transmission lines 21 and 22 and transmit the same data cyclically and simultaneously to both lines 21 and 22 according to a fixed order and in the opposite directions to each other. Each controller contains interlocking switches 41 and 42 for transmission/reception of input/output 21-1, 21-2, 22-1 and 22-2, a reception switch 43 for data supplied from both lines 21 and 22, a data receiver 36 which monitors the output of the switch 43 and controls its working and a data transmitter 37. When the output of the switch 43 is normal, the receiver 36 switches the switch 43 periodically. Then the output of the switch 43 has a drop- out when the data transmission line has a fault. Thus either one of controllers 11∼15 switches the switch 43 according to the place where the fault occurs.