Abstract: A loop communication system includes a loop controller originating and terminating parallel output multiple bit message frames over a pair of trunks to remote stations, each of which includes a multiplexing connection means to the incoming trunk segments, with means to selectively and operably connect one of the incoming trunk segments to a frame processor. A pair of transmitters are also provided at the interface means and actuated to synchronously and correspondingly retransmit the process signal over the two outgoing trunk lines or segments. A logic circuit directly transmits the signal from one receiver and blocks the other until the first fails. The message is processed and retransmitted by a pair of transmitters over the two outgoing trunk segments. Suitable indicators coupled into the logic circuit signal the failure. A contact status circuit initiates transmission of the failure location via message frames to the controller.