PURPOSE: To inform the presence of a reception error at a terminal equipment quickly to a controller with respect to a simultaneous communication data by allowing a terminal equipment to output an opposite level to that of a transmission line in the absence of a transmission data to the controller when a data reception error is detected.

CONSTITUTION: A high level is normally outputted to an output port and a low level is outputted only for a prescribed time when a reception error takes place in the reception of a simultaneous communication data. A controller regards the state to be normal when the reception data is at a high level even after a specified time is awaited. Since the controller receives a low level for a time when one set of terminal equipment has an error, the presence of the reception error is recognized and any processing is implemented.