PURPOSE: To reduce the possibility of misconnection by using two kinds of positive and negative signals so as to simplify a wiring work.

CONSTITUTION: In sending a data from a central equipment 11 to a terminal equipment 15, a high level control signal is given to a demultiplexer 19, a terminal 19a is selected, a data signal is sent as it is to a transmission line 20 and a positive signal output circuit 13 outputs a positive signal to the terminal equipment 15 as it is. In sending a data from the central equipment 11 to a terminal equipment 16 next, a ground level control signal is outputted to the demultiplexer 19 to select a terminal 19b, a data is outputted inversely, a negative signal is inverted by a negative signal inverting output circuit 14 and the result is outputted to the terminal equipment 16.