Abstract: A parent station output section changes a duty ratio between a period of a level other than a predetermined power-supply voltage level and a subsequent period of the power-supply voltage level according to each data value of a control data signal to convert the control data signal into a serial pulse voltage signal and output it onto a data signal line. A parent station input section detects a supervisory data signal superimposed on the serial pulse voltage signal transmitted over the data signal line as the presence or absence of a current signal generated by contention between the supervisory signal and the power-supply voltage on the rising edge of the power-supply voltage.