Patent ID: 7302184

Claim:
A data communication system comprising: a terminal unit for generating signals responsive to status of a plurality of switches; a control unit for controlling operation of a predetermined load according to the signals transmitted from the terminal unit; and power supply lines for supplying power required for operation of the terminal unit from the control unit to the terminal unit; said terminal unit comprising light signal-transmitting means for generating light signals according to operation of the switches and spatially propagating the light signals to the control unit, and electronic signal-transmitting means for generating electronic signals according to the operation of the switches and transmitting the electronic signals to the control unit through the power supply lines; and said control unit comprising light signal-detecting means for detecting the light signals transmitted by the light signal-transmitting means and controlling the operation of the load, and electronic signal-detecting means for detecting the electronic signals transmitted by the electronic signal-transmitting means through the power supply lines and controlling the operation of the load when the transmission of the light signals from the terminal unit is discontinued; wherein the power supply lines comprise a plus and a minus line for supplying DC power from the control unit to the terminal unit, the minus line being grounded in the control unit through a first resistor, and the electronic signal-transmitting means located in the terminal unit comprises a plurality of second resistors having different values of resistance, the second resistors being connected in series to the respective switches, and series circuits of the respective switches and the respective second resistors are interposed between the plus line and the minus line, respectively.