PURPOSE: To prevent the destruction of an input element even if the connection of a connector is inferior, by providing a means, which detects the voltage between electric power supply terminals on one of two boards connected by the connector, and providing a signal output element means on the other board to protect the input element.

CONSTITUTION: When the detection voltage of a detecting circuit 4 of the voltage between power supply terminals which detects the voltage between power supply terminals 1a and 1b of a data collector part is lower than a prescribed value, a transistor(TR) 3 is not turned on but is turned off still, and an NOR gate 50 is closed. Consequntly, a signal output element control circuit 5 of a power supply interface part inhibits the conduction of a TR2 as long as there is a possibility that a voltage which destroys a TR1 is generated in both ends of a diode D. When said detection voltage exceeds the prescribed value, the circuit 4 turns on the TR3. Therefore, conditions of opening of the gate 50 are lost, and the TR2 is turned on and off in accordance with the signal from a sensor. Thus, an excessive voltage is prevented from being applied to the gate of the TR1.