Patent ID: 8138813

Claim:
An interface circuit, comprising: a connector terminal configured to receive an input signal in which one of signal levels is substantially direct current; an oscillating circuit configured to generate and output an input level determination signal having a frequency higher than a frequency of the input signal; an output control circuit having a first enable terminal connected to a first power supply node and a second enable terminal receiving the input signal via the connector terminal and an input terminal connected to the oscillating circuit to control whether to output the input level determination signal or not, based on a signal level of the input signal; an AC coupling element coupled to the output control circuit to transmit the input level determination signal output from the output control circuit; and a receiving circuit coupled to the AC coupling element to receive the input level determination signal, the receiving circuit comprises a clamp circuit that limits an upper limit value of the input level determination signal and includes a protective diode connected between an input node of the receiving circuit and a power supply terminal, wherein the oscillating circuit continues to output the input level determination signal when the output control circuit stops outputting the input level determination signal.