Input/output circuit

An input/output circuit for sending and receiving a high-level signal and a low-level signal via a coupling capacitance provided on a communication line,

• the input/output circuit comprising:
• a receiving portion including a first detection circuit arranged to detect one of the high-level signal and the low-level signal and a second detection circuit arranged to detect the other of the high-level signal and the low-level signal;
• a transmitting portion including a three-value output circuit configured to output one of a plurality of signals consisting of a high-level signal, a low-level signal, and a high impedance signal;
• and
• a control circuit configured to control the receiving portion and the transmitting portion, wherein the control circuit judges a level of an inputted signal depending on detection results of the first detection circuit and the second detection circuit in a receiving state and controls an output value of the three-value output circuit in a transmitting state.