A connection status detector of a controller determines that a two-wire cable is forward connected, in a case where a synchronization pattern of reception data completely coincides with a synchronization pattern for forward connection check. The connection status detector determines that the two-wire cable is reversely connected, in a case where the synchronization pattern of reception data completely coincides with a synchronization pattern for reverse connection check. A transmission data inverter reverses an uplink signal output from a transmission data processor, in a case where it is determined that the two-wire cable is reversely connected. A reception data inverter reverses a downlink signal output from a reception driver, in a case where it is determined that the two-wire cable is reversely connected.