The present disclosure describes a novel method and apparatus for using a device's power and ground terminals as a test and/or debug interface for the device. According to the present disclosure, messages are modulated over DC voltages applied to the power terminals of a device to input test/debug messages to the device and output test/debug messages from the device. The present disclosure advantageously allows a device to be tested and/or debugged without the device having any shared or dedicated test or debug interface terminals.