An interface circuit configurable in input and output modes comprises a current source; an input/output node; a diode to prevent current from flowing from the input/output node to the current source; a first switch to short-circuit the diode when closed; and a second switch to form a ground path from the input/output node when closed. In input mode, the first switch is closed and the state of a connected sensor circuit may be measured. In the output mode, the first switch is open, and the second switch may be closed to activate an output circuit. In the output mode, power dissipation may be monitored, and the diode may protect the current source and measurement node from current flowing through the output circuit. In the input mode, closing the first switch may prevent the diode from introducing measurement errors due to non-linear attenuation of perturbative AC signals.