A data processing device with a power supply and data signal interface circuit has a switch for connecting an external line and an internal node. The power supply and data signal interface circuit also includes a controller for applying an enabling voltage to the switch enabling the switch to supply current between the external line and the internal node in the presence of power supply to the controller and in the absence of the overvoltage condition on the external line. The power supply and data signal interface circuit also includes a voltage reduction connection from the external line for applying a control voltage to the switch in the absence of power supply to the controller. The control voltage from the voltage reduction connection limits a voltage applied to the internal node through the switch in the presence of the overvoltage condition.