Patent ID: 7054124

Claim:
A method comprising: a) providing at least two supply voltages with respect to a ground potential to a circuit unit; b) connecting signal connections of the circuit unit to signal lines for transmission of signals between the circuit unit and one or more external circuit units; c) propagating a reference voltage from a control connection of the circuit unit to an overvoltage protective device, the overvoltage protective device connected symmetrically to two resistance element connections of the signal lines; d) propagating a second reference voltage signal from a second control connection of the circuit unit to the overvoltage protective device, the second reference voltage signal derived from at least two further supply voltages with opposite polarity by means of a second switching module; e) tripping the overvoltage protective device responsive to detection of an overvoltage on at least one of the two signal lines with respect to the reference voltage, thereby causing the overvoltage protection device to dissipate the overvoltage to the ground potential, f) switching the circuit unit to select one of the at least two supply voltages using a mode switching module that is integrated in the circuit unit, and wherein the reference voltage depends on the selected one of the at least two supply voltages.