The present invention relates to a circuit which is connected to a two-conductor control system for a variable analog DC input and that also enables bidirectional digital communication along the two conductors for diagnostic operations of a transducer. The novel circuit includes a switch circuit that has a first position that provided the ability to accept both the variable DC analog signals and the bidirectional digital communication signals by presenting a first impedance for the DC signals and a second switch position for providing a second substantially higher impedance while using the same two-conductor system. The novel invention also includes an auxiliary analog input signal to the circuit which allows further control as a current feedback to a control algorithm in a microcontroller.; An auxiliary process transmitter can sense pressure, temperature, flow or some other process related variable and couple it to the circuit for control of the transducers. Finally, the novel invention includes a novel voltage regulator and a capacitive voltage supply for utilizing the voltage on the two conductors from the controller to also power the device.