A valve controller is disclosed. The valve controller includes an interface circuit having an input for connection to a controller, a processor in data communication with the interface circuit which is operable to generate a control signal for controlling a valve position, and a control device operable to control the valve position in response to the control signal from the processor. The interface circuit includes a power extraction circuit connected to the controller and is further operable to generate a DC operating voltage for use in powering the interface circuit and the processor. The interface circuit includes a current sensor connected to the controller and is further operable to generate a measure of a current from the controller, the measure being used in controlling a device associated with the interface circuit. The interface circuit also includes a digital communications circuit connected to the controller and is further operable to transmit and receive communication signals. The digital communications circuit includes an impedance controller having an operational amplifier connected to control an impedance presented to the controller by the interface circuit.