Some embodiments are directed to a process device comprising a process variable sensor configured to generate an output signal indicative of a sensed process variable; loop current output circuitry configured to control a loop current on a two wire process control loop to a value based on the output signal; loop current measurement circuitry coupled to the process control loop and configured to generate a measured loop current value based on the loop current; and loop current verification circuitry configured to approximate the loop current value based on the output signal and properties of a low pass filter, and generate a diagnostic signal based on a comparison of the approximated loop current value and the measured loop current value.

An automation engineering two-wire field device, having: a sensor element for capturing a process variable; a connection terminal for connecting a two-wire line; field device electronics supplied with an operating power by a terminal voltage and a loop current from the two-wire line and configured to communicate the captured process variable by setting the loop current to a value between 4 mA and 20 mA; and a diagnosis unit configured so as, at least in the measurement mode, to capture at least one applicable value for the terminal voltage for each of at least two different values of the loop current set to between 4 mA and 20 mA, and to take exclusively the values captured in the measurement mode as a basis for making a statement about a minimum value of the terminal voltage for a maximum value of the loop current greater than 21 mA.

Control electronics for a two-wire process device, which two-wire process device can be connected to a two-wire loop. The control electronics comprises an input circuit and a control unit connected to the input circuit, wherein the input circuit is used in a first operating mode to set a current and/or voltage in the two-wire loop by means of a control signal transmitted by the control unit. The input circuit is used in a second operating mode to automatically, in particular independently of the control unit or independently of the control signal transmitted by the control unit, to set the current and/or the electric voltage in the two-wire loop.