What's the simplest panel meter or controller you can recommend that receives and normally retransmits a 4-20mA signal, but has a digital input so that when it receives that input, switches to an on- board potentiometer (or buttons) as a manually-controlled 4-20mA signal?

 

A 4-20mA repeater/isolator mirrors the input value on the output value. But I've never seen a repeater with an auto/manual function.

A standard, stand-alone "PID" controller with
- a 4-20mA control output that the PID normally controls in auto mode
- an auto/manual function that allows for manually controlling (typically with up/down pushbuttons) the percentage output of the control output.
should replicate the input signal and provide override capability.

It needs scaling and tuning constants so that the control output signal mimics or follows the process variable input.

I just tried it with a Honeywell UDC2800 controller with a current output.

The 4-20mA signal is scaled a linear 0-100 (percent) with PID tuning constants of a gain of 10 (or PB of 10), zero rate action and a reset action at its lowest setting 0.02min/repeat.

In auto mode, the control output does not have the near instantaneous response of a repeater. A step change from 25% output to 75% output takes 10-11 seconds and overshoots 0.2 units and then it takes another couple seconds to drop to SP.

In manual mode, the output response is near instantaneous with the changes via the keypad.

Be aware that the really inexpensive 1/16 DIN temperature controllers typically do not have a key/button for auto/manual mode, so they require navigation through the setup menu to some point where manual mode is invoked and then there's some parameter for setting the output percentage value.

The higher end PID controllers will have an auto/manual button that makes the task you're describing a whole lot easier.

 

You can get hand held meters called process calibrators or loop calibrators, makes include chauvin-arnoux or Druck (personal favourite) and they can read and decode, or produce and impersonate, a 0-20ma, 4-20ma or 0-10v control loop. But they aren't "controlled by a logic input". However, very handy for setting up, calibrating and testing control loops and sensors / transmitters / PID process controllers / VSDs etc.