GSV for ControllerDevice

I gave the plant a status screen for all the IO racks and I added a controller status to show RUN/REM/PROG, but then I realized if the PLC is in program mode, it won't be executing the GSV to update the status of the controller mode, so what is the point of even having this in the ControllerDevice object?
Is there actually a way to poll this when it's in program mode?

The only method that I have used to show operations when a processor has stopped (fault or program mode) is another PLC exchanging data, and detecting a heartbeat lost.

PLC A puts data for it's HMI into a data file, N43, which is read by the HMI and PLC B. One of the registers in N43 is a heartbeat, which increments regularly, like every second.

If PLC B reads the heartbeat register in PLC A and sees no change in the number for 3 seconds, PLC B writes 0's into the whole N43 data file in PLC A. 0 in the status word for each of the devices happens to mean power lost, communication lost, not running, in maintenance.

It's not elegant, but it works.

It gets complicated when you have more than 3 PLCs. PLC B monitors PLC A, PLC C monitors PLC B, PLC A Monitors PLC C.

THIS NEEDS TO BE VERY WELL DOCUMENTED! It is quite confusing to have another PLC change data within the PLC you just switched into PROGRAM MODE. This is the only time I WRITE to another PLC instead of READING.

You're correct that the PROG mode value for the ControllerDevice object is a bit of a paradox, but it makes sense when you consider that the object itself can be accessed over a network as well as via the GSV instruction.

You could have another controller access the object via MSG instruction (RA KB Answer ID 779392, TechConnect required).

If you do have another controller consuming tags from this controller, modern firmware (v17+) allows you to include the controller run/prog status as part of the connection header. (KB ID 60813).

Some HMI drivers can be configured to query that object and report its value.

RSLinx Classic and RSLinx Enterprise, for example, have pre-defined diagnostic items, including @Mode.