Thank you so much for your prompt response.
In the event it has not already been brought to your attention: A correction needs to be made to both Piston & Turbine Duke manuals which do not list the Flight Director variable as "L:var_FlightDirectorVisible," but rather, "K:TOGGLE_FLIGHT_DIRECTOR."
In pursuing this matter with RSG, this is the response I received:
Since a large number of the variables for the Duke's use L:vars you would need to program the GFC500 autopilot using software like Spad.next. Our current device interface is not able to send L:Vars.
Additionally the Duke's internal coding (at least on the Turbine version) has a flag in it to maintain the activation of the MSFS Flight Director and HDG modes on the Century 4 autopilot. The L:var flag that is listed in the manual for the flight director will only turn on/off the FD needles on the attitude indicator in the 3D cockpit, while the lights on the GFC500 will always stay on for the flight director.
I do not have Spad.next, so I am unable to test it with the GFC500. But I do have FSUIPC, and unfortunately, the GFC500 does not show up as an addressable device.
I will pursue this matter further with RSG to see if they might consider updating their interface to include L:Vars. I would be open to other solutions you might have.
Thank you for your time, and for providing such awesome products.