AXP322 remote control diversity

Anyone out there figure out how to control a AXP322 xponder from BOTH a IFD540 AND IFD440? Currently if you have the 540 controlling the transponder, and you loose the 540, you loose the transponder too. Seems like you should be able to wire in the 440 as a backup for controlling the transponder.

It makes for a difficult situation when the 540 goes out and you are trying to fall back and ATC gets on your case for a faulty transponder. Makes the workload higher PLUS ATC doesn't have a good radar track in case things are really going bad.

Also, seems like you should be able to feed GPS position data to the transponder from either unit so that ADS-B keeps working.

Anyone work out this problem?

I don't think this is unique to the AXP322.  Most ADS-B transponders only have one data input port for the GPS position.  So in most cases, the ASD-B transmitter is "captive" to a single GPS receiver onboard the aircraft.  Also, most remote-mounted avionics only have one input port for a control head.  So the remote box is always captive to a single point of control.

For aircraft with dual transponders, I could understand having transponder #1 controlled through IFD #1 and transponder #2 controlled through IFD #2.  Aside from that, I would not be too worried about it.  Equipment failures are a legitimate emergency from the ATC perspective, even if the effect on the flight is minimal.  So if I were to lose my ADS-B as part of a larger equipment failure, I would expect ATC to accommodate my needs for that flight (once I reported the failure to them).

Maybe I missed the point.

Are we talking about the GPS groundspeed data necessary to perform mode-switching in a standard transponder?  Or are we talking about a GPS position that is necessary continuously throughout flight to populate the ADS-B Out data transmissions from a 1090ES or UAT transmitter?

If we're talking about mode switching, I have to ask do you normally check your transponder during the takeoff roll to see that it switches to an active mode?  I think my attention is well-spent on other things in those moments.

If the concern is a GPS that was working at the ramp, then fails prior to flight (rather than failing once in flight), I suppose I understand the scenario.  But the opportunity seems vanishingly small.  If this keeps you up at night, then just don't configure your transponder for automatic operation.  In my case, that isn't even an option.  I have an old-school transponder and I just leave the mechanical mode switch in ALT all the time.  Worry-free.

That's different than having it power up, work normally and acquire position, then fail at the runway hold-short line.

If the GPS fails to acquire at start-up, well that's less of a problem than finding a bad mag at the run-up pad.