What you need to get is the same serial data source as output by the GNS 430. I don’t know the format but that is what they interface to the GTX330.
Well obviously if you emulate a Garmin GPS then it will work 
It appears that it is not simple.
The data going GPS to transponder, for ADS-B purposes, is not an “open” data stream. It is some kind of proprietary protocol. Avidyne have not been able to reverse engineer it, apparently. It may well be that the data is “open” (and may be published, or can be easily seen with a data analyser) but there is a cryptographically strong hash at the end of each packet which the GTX330 verifies before accepting the packet. That’s something that’s trivial to do in software and would ensure that only Garmin GPSs (of a certain era or later) can be used with a Garmin transponder. It would not affect certification because however distasteful this is, it is not for the FAA or EASA to prevent anticompetitive behaviour. Garmin did this on their crossfill protocol recently, to make sure that 3rd party flight plan loading products like Voiceflight do not work on the GTN boxes, and I believe they also don’t work on the most recent GNS firmware.
In any case, all that Garmin need to do is to write their STC so that only Garmin transponders are shown in the wiring diagrams (for ADS-B OUT purposes) and then they can claim that they are merely enforcing the STC compliance I am sure if they got half a chance they would verify (cryptographically or with a DNA sample) that the installation was done by a Garmin dealer
Getting the GND/AIR switching to work should be much easier, because the GTX only needs to extract the ground speed value out of an RS232 or ARINC429 data stream. But that’s a fish not worth frying because it is easily done with a differential pressure switch. I already have that. It was installed (“bodged” would be more accurate) during the TAS605 installation. There is a pic of the switch in that writeup. In fact I vaguely recall that Garmin did a GTX330 firmware update to make it “compatible with the KLN89” which sounds like a sneaky fix for the KLN94 issue, 9 years after they first published the interconnection 
Ah I didn’t realise the serial data from the GPS was encoded.
Ah I didn’t realise the serial data from the GPS was encoded.
It is not. The datastream has been modified a number of times. It is just not always possible to keep software backwards compatible.
For crossfill the same is true (this is not an open standard as far is I know). Garmin does not crossfill between certain software levels as well.
Feedback from the USA is that the IFD540 will not work as an ADS-B compatible source for a GTX330.
Would be a good reason for me to get rid of the GTX 330 and get a transponder which does not block any competitor’s devices.
Garmins sublte protectionist attitude towards other manufacturers was a primary reason for me NOT to choose Garmin as a Mode S Transponder supplier apart from their high pricing.
Sure, but back when the Mode S “wars” were being fought, the choice was the GTX330 and its nice buttons, or the King KT73 and its 1970s-style knobs. Also the GTX330 was a few hundred quid cheaper, even by the time you did the rewiring for it (the KT73 was plug compatible with the nice KT76C).
However, as I’ve said before, ADS-B OUT is almost totally pointless in Europe (to spend significant money on) because virtually nobody can use it for TCAS purposes. Currently…
I’ve just read on US sites that Garmin have stopped selling the GNS530W installation tray.
This is widely interpreted as a move to devalue used GNS530W boxes, in anticipation of a lot of people pulling out the 530W and pushing in the (plug compatible) IFD540.
Most people are not exactly happy about this move…
The Garmin 530W STC is conditional on the installation using the Garmin 530W tray. It specifies the Garmin P/N for it. So the obvious workaround – using the Avidyne IFD540 tray, about $600. for the GNS530W – is technically illegal because the installation won’t comply with the STC.
Whether anybody will be able to tell you did that is doubtful, but I guess the bigger avionics installers are not going to co-operate on that, partly because they want to make their 25% margin on the supply of a new GPS rather than just get a labour fee for installing a customer-supplied used 530W.
The “legal” solution to using the Avidyne tray to mount a 530W is a Field Approval, which in the USA is probably quite easy to get since it is so obviously like-for-like.
Avidyne should have anticipated this move, because it is pretty obvious that the whole “plug compatible with the GNS530W” concept relies on the old GNS530W tray being left in place, which means that your old GNS530W will be sold into the used market without a tray. They should have obtained an STC for their own tray for the 530W.
Avidyne has suggest to use their rack on an installation where the customer wants to sell his GNS is a complete unit. The rack swap would be less expensive then the approval of for using their rack with the Garmin.
However, the GNS530W STC prohibits the use of a non Garmin tray (rack).
I wonder how easy it would be to swap the two trays. If the connectors just drop out from behind, then it’s easy. If they are fixed from the inside (and would need to be un-wired) then it’s not.
IMHO this is largely academic, on an N-reg for sure, because using the AML STC you just push the unit into the hole, and get an IA to fill in the 337 and send it with the STC to AFS-750 in Oklahoma, and the job is done. You don’t need any “company” involvement. But I would guess the typical owner of an IFD540 will need a lot of configuration assistance.
Just to be clear, Avidyne recommends installing the IFD in the Avidyne rack and swap out the Garmin rack, to keep that with the GNS. So it can be sold as on complete package. This suggest was made in general, not specific for European market.
