I found this on a US site, for a KLN94 → GPS175 change
I thought this might help anyone else going from a Bendix/King KLN-94 to a Garmin GPS-175 to get the OBS resolver working with the existing KLN-94 wiring:
KLN-94—————————GPS-175
P942——————————-J1751
37 OBS RETURN——2 OBS STATOR H (GND)
34 OBS OUT—————3 OBS STATOR C
36 OBS COS————-25 OBS STATOR F
35 OBS SIN—————-44 OBS STATOR D
-————————————24 OBS STATOR G (GND) N/C
-————————————45 OBS STATOR E (GND) N/C
The only other issue when switching is the tray mounting screw holes from the KLN-94 do not line up with the GPS-175 tray so you will need to drill new holes.
The GNX 375 would make a neat swap for a KLN94 and Mode S transponder, but not sure if Europe is ever likely to go ADS – B – Out/In, for sub 5.7 ton aircraft. So not sure it makes sense economically.
A TB owner in the US replaced a KLN94 with the GNX375 – exactly what I would expect to be a common application. This was posted on the US Socata site
He says it drives the KMD250 MFD OK (he cannot get database updates for it though). I guess he has not tried it in the OBS mode, which is where replacing a KLN94 with a GNSx30 box reportedly failed to work when driving the KMD550 normally fitted to later TBs.
His report:
The ADS-B solution for N205GT was to remove the old BK KLN-94 and transponder replacing it with a Garmin GNX 375, an economical solution to get GPS Nav up to date and comply with the Jan.1 mandate in one box. It works great, interfacing with the King indicators and S-Tec 30 Autopilot with no mods. One hitch, we had planned to keep the old BK KMD-250 as an auxiliary display. The GNX 375 will display course info on it and the old TKAS installation is still active, useful for alerting to the 20% that’s not ADS-B compliant yet.
Garmin GNC 355.
A simple GPS/Com has just been released. Not sure why the 8.33 version is 700 dollars more expensive.
https://www.avweb.com/aviation-news/garmin-adds-ifr-approved-gps-com/
Peter wrote:
The KMD550 with its 320×240 display
The GTN 175 has a resolution of 732 X 240. The screen size is 4.8" diagonal verses 5.0 in. diagonal for the KMD 550.
Yes, and the video posted by Bosco shows how it can work as a user interface for the GPS175.
However I dislike “gadgets” for the reasons given.
In a homebuilt it may well be a popular option to install this box. An IFR GPS is unavoidable for flying GPS approaches, it appears, more or less, and a secondhand KLN94 doesn’t do LPV. These people can permanently mount an Ipad in the panel, with proper cooling. I would probably root it and install a startup script which makes it start up fully just with the power coming on. But in most certified planes you don’t have the room.
In general, the iPad is a great MFD.
Peter wrote:
In IFR flight, in the Eurocontrol system, one is constantly working the route ahead tactically, asking for shortcuts, etc, often juggling shortcuts in conjunction with avoidance of weather.One needs a good view of the route ahead.
…
I would never want a single GNS430, GTN650, GPS175, etc. For VFR hacking, sure (because one is mostly OCAS and even the briefest CAS bust is a massive hassle, so one needs to run special mapdata or apps) but not for busy single pilot flying in busy airspace etc.
I have a single GNS430, and I don’t think it’s nearly as bad as you say. It could certainly use a bigger screen, but there are very few tactical navigation tasks that couldn’t be managed on a 430 by a couple of hits on the range buttons if the flight has been prepared diligently enough.
In IFR flight, in the Eurocontrol system, one is constantly working the route ahead tactically, asking for shortcuts, etc, often juggling shortcuts in conjunction with avoidance of weather.
One needs a good view of the route ahead.
Concurrently, the “small box” (the KLN94, the GPS175, etc) is used for textual data e.g. displaying the LFOB (landing fuel on board), the ETA, distance to run, etc.
The KMD550 with its 320×240 display
is antiquated but absolutely priceless. Without this overview, the job would be much harder.
I would never want a single GNS430, GTN650, GPS175, etc. For VFR hacking, sure (because one is mostly OCAS and even the briefest CAS bust is a massive hassle, so one needs to run special mapdata or apps) but not for busy single pilot flying in busy airspace etc.
We had a thread on MFDs in general, in which everyone expressed the standard views (they are 20th century stuff) 
including the correct observation that most of them have been discontinued, but I can’t find it. Avidyne might still make some of theirs (and have apparently no resources for new products anyway) but Garmin compatibility was never possible to establish (OBS mode in particular) despite extensive enquiries by myself in years past. Garmin still seem to make the GMX200 and presumably mapdata support is still available and will be for many years. Anyway, MFD mapdata doesn’t really need updating because – at least in the King world – the entire route including waypoints and their names – gets sent from the GPS to the MFD.
No way would I use a tablet for this job. There is barely room in the TB20 for stuff like that, the battery goes flat, it overheats and shuts down, etc.
Unfortunately the GMX200 cannot replace my KMD550 because it is 5" high versus the KMD’s 4". And I have no spare space, so this isn’t going to run, sadly.
Bathman wrote:
Peter why do you need a GMX200?
I have the same question, what functional requirement is satisfied by the GMX200, or for that matter an MFD?
