Alright, so I have the KLN-90B hooked up and working with the D-180. It seems that the EFIS receive and transmit lines are working properly. In the installation guide for the KLN-90B, the manual talks about 2 separate ARINC-429 inputs (2 sets of twisted pair). These are "EFIS-429 receiver input" and "Air Data 429 Receiver Input".
I have wired my GPS so that the Dynon's 429 Transmit Pins (13/25) are connected to both (spliced) the KLN-90B's "EFIS" and "Air Data" Pins. i.e. Dynon 13 (ARINC 429 TX B) is connected to both KING 4 (EFIS RX B) and 8 (AIR DATA RX B), and Dynon 25 to KING 5 and 9.
For some reason, this configuration worked flawlessly for about 3 minutes, during a ground run. I could actually change the altimeter setting on the Dynon, and see it change on the GPS. But then the next day, the GPS began throwing "AIR DATA 429 FAIL" and "ALTITUDE FAIL" messages.
So I know that hooking up the ARING 429 TX pins from the Dynon to the AIR DATA 429 RX pins on the GPS works, but perhaps it's not smart to do it via the same pins that are connected to the "EFIS 429 RX" pins on the GPS. Are the 2 separate sets of ARINC 429 TX pins on the Dynon (13/25 and 12/24) independent? I'm thinking another possible solution, since I am not currently using the other transmit pins, would be to wire those up to the Air Data RX pins on the GPS, so they are separate lines.
Any other ideas? Like I said, the GPS receives OBS information from the Dynon, but it's not receiving any of the "Air Data" inputs.