Hi Scott- Since no one else has chimed in, I'll give it a shot.  From the SkyView installation manual, revision AD, page 11-11:

"Direct Serial GPS Position Input (Pin 3) Prior to 2016, the only way for a SkyView system to comply with the FAA 2020 ADS-B Out mandate was to connect a serial output from a certified GPS to the Direct Serial GPS Position Input (Pin 3). As of January, 2016, Dynon Avionics offers the SV-GPS-2020 GPS Antenna/Receiver Module which provides high-integrity GPS data compliant with the FAA 2020 ADS-B Out mandate. The SV-GPS-2020 transmits high-integrity GPS position data to the SkyView display(s), which then transmit the high-integrity GPS position data to the SV-XPNDR-261/262.
Thus, if an SV-GPS-2020 is installed, this input is not used. "

As it says, since you have the SV-GPS-2020 installed on your system, pin 3 is not used.  Your 261 transponder will get all the position data it needs to be 2020 compliant thru whichever serial port (#1 - #4, since #5 is generally recommended for connecting your SV-GPS-2020) you chose to connect it to the rest of your system.

If you had the older (non-2020 compliant) SV-GPS-250 for your system, but also had a WAAS IFR GPS navigation unit (Garmin 430/530W, Avidyne 440/ 540, etc.) installed, you could route a position signal from that WAAS to pin 3 to become 2020 compliant.

Ken

On 11/27/2019 4:37 PM, scottbcs@... wrote:

Thought I'd give the new canard zone a try with a question I've been searching on today without success:

 I've got a Dynon Xponder 261 (2020 ADSB-OUT compliant) and a Dynon HDX display and Dynon's GPS 2020.  The wiring for the Xponder shows a serial interface with the HDX in both directions and it appears to be working in the sense that it provides ALT out, but I've not checked with others in flight yet to see if I'm showing up on their ADSB-IN displays.  

Here's my question:  The Xponder wiring diagram also shows an optional GPS data input to the Xponder that I've not wired yet.  Does the Dynon Xponder 261 get it's GPS information from the HDX (which most definitely has the GPS wired to it) over the HDX<->Xponder serial port, or do I have to wire this external GPS input in addition to the HDX serial lines to get the full benefit of GPS over the ADSB-OUT functionality of the Xponder?

Thanks,

Scott Fish
Cozy MKIV  N78CZ