G5 and KFC225

Here’s the interface. The drawing is from the STC manual for the certified version.

this is the second page and might also help.

The GAD29B ist the interface between the Garmin CAN bus that connects all/most Garmin devices and Arinc 429. It is a separate box.

Peter wrote:

Is a GAD29B inside the G5? That connection looks like a fake heading bug. The GAD29B then looks like a roll steering converter, taking in ARINC429 and outputting a fake heading bug.

The GAD29B is a separate unit that converts GPS/NAV ARINC429 to CANBUS to provide display of GPS and VOR/ILS data on the G5. There are separate drawings showing the complete 429 interface including the VOR/ILS bus.
It also converts CANBUS back to analogue heading and course error for the autopilot (the G5 has no dedicated analogue interfaces). It can provide the pseudo GPS roll steering for those autopilots that don’t have the ARINC interface although not necessary with the KFC225.

Note 4 to the diagram from @Wigglyamp might also be relevant.

Peter wrote:

So what does this actually do, given that a KFC225 can accept ARINC429 roll steering directly from a GPS box (GNS GTN IFR etc).

It allows the autopilot to receive Heading and Course error if using a G5 as an EHSI. In your case with a Sandel SN3500 it wouldn’t offer you any benefit as an EHSI over what you have an in fact would be less useful as it can’t display DME or ADF.
If you has a G5 as an attitude indicator it also wouldn’t benefit you as it can’t replicate the autopilot attitude source or provide the FD bars. If you wanted to replace the KI256 in your TB, you’d need the GI275 (ADAHRS version) instead.

For an experimental RV (without the KFC225 of course), it’s the perfect solution as it will drive the Garmin autopilot.