The unit manufactured by collins features 2 connectors for 2 separate transponder unit comunication. Here is the pinout: (Thanks to my friend Curd for providing it to me)
So as you can see both functions are the same. So of our interest we have:
5VAC for panel backlight.
115VAC and return for unit power
Fail logic input
ARINC data channel
Here are few shots of the unit powered up. Note when selected transponder 1 you see ---- cause I didn't power the xpdr 1 input.
And here is a video of it during power up test