Ideally they should tie back to grounds that are common for the systems they control... My Push-to-Talk and Freq. Swap buttons tie back to the PTT, FSWAP, and GND pins on the back of my Icom, and my transponder Ident button ties back to the IDENT and GND pins on the back of my Garmin 327.
If you join all the grounds, you run the risk of getting noise from one system into another... At least that is how I understand it.
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Rob Prior
1996 RV-6 "Tweety" C-FRBP (formerly N196RV)
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