(Posted this on the aeroelectric list.  Probably worth posting here as well.  Guess the ultimate question is: what works?) 

After Wayne Sweet suggested the Garmin manual was off regarding shielding, I called the company.  Ready for this?

1. Connecting a UPS SL30 Nav Com to a MD200-306 CDI indicator: UPS manual (before the Garmin merger) says ground shields at both ends.  They are SPECIFIC.  For example, they have footnotes that explicitly say to ground a both ends, while they also have footnotes saying to let float the Nav/Com end of the connection to the audio panel.  Couldn't be more explicit.

However, the Garmin tech rep says to ground the shields only at the Nav/Com end.  He says "I don't know why the manual says that?"

2. Connecting a GNS430 GPS/Nav/Com to a MD200-306 CDI indicator: Garmin manual says ground shields at both ends.

However, the Garmin tech rep says to ground the shields only at the Nav/Com end.  He says "I don't know why the manual says that?"

3. Connecting a GNS430 to an altitude serializer: Garmin manual says ground the shield at both ends.

However, the Garmin tech rep says to ground the shield only at the Garmin end;

while the Microencoder rep says ground the shield at the Microencoder end (the "sending" end).

Almost as bad as economists (my field).  OK scientists, there either IS a right answer to each question, or there are tradeoffs that are worth understanding.  Inquiring (if somewhat mystified, and a little irritated) minds want to know.