"Johnhh" wrote in message
...
Larry, you bring up and interesting question that I am pondering. I am
going to be hooking my Garmin 176 up to my Simrad autopilot. The Simrad
nicely isolates the Nmea common from battery neg. Unfortunately, the
Garmin
only has 4 connections, 12v+, Nmea in, Nmea out and common shared between
the Nmea and 12v -. If I connect this up, I'll be crossing the ships neg.
bus with Nmea common. This does not seem like a good idea to me. Do I
need
to (or should I) be looking into some sort of optoisolator?

No.

NMEA is laid out to prevent ground loops in the datalines. This is achieved
by having every input isolated. The outputs need not to be isolated. And if
everyone adhered to that spec, there would not be any problem.
In your case, youll be fine since that situation is almost exactly to the
NMEA spec.

Meindert