Oh boy, on a scale of one (none) to ten (good), my understanding is proably
about a 3, so bear with me. To complicate it further, I have lots of other
Nmea devices to hook up berfore I'm through. Radar, computer, DSC radio,
second or alternate GPS, but we don't need to get into that.

When I hook up the one GPS, both the NMEA in and out are going to be
grounded at or near the GPS. If I understand correctly, you are saying that
that is ok provided that it is only grounded at one point, correct?

Now, if I run shielded signal wire, where should the shield be grounded, or
does it matter?

Thanks
John


"Meindert Sprang" wrote in message
...
"Larry W4CSC" wrote in message
...
"Meindert Sprang" wrote in
:

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

But, alas, he's hooking one side of his NMEA dataline to GROUND! It's
not
isolated at all at that point.

If the output had been differential, there would have been a path to
ground
as well though the internal impedance of the driver an output filters.
This
situation makes no difference since the other end at the autopilot is
completely isolated. So any induced HF on both the data line and the
ground
line are in phase and therefore generate no differenctial voltage at the
isolated autopilot end.

Read a textbook on transmission lines if you don't believe me.

Meindert