On Mon, 25 Aug 2014 16:16:47 -0400, "Sir Gregory Hall, Esq."
wrote:

OMG! You are stupid. Here's a clue: The ONLY reason direct
current works is when it is able to complete a circuit from
the positive terminal of the 12V battery to the negative
terminal of the battery. NMEA is but an extension of the
mother board circuit in any VHF or GPS. It must be able
to complete a circuit from positive to negative.

===

Stupid is as stupid does. NMEA 183 does not require a ground
reference, just two legs of a circuit. As I understand it, it is
sometimes coupled using opto isolators to prevent an
accidental/unwanted ground reference.

I would direct you to the following sentence found he

http://en.wikipedia.org/wiki/NMEA_0183

"Although the standard calls for isolated inputs and outputs, there
are various series of hardware that do not adhere to this
requirement."