On Tue, 10 Apr 2007 17:10:04 -0400, "Roger Long"
wrote:

Goofball_star_dot_etal wrote:

it is odd. Is your radio model here?
http://www.standardhorizon.com/?cmd=...s&DivisionID=3


Yes, it's this one:

http://www.standardhorizon.com/downl...Name= GX1255S

I left the manual on the boat because I didn't start wondering until the
drive home.

Looking at the PDF version (thanks for the link), I see that the radio calls
both wires "NMEA IN", one is + and the other - so connecting to ground makes
sense. But, why wouldn't they just do that inside the radio and have you
just connect one wire?

Officially, an NMEA-0183 input should be optically isolated, so your
two leads are the anode and cathode of an LED in an optocoupler.

Also officially, an NMEA output should be differential - there should
be a +out and -out which would connect to the +in and -in. However
most manufacturers seem to economize, and use a single-ended output,
and many also use a single-ended (non-isolated) input.


Looking at the table, it looks like connecting to ground is the common for
many GPS types but not universal. Maybe not having a connection to ground
inside the radio is necessary for the other GPS types to work properly.

Not having an internal ground makes the input a "proper" NMEA input.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca