Jon Gauthier wrote in
:

is
it best to just leave the NMEA (-) leads floating, or run them to DC
ground?


NMEA is the RS-422 databus....*******ized. RS-422 uses twisted pairs to
reduce interference into and out of the cabling. So does your landline
telephone. Any signal crossing the twisted pairs will show up from the
wires to ground, say your HF SSB transmitter RF. But, between the balanced
wires (from wire to wire), anywhere along the wire of any length, the
induced signal is cancelled out by the twisting and because both wires go +
and - in polarity together, not 180 degrees out-of-phase like the true RS-
422 signals do. Equipment listening to the bus knows the difference.

RF noise, generated by the data pulses on a true RS-422 bus, causes equal
and opposing radiation to happen. Because the wires are twisted, this
effectively cancels any RF radiation or nearby inductance pickup of these
pulses rich in harmonics.

Professional microphone circuits are all 600 ohm balanced lines for these
same reasons. Any induced 60 Hz hum from nearby electrical wiring is
cancelled out long before it get to the amplifiers, even on long mic lines.

Unfortunately, NMEA accountants must have found out it costs $1.08/unit
more to do it the right way. ANY grounding of the - signal line negates
the balanced line advantage over the entire system. THANK YOU, NMEA!!



--
Larry