Larry wrote:
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!!




I'm a telecom/datacom engineer, long since divorced from the analog
world, but I am familar with the concept of inductive coupling by
twisted pairs to cancel out the effects of capacitance created by long
parallel conductors with an insulator between them. I wonder how many
loading coils are still out there in old local telco outside plant.

Yes, professional microphone circuits are balanced, but there's still a
lot of "professional" equipment out there that hasn't implemented the
concept properly. I used to do sound work for live bands back in the
80's and a bit in the 90's. I routinely had to run ground busses around
and lift various patch cords' shields to eliminate 60Hz hum. Roadies
couldn't understand why this cable worked fine plugged in one way but
screamed like hell in the other direction.

Biggest problem is that guitar amps run unbalanced (I assume that most
any guitar you buy today does not have a balanced lineout - unless you
count the relatively new Gibson Ethernet model with standard 10/100
Ethernet ;-), as well as most effects units. I have some digital delay
units from Alesis and equalizers from Rane that support both. I bought
my wife a new Yamaha keyboard last Christmas - and it only supports
unbalanced. It's not only the NMEA that's shortsighted...

Anyway, I don't have HF SSB on my boat, and my only differential NMEA
units are both listeners only (Navico WP5000 auto and a Standard-Horizon
DSC VHF for GPS input), so I'll go with Peter's suggestion and ground
those listeners' (-) leads...

BTW, I also found out that Standard's old SL45 speed/log unit does
support NMEA 0183 (VHW only - not water temp MTW). So if anyone has one
and doesn't want to face the prospect of filling a 4" hole in a
fiberglass bulkhead, you can still get speed over water to a laptop or
NMEA repeater.