Ground the NMEA negative?

Rivers & creeks

Rivers & creeks

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I had a Garmin GPS feeding the NMEA system which I've now replaced with a Matsutec. The Garmin required battery negative to be joined to the NMEA -ve out from the GPS so I attached that to the NEMA -ve mini bus bar. Since then the Garmin's gone but I'd like to leave the battery -ve connection as it RF grounds another piece of kit. Is that a problem?
 
Lazy Kipper said:
I had a Garmin GPS feeding the NMEA system which I've now replaced with a Matsutec. The Garmin required battery negative to be joined to the NMEA -ve out from the GPS so I attached that to the NEMA -ve mini bus bar. Since then the Garmin's gone but I'd like to leave the battery -ve connection as it RF grounds another piece of kit. Is that a problem?
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That should be fine.
 
Not a problem as such as in nothing nasty will happen but cable of that size would not provide an effective RF ground, 8awg (8.36mm2) is considered the minimum for an effective RF ground. The original Garmin connection to the DC- has nothing to do with RF grounding. Also worth a mention that the RF ground should be non current carrying.
 
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Newer NMEA 0183 kit uses balanced lines where neither needs to be connected to boat negative. However as I understand it, it can cope with one side being connected to the negative and still function fine.
 
David2452 said:
Not a problem as such as in nothing nasty will happen but cable of that size would not provide an effective RF ground, 8awg (8.36mm2) is considered the minimum for an effective RF ground. The original Garmin connection to the DC- has nothing to do with RF grounding. Also worth a mention that the RF ground should be non current carrying.
Click to expand...

Thanks for that. I built a five section bus bar for the three Seatalk cables and NMEA +/- so that I could easily add more listeners to the NMEA GPS talker and more instruments to the Seatalk system. The NMEA -ve had a connection to the battery -ve because the previous Garmin GPS required that and so I attached the SPX30 RF ground stud to that via good sized cable. As the whole -ve side is all interlinked I assumed it would be OK.
 
Here's the lowdown on NMEA-0183 electrical specification so you can understand what is going on and why it may or may not work.

NMEA had a brain fart half way through 0183's life and changed the electrical specification. Consequently, it's a mess. Up to version 2.something 0183 was single ended, like RS-232. This means there is a data line which has a voltage level that goes positive and negative with respect to the other line which is ground, as in device ground, connected to your battery's 0V terminal.

Then in their wisdom NMEA decided to change to a differential system, like RS-422 (but not RS-422, just a bit like it). This has a data+ and a data- lines and no ground. For one state the + line will have a voltage on with reference to the other, and these will be reversed for the other binary state. Being differential these lines could be floating with respect to your boat ground, but in reality the line drivers are powered by your boat battery so when a data line is low it is effectively battery ground. To confuse things further many manufacturers went to the newer standard for data format but kept to the old standard for the electrical bit, i.e. data and ground. Garmin was famous for doing this for years, they might still. My Garmin GPS has data in, data out, power and ground.

So if your talker uses the newer differential electrical specification and you connect NMEA- to ground, in one of the binary states your talker will be applying volts onto this line but you have grounded it - a short circuit. This will cause the following to happen - you might blow your talker's output driver, or alternatively it will have current limit protection and you will get away without damaging anything.

But will the comms work if you do this? It might, but it might not. NMEA-0183 listeners are usually a transistor driving an opto-isolator. If you have grounded your data- line on a differential system, and the driver can cope with this abuse, you might still get data through because in one binary state everything is fine and the transistor is switched on, and in the other when the voltages on the data lines should have reversed there will instead be no voltage difference and the transistor will still be switched off, allowing more by luck than anything, the data to get through.

So in summary, if you have a differential output you should not ground your data- line because you may damage your talker, but you probably won't as it will be protected, and your data may still get through (but also may not). However, it's mis-treating your kit and negates the advantages of the differential system as well (better noise immunity).

Confused? The whole world is about this one, and it's NMEA's fault for bodging NMEA-0183 and not moving to NMEA-0184.
 
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Thanks Angus, at first i was confused, now I'm petrified
Kept thinking i could never join my little toys inside,
But then i spent so many nights
thinking how was i doing it wrong
And i'm still not strong
But i hope to get along
And now you post, are you from outer space?
I just logged on and now ive a sad look on my face
i should have changed that stupid pilot
i should make you come and show me
If i'd known for just one second how nmea bothers me
go on now, go, please just show,
Just turn around now,
Cause your welcome on my boat
I hope you are the one who gets me back out on the sea,
I fear i might just crumble,
But one day it might just rumble!

Oh yes Angus, I. I will go live
As long as i know how to sail
I know i'll stay alive
Ive got all the wiring plans
Ive got cables in my hands
And i'll go live,
I will go live, Hey, Hey
 
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