Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 07-03-2014, 11:38   #1
Moderator
 
nigel1's Avatar

Cruisers Forum Supporter

Join Date: Oct 2009
Location: Manchester, UK
Boat: Beneteau 473
Posts: 5,180
Grounding NMEA 0183 Shield Wire

Soon be installing AIS and a multiplexor, connecting to a chartplotter.

Have a few questions regarding the shield wire on the NMEA cables.

1) From various bits of research, the shield wire should only be grounded at the talker end of the circuit, this is to avoid ground loops (there will be a question on ground loops later)

2) Will it be OK to connect all the shield wires to a common connection point, say a bus bar, or a copper strip?

3) If item (2) is OK, where do I connect the common ground point to. Is it OK to ground to the engine block, or can it be connected to the battery return cable at the switch panel

4) The multiplexor, this will be a listener and a talker. The NMEA plug on the chart plotter contains both input and output NMEA pairs, plus a shield wire. I assume that the shield wire in the plug somehow is grounded through the plotter. The multiplexor will receive NMEA from the chartplotter at a baud rate of 38,400, then converts that to 4800 baud rate so I can get NMEA data to DSC and radar. Should I ground the shield wire of the cable from the multiplexors 4800 output port?

5) What problems do ground loops cause, I donít want to make a ground loop, just interested in what havoc it can cause.

I want to keep the present system for the time being, so please, no suggestions to bin it, or sell it, and fit a N2K system or similar, thanks
__________________

__________________
Nigel
Beneteau 473
Manchester, UK
nigel1 is online now   Reply With Quote
Old 07-03-2014, 12:41   #2
Moderator
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Albany Ga.
Boat: Island Packet 38
Posts: 17,032
Re: Grounding NMEA 0183 Shield Wire

We aren't talking about landing tailwheel aircraft are we?
Best example I can think of a ground loop is the hum you will sometimes hear in an amplified system, ground loops can introduce noise, interference into an electrical circuit
__________________

__________________
a64pilot is offline   Reply With Quote
Old 07-03-2014, 18:05   #3
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Grounding NMEA 0183 Shield Wire

At 4800 bps over a few feet, forget about shields completely , even at higher speeds the shield does little

Dave


Sent from my iPad using Tapatalk
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 08-03-2014, 17:48   #4
Marine Service Provider
 
Paul_DY's Avatar

Join Date: Jun 2012
Posts: 39
Re: Grounding NMEA 0183 Shield Wire

Theoretically you should join all of the shield connections to a common ground (usually Earth on land). On a boat you do not have an earth connection so what tends to happen is you connect all of the grounds together and then connect this to any metal skin fittings, SSB grounding plates, sacrificial anodes, etc.

You do have to be careful though because if you get any stray currents in the system it can increase the speed of electrolytic corrosion and for this reason some people prefer to isolate everything.

I agree that grounding the screens of NMEA0183 signal wires is not really necessary but it is good practice to ground the screens of data cables at the transmitting device to reduce noise emissions. Also clip on ferrite cores are worth fitting as they suppress high frequency harmonic noise.

Earth loops are usually only a problem in radio and audio systems where you get that irritating hum or background noise although it can also cause flickering or distorted video on monitors and TVs.

You will never get an earth loop causing data corruption in an NMEA data link,

If your engine block is connected to the negative of the battery then you definitely do not want to connect this to the ground.

The issue of grounding on boats is confusing and there are differences between US installations and European installations, so I would recommend talking to a local NMEA qualified Dealer about the specifics of your installation.
__________________
Paul Sumpner
Digital Yacht
Paul_DY is offline   Reply With Quote
Old 08-03-2014, 18:04   #5
Moderator
 
nigel1's Avatar

Cruisers Forum Supporter

Join Date: Oct 2009
Location: Manchester, UK
Boat: Beneteau 473
Posts: 5,180
Re: Grounding NMEA 0183 Shield Wire

Thanks for the explanation Paul, cleared up a few things for me.
About the ferrites, I have those clipped around all the power cables, but is it good practice to place on data cables as well?

Thanks
__________________
Nigel
Beneteau 473
Manchester, UK
nigel1 is online now   Reply With Quote
Old 09-03-2014, 07:18   #6
Marine Service Provider
 
Paul_DY's Avatar

Join Date: Jun 2012
Posts: 39
Re: Grounding NMEA 0183 Shield Wire

You certainly won't do any harm adding ferrites to data cables and everything you can do to minimise noise in a system has to be a good thing.

In all likelihood, assuming that your electronics are from reputable marine manufacturers, you should not have any noise issues and you will not detect any benefits from all of the grounding and fitting of ferrites. However, it is better to do this work during installation than try and do it retrospectively after finding a problem.
__________________
Paul Sumpner
Digital Yacht
Paul_DY is offline   Reply With Quote
Old 09-03-2014, 11:20   #7
Moderator
 
nigel1's Avatar

Cruisers Forum Supporter

Join Date: Oct 2009
Location: Manchester, UK
Boat: Beneteau 473
Posts: 5,180
Re: Grounding NMEA 0183 Shield Wire

Thanks Paul,

Appreciate the advice
__________________
Nigel
Beneteau 473
Manchester, UK
nigel1 is online now   Reply With Quote
Old 09-03-2014, 14:14   #8
Registered User

Join Date: Jun 2013
Location: canada
Posts: 1,695
Re: Grounding NMEA 0183 Shield Wire

so devices don't have a "shield" and the bare wire is actually data neg. depends on the stuff.

I've never bothered shielding nmea 183 and never seen an issue.
__________________

__________________
smac999 is offline   Reply With Quote
Reply

Tags
nmea, nmea 0183

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 05:45.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.