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  Topic Review (Newest First)
09-18-2008 08:15 PM
sailingdog Actually, I found the hard way that you can read an RS-422 signal on an RS-232 port fairly well. If you read the technical literature, and many of the actual implementations of NMEA 0183, it is, in fact, an RS-422 signal.
09-18-2008 06:06 PM
hellosailor SD-
I've always heard NMEA referred to as RS-232, not RS-422. The former can be implemented on 3 wires and swings from +5 to 0 volts, where the latter is a differential system which swings from +5 to 0 to -5. Macintoshes typically used an RS-422 serial bus, instead of the RS-232 used on the PC, because the more expensive bus allowed longer runs and higher speeds.

I treat NMEA as RS-232, and it works. It couldn't do that if it was RS-422. Typical GPSes (Garmin, etc.) have RS-232 poinouts, and don't support 422.

The biggest confusion over connections, is that most manuals fail to make clear that "RX" and "TX" (aka Receive and Transmit) are relative terms, not absolute. So the RX wire from one unit, goes to the TX wire of the next, when there's only a pair of them. Not "same to same" as most folks would expect.
09-18-2008 05:25 PM
sailingdog It really depends on which Garmin you're sending the data into. Different models will recognize different NMEA sentences. Most of the smaller standalone Garmin chartplotters won't recognize much more than Velocity through water, Water temperature, Wind speed and heading, or so.
09-18-2008 05:21 PM
jarcher Thanks everyone... Actually, I was not too concerned about the electrical specifications, which i should have made clear. My concern is whether or not Garmin can accept and utilize properly the NMEA sentances emitted by the Nexus server.

The Nexus can be the navigating device or let another device be the navigator. If it is the navigator it will emit lots of useful NMEA and I was wondering if that can be fed into a Garmin chart plotter such that the Garmin will do something useful with it.
09-18-2008 08:24 AM
sailingdog Tommays-

NMEA 0183 is actually preferentially defined as RS-422 protocol per this PDF. To quote another PDF on NMEA 0183 protocol:

Electrical specification

The latest specifications for NMEA 0183 (version 2 and
onwards) should, as a minimum, meet the requirements
of the computer standard “RS22” (Standard EIA-22-A).
They use +5 / 0 volt signalling, which is low voltage and
easy to interface to computer equipment.
09-18-2008 07:44 AM
tommays In 0183 the GPS transmit wire allways connects to the receive wire of the decive and a common wire (Single ground)

Anything giving info to the GPS would have to have its transmit wire connected to the GPS recieve wire and the common wire (single ground)

The 0183 is really just RS-232 (old mouse) given a marine name

Even in a limited manual they should tell you the color code of the wires

Then as mentioned you get into the proprietary data BUT if it is a 0183 device then it should work but there is really no way to check other than it works

i gotta read the date
09-18-2008 07:44 AM
sailingdog Not likely that he'll respond since his post is TWO YEARS OLD. Please check dates on posts before replying. Also, highly recommend you read the post in my signature.

As for interfacing Garmin NMEA ports... they have an odd design idiosyncracy—they use the 12 VDC ground for the signal ground. When you connect the NMEA wires from the other device, you need to connect the negative wire from it to the Garmin's 12 VDC ground. It may be because NMEA 0183 was originally designed as a RS-422 signal and many implementations of it actually are RS-232 signals. Give that a try.

Originally Posted by jarcher View Post
Were you able to accomplish this?
09-18-2008 05:09 AM
Originally Posted by Silmaril View Post
I had a heck of a time getting my Garmin to talk to my Nexus network, compounded by the fact that Garmin is notorious for mixed up wiring on the NEMA connections.
Were you able to accomplish this?
02-19-2006 09:50 PM
It's in the "Words"

Well, there is NEMA 0183 and there are it's "Versions" 1.0, 1.5, 2.0 etc. That sets the standards for electrical connectivity to the communication "bus". All devices on the bus communicate and share information equally.

Then you have to see what "words" or "phrases" each device recognizes. Raymarine uses proprietary "phrases" for its "Seatalk" interface. That way it can handle all the data on the interfaces and between their instruments. The problem arises when you try and interface a non-Seatalk device that does not regonize the phrases that Raymarine sends/recieves. Your new plotter may or may not recognize all the Raymarine phrases. In the communications appendices of both the Raymarine devices and your new plotter, down around oh say page 200 (just kidding) you will see a listing of the interfaces supported and the phrases recognized. You will need to make sure the Baud Rate, and NEMA version are the same for all devices, and then see what phrases are common between the new plotter and the Seatalk format.

Have I lost you yet? It's really quite complicated, and I think that instrument vendors do that specifically to avoid the "mix and match" approach to onboard data systems and have you buy everything from "Them" and not other vendors.

I had a heck of a time getting my Garmin to talk to my Nexus network, compounded by the fact that Garmin is notorious for mixed up wiring on the NEMA connections.

Good luck! Unfortunately the manufacturers are ZERO help trying to connect to things outside of their own product range!
02-19-2006 06:56 PM
NMEA 0183 how to connect my new plotter

I just bought my boat (used) and want to replace the old Raymarine 520 plotter (greyscale) with a new NAVMAN Tracker 5600 (color). All other instruments on the boat are Raymarine (Wind, depth-speed, & Autopilot). I want to understand and learn how to make the connections so the plotter reads the inputs of the other instruments and also connects to the Autopilot. Is this just a matter of creating sound connections or do I need more? The Raymarine plotter seemed to had a bunch of wires coming in, at least I didn't see any additional box (multiplexer???) B.t.w. these connections were made in a horrible matter and secured with electrical tape ....) when I took the instrument of the Nav-Pod today. I expect the new plotter to arrive within a week or two and want to learn what needs to be done so I can install it the proper way. The NAVMAN manual gives me the colorcodes for the wires, but of course doesn't go into any specifics ....

Any help would be greatly appreciated.


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