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Author Topic:   NMEA Circuits
btb posted 04-28-2008 10:01 AM ET (US)   Profile for btb   Send Email to btb  
I have a GPSMAP 276C which has NMEA in (1), NMEA in (2) and corresponding NMEA outs, 1 & 2.

What is the difference between circuits 1 & 2?

I plan to hook this up to a Furuno FCV620 which also has two circuits which they label TD-A and TD-B (presumably Transmit data A&B?) and RD-A and RD-B (presumably receive data A&B?).

What's the difference between circuits A & B?

Other than this information in the respective manuals, no other mention is made of how to use these circuits.

Any info gratefully received!



Bella con23 posted 04-28-2008 01:25 PM ET (US)     Profile for Bella con23  Send Email to Bella con23     
The reason for the two inputs and two outputs on your GPS unit allows you to connect the GPS unit to two different other electronic components.

You may have a radar hooked up to circuit (1) and a fishfinder connected to circuit (2). Having two circuits prevents the electronic units from interfering with one another.

jimh posted 04-28-2008 07:27 PM ET (US)     Profile for jimh  Send Email to jimh     
Each output and each input is a separate serial data connection. You probably can configure the outputs to send a variety of sentences using the NMEA-0183 protocol by using the menu options of your devices. The better marine electronic devices contain explanatory material with enough data to allow a reasonably knowledgeable technician to interconnect the devices without any problem.

In general in the public doman not too much is known about NMEA because, even though it is a "open standard", the information is essentially kept secret by making the cost of obtaining it ridiculously high. If you want to get a few pages of information it is hundreds of dollars. NMEA is also an organization that promotes itself to marine electronic technicians and encourages them to join and become certified. I guess if they keep enough information out of the public view they can create work for their members.

NMEA is the antithesis of the internet, where all the standard protocols are clearly defined and available in open and free reference material.

Many people have developed some information about NMEA protocols by gleaning the tidbits of data that some manufacturers have published.

If you want to learn about NMEA protocols read this document

which is provided by a German website. I assume that probably keeps the NMEA organization from hounding them about giving away their secrets.

NMEA rationalizes their tight hold on the information by claims that in order to prevent harm to NMEA certified devices that they must discourage anyone from trying to home brew a NMEA device. It is rather short sighted of them. A good example is their standard called NMEA-2000. Here it is 2008 and the NMEA-2000 standard is barely getting off the ground. And what you are trying to connect is the old NMEA-0183 standard that goes back a decade longer--and it is still a big secret as far as they are concerned.

jimh posted 04-28-2008 07:31 PM ET (US)     Profile for jimh  Send Email to jimh     
The other laugh about NMEA is demonstrated by your situation. The manufacturers cannot even agree if they are going to refer to the inputs and outputs with letters or numbers. Some standard.

Also, there are many good sources on the internet of information about interconnecting NMEA. It really is simple:

--connect outputs to inputs

--if the output is a single-ended circuit, and the input is differential input, connect the inverting input to the power supply reference of the single-ended output

Chuck Tribolet posted 04-28-2008 07:55 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Jimh is using EE-speak again. That's usually MY trick.

Single ended: There's only one input or output line. The
negative power supply lead supplies the other half.

Differential: the input or output has two lines, + and -.


jimh posted 04-29-2008 12:29 AM ET (US)     Profile for jimh  Send Email to jimh     
My philosophy is you can't turn fishermen into serial data interconnection wizards. I just refer people who want to really learn about serial data interconnections to information about serial data interconnections.

Try this collection of information. It is excellent:

The NMEA 0183 Information Sheet

Everything you wanted to know about
NMEA 0183

(but were afraid to ask) The%20NMEA%200183%20Information%20Sheet.pdf

bluewaterpirate posted 04-29-2008 08:31 AM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
Use Com 1 from the 276c to TD A on the 620. Com 2/TD B can be used to connect other units at a later date. Here's the wiring diagram. 39428740/file.jpg


btb posted 04-30-2008 12:35 PM ET (US)     Profile for btb  Send Email to btb     

thank you again for your input.

So to connect my single ended Garmin to the differential Furuno, I also have to connect the black on the Garmin to the Blue and Green on the Furuno, in addition Tom to what you showed - the White on the Furuno to the Yellow on the Garmin and the Yellow on the Furuno to the Blue on the Garmin.

I posed the same question to Furuno tech support and they back up what you say here. They responded the same day I posted a question, with a thorough and complete answer - that was two days ago - they were very fast and very thorough.

At the same time, I posted the same question to Garmin tech support - haven't heard from them yet.

So we are all in violent agreement! Hurray. I can't wait to try this out!

Now all I have to do is install it in my boat, which will have its challenges, but I am sure I will prevail. The unit arrived yesterday.

Thanks again!


Chuck Tribolet posted 04-30-2008 09:12 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
btb: yes, BUT. Make sure ALL the wires follow basically the
same path. And a few loose twists wouldn't hurt. If you
route the minus/ground site very differently from the plus
side, you are making a big loop antenna (jimh: yes, I know
that's a gross oversimplification, but it's not EEspeak and
and get's the point across).


btb posted 05-01-2008 09:54 AM ET (US)     Profile for btb  Send Email to btb     
Chuck - I have a couple of options: 1) wire all the "grounds" together close to the Furuno (in my port console), and use the existing power ground wire which connects back to the main ground "bus" in the stbd console, or 2) take an extra dedicated "signal-ground") wire back to the chartplotter in the stbd console along with the other signal wires.

In option 1, I am using the ground/negative power wire, in option 2 I am still using that wire for power ground/negative but running the signal common wires back to the Garmin as well.

Of course, all these "signal ground"s and power grounds meet up at the garmin anyway as its circuits are single ended.

So is 1) preferable over 2) or vice versa, or doesn't it matter? (My ignorant non EE thought process says if all these power ground/signal ground wires are all joined together electrically at one stage or another anyway it doesn't matter, other than I know if there is more than one route to ground I can get a "hum loop" - at least thats what the stereo guys always say).

Regardless whether I choose option 1 or 2, the wires will follow essentially the same path.

Here's the setup forced on me by the boat (16 SL dual-console): Most electrics and the Garmin chartplotter are in the stbd helm console (Its a bit of a mess in there - mostly miles of bundled up original Boston Whaler and Yamaha wiring). A previous owner somehow ran a power line from the stbd console to the port console to run a tape player, the VHF, and the Garmin 240 fish finder (the 240, in his setup was "standalone" - i.e. not linked to anything else). There is also a pretty hefty - say 12 gauge - pair of "speaker wires" which appear to be run alongside the power red/black wires from console to console. I have thrown out the tape player, tidied up the port console wiring which was also a mess, and used the "speaker wire" to run fishfinder signals back to the garmin chartplotter - and this actually works.

When I install the Furuno to replace the sad 240, I need one more wire than I currently have to carry the nav signal from the chartplotter to the Furuno, so I have to find a way of getting another wire from console to console. What I was thinking of doing is running a length of cat 6 (ethernet) wire, mostly because if has plenty of wires and is relatively small diameter. The Challenge in all this is getting the wire routed - I have no idea how this was achieved for the existing power and "speaker" wires by the unknown previous owner.

So there you go - that's my set up, for better or for worse!

btb posted 05-01-2008 09:58 AM ET (US)     Profile for btb  Send Email to btb     
Wow - I just figured something out:

My existing setup has one redundant wire in it - I am currently using one of the "speaker wire" pair as a "signal ground" from the 240 fishfinder to the garmin - yet its redundant as everything is connected to the power-ground anyway.

So I can use THAT wire to take the nav signal to the Furuno - no new interconnecting wire needed.

Problem solved?

btb posted 05-02-2008 12:59 PM ET (US)     Profile for btb  Send Email to btb     
New power cables plus the Cat 6 cable was pulled through last night - will connect it all up over the next couple of days.
gorji posted 07-05-2008 08:19 AM ET (US)     Profile for gorji  Send Email to gorji     
I would like to ask a related question on this topic rather than create a new post:

I also have a Garmin 276c. I want to hook it up to my Raymarine 54 VHF radio (fixed mount).

I have the connection dongle from my Raymarine which can connect to the Garmin. The Raymarine side has a yellow (+) and green wire (-) for connection to the Garmin.

The Garmin end connects to the GPS' power inlet; other end is a bunch of colored wires: Ground, Alarm, data out 1, data out 2,data in 1, data 2 in, voice and power.

Can I connect the data outs from the garmin to the VHF radio without doing any harm? I will cap the other wires (Garmin). I have selected the garmin COM1 and COM2 to NMEA IN/OUT.

OR is the - wire on the Raymarine side for grounding and should be connected to the ground wire of the Garmin?

The instuction manual for both units are not helpful.

I appreciate your help.

Thank you

jimh posted 07-10-2008 11:49 PM ET (US)     Profile for jimh  Send Email to jimh     
The term "dongle" usually has a more specific meaning than you might suspect, and its use here is in conflict with that meaning. A dongle is usually a device which is connected to an I/O port of a computer as a hardware verification key of legal authority to execute some software. In terms of marine NMEA serial data communications, as far as I know there generally are no dongles needed.

Dongles are used in some cases when a proprietary and closed marine instrumentation system is connected to other devices. For example, when connecting to a Mercury SmartCraft network, a dongle is necessary. This is because Mercury does not allow interconnection of devices with their network unless the devices are licensed and certified. This provision is often accomplished by providing a dongle--a hardware device--in which some proprietary translation or authorization takes place.

To connect NMEA-0183 serial data devices together no dongle is needed. NMEA-0183 devices can be wired directly to each another. This is the basis for the information being sought here.

The inquiry was not clear on exactly what connection was desired, but let's assume that we want the chart plotter and GPS instrument to send information about the vessel's position to the radio so that the radio can (in an emergency) transmit the data.

The connection you propose is very simple. Let's identify the participants. The CHART PLOTTER is the TALKER. The RADIO is the LISTENER. The plotter will send NMEA sentences of serial data to the radio. The electrical details of the connection are very trivial:

--connect an output from the TALKER to an input of the LISTENER.

The TALKER is a single-ended output. There is only one signal wire. The signal is provided in reference to the ground. This is probably called "NMEA out" and is denoted with a certain color of wire. The ground is almost always the chassis, the negative power lead, or the battery negative bus.

The LISTENER is most often a differential input. This means it has two wires, and it is sensitive only to the difference in voltage between the two wires. Connect the non-inverting input wire (usually denoted as the "+" input) of the LISTENER to the output wire from the TALKER. Connect the inverting input (usually denoted as the "-" input) to ground. Here the plus and minus notations do not refer to a polarity, but refer to non-inverting and inverting inputs of a differential amplifier input. If the LISTENER is not a differential input, then you can forget about the non-inverting input and connecting it to ground. That is already done for you.

Your equipment manuals will make clear what the wire colors are for these three wires. For ground you can use the chassis, the negative power lead, or the common buss for the battery negative terminal.

Once you have made the electrical connections, you must configure the actual signals which will be sent and received. This is controlled by the software of the devices. This is also a trivial step. You simply configure the TALKER to send the NMEA sentence that the LISTENER wants to hear. In the case of a chart plotter talking to a radio, you most likely will want to send position information. There are several sentences available. The RMC sentence is the most commonly used one.

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