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Old 13-02-2010, 08:54   #1
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NMEA Wiring

Hello you collective geniiiiiiiiiiiiiii.

Now for those familiar with NMEA 0183 this is going to sound like a particularly stupid question and I know myself that its very possible that this is going to be one of those 'can't see the wood for the trees' problems but I just don't get this whole NMEA business. I can't find anywhere (and I've looked, and looked, and then looked some more) a simple and clear explanation of how to wire one piece to another availing of this mysterious NMEA business. I've read about baud rates, listening, hearing, +, -, ports, senders, receivers and multiple ports but no simple connect this wire to that wire. I know it can't really be resolved that easily but I feel if someone could explaine the simple basics to me I might stand a fighting chance of figuring out what I'm supposed to be doing.

I sent the following message to raymarine on their site;

I have purchased an A-Series multi-function display, a raystar 125 GPS receiver (A-series is down below in a steel boat) and a Ray49E marine VHF radio.

The VHF is in place and working although not with DSC, the GPS receiver is fitted but not wired up and I'm looking at the A-series.

I expected, as all items are raymarine, that connecting them would be straight forward but having spent all day reading manuals it would seem anything but. I do not want to use seatalk as I have a log and depth-sounder I want to integrate using NMEA.

Can you provide a simple diagram or explanation of how these 3 devices should be connected? The RS125 has one NMEA out and one NMEA in, the A-Series has multiple in and out NMEA wires, the VHF has connections for the GPS and the chartplotter (pg 19 in manual) all seem to conflict and there is no explanation how they should work together?

Is it necessary to connect the VHF to the RS125 and the A-series or just to one or the other?

Which wires from the supplied wiring with the A-Series connect to the supplied wiring with the RS 125?

Does the RS125 need to be connected to a separate source of power? I thought it would take this from the A-series?

What is RTCM on the RS125? What do I do with it?

Raymarine responded with the following;

You cannot use SeaTalk as the A-Series does not support SeaTalk.

To get NMEA to work correctly you need to configure the NMEA connections as below. You will need to refer to our manual and the manual for your third parties product. Below is the NMEA standard, they way it is meant to be wired as our devices meet that specification. If your third party unit does not have the following wires, then that is a non standard NMEA connection and you will have to contact them on how to wire it to a standard NMEA setup.

Basic NMEA Connections

NMEA Talker NMEA Listener
AKA Output >>>>> AKA Input

+ve Output >>>>> +ve Input
-ve Output >>>>> -ve Input

In other words the Talker (Output) is the unit which holds the information you want and the Listener (Input) is the unit that you want to input the information to.

Also today's modern units have multiple NMEA ports on them. These are unique to each other so you are able to connect multiple devices without the use of a Multiplexor. Normally these are labeled Port 1, Port 2 etc and each port can have it's own input or output. To connect to displays with Multiple connections you choose just one Port for each device used.

So maybe its just me but thats about as clear as mud? Am I alone in my confusion? I just know there is someone out there that can explain this in a way a simple sailor can understand.
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Old 13-02-2010, 09:17   #2
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Think of any two devices as two humans talking. There is only one conversation taking place, alternating between mouth (output) and ear (input). this alternates to make conversation possible. + and - are voice and hearing modes, so are connected.
Hope this helps.
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Old 13-02-2010, 09:25   #3
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Hi Blue Stocking, so am I correct in saying that by following through on this logic that a '+' should connect to a '-' ? A listener to a talker?

If this is correct then how do I connect the GPS receiver to both the chartplotter and the VHF as the GPS receiver has only one NMEA + wire and one NMEA - wire?

Also the chartplotter has 8 different NMEA wires hanging out the back of it, how do I know which ones I should be connecting to? Don't want to blow the thing up!

Finally the VHF has a provision to connect to the GPS and also the chartplotter but no mention of whether I should connect to both or just one or the other?
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Old 13-02-2010, 09:52   #4
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Quote:
Originally Posted by ianhef View Post
Hi Blue Stocking, so am I correct in saying that by following through on this logic that a '+' should connect to a '-' ? A listener to a talker?

If this is correct then how do I connect the GPS receiver to both the chartplotter and the VHF as the GPS receiver has only one NMEA + wire and one NMEA - wire?

Also the chartplotter has 8 different NMEA wires hanging out the back of it, how do I know which ones I should be connecting to? Don't want to blow the thing up!

Finally the VHF has a provision to connect to the GPS and also the chartplotter but no mention of whether I should connect to both or just one or the other?
If this is correct then how do I connect the GPS receiver to both the chartplotter and the VHF as the GPS receiver has only one NMEA + wire and one NMEA - wire?
The + NMEA wire of the GPS need to be connected to both the + NMEA input wire of the VHF and the + NMEA input wire of the chartplotter. So you need to connect both + wires to the single + wire of the GPS. Do the same for the -wires.


Also the chartplotter has 8 different NMEA wires hanging out the back of it, how do I know which ones I should be connecting to? Don't want to blow the thing up!
The manual for the chart plotter will tell you the colour code for the NMEA in + and - wires.+ and - need to be the same channel.

Finally the VHF has a provision to connect to the GPS and also the chartplotter but no mention of whether I should connect to both or just one or the other?

The most important is to connect the GPS NMEA out to the VHF NMEA in. This is how you send position with a distress call.
Connection to the chartplotter is helpfull as it will show the position of another vessel in distress. Connect the VHF NMEA out wires to The NMEA in wire of the chartplotter, but this needs to be a different channel from the GPS connection.
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Old 13-02-2010, 10:08   #5
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You must connect a "talker" (output) to a "listener" (input) but the + connection on the "talker" must connect to the + connection on the "listener". Same thing for the - connections.

I'm not sure what it is you don't understand about Raymarine's instructions. Perhaps you need to hire a marine electrician or electronics engineer to help you. You can work with him and learn.
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Old 13-02-2010, 10:12   #6
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NMEA (actually NMEA-0182, as NMEA-2000 is totally different), uses what is called a differential connection. A NMEA sender puts out its signal on a pair of wires, with one usually called "+" and the other "-". Together, this +/- pair carries the data, and the + and - signals are mirror images of each other. This is done to reduce interference.

A NMEA sender can be connected to several NMEA listeners, just connect the sender + to the listener +, and the - to the -. You cannot connect a sender to a sender.

For connections shorter than a few feet it doesn't matter much where you make the connections to the multiple listeners. I usually use a terminal strip, but you can also just splice the wires together. There are ways to make reliable connections on a boat, and ways to create trouble for yourself, so if you aren't comfortable with this do ask for advice. If you are just trying to see if it's going to work, you can twist the wires and use electrical tape, but this is guaranteed to eventually fail so don't leave it like that.

The NMEA +/- pair is usually covered with a shield, and you will see different opinions as to the best method of connecting the shields. You can connect them together at the same location there you are connecting the sender and listener(s), and that will usually work well.

There are also baud rate (data speed) considerations. Until recently, all NMEA signals were sent at 4800 bits/second. Unless you have AIS equipment connected, it is very likely that everything will be at 4800 bits/second in your system, and your connections will work fine. If AIS is involved, you will find the AIS (and possibly other) data is being sent at 38400 bits/second, which cannot be directly connected to a listener configured for 4800 bits/second.

Your Raystar 125 GPS receiver will need to be connected to a power source. the NMEA data connections do not provide power. There should be power wires in the cable, along with the data wires.

SUMMARY: Connect NMEA+ to NMEA+, and NMEA- to NMEA-
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Old 13-02-2010, 10:25   #7
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Wiring diagram here you go http://www.actisense.com/Downloads/Documents/OPTO-2/OPTO-2%20Install%20wiring%20guide%20issue%201.10.pdf
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Old 13-02-2010, 11:26   #8
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Your Raystar 125 GPS receiver will need to be connected to a power source. the NMEA data connections do not provide power. There should be power wires in the cable, along with the data wires.

SUMMARY: Connect NMEA+ to NMEA+, and NMEA- to NMEA-[/QUOTE]

Thank you ALL very much, its amazing how difficult it is to get the simple information, this is as I thought but couldn't find it written this simply anywhere. This makes the reply from Raymarine easier to understand.

Just one thought, so if the NMEA + from the GPS receiver is connected to both the NMEA + from the VHF and the NMEA + from the chartplotter does this then mean by defalult you have connected the VHF and chatplotter NMEA + or is there some method of isolating them? Does this even matter?

Anyone want to hazard a guess on the RTCM? I'm pretty sure this is differential correction for the GPS but haven't found confirmation of this yet.

Deepfrz, the problem I was having with the Raymarine instructions is they don't explaine why the chartplotter or the VHF need to be 'talkers' at all, as far as I was concerned all they needed was to be told the position, I couldn't see why or where this two way street was needed.
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Old 13-02-2010, 12:30   #9
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The A series has 2 bi-directional NMEA 0183 interfaces, its receives and transmits the following sentences]

NMEA 0183 Input messages
APB, BWC, BWR, DBT, DPT, GLL, MTW, RMB, RMC, RTE, VHW, VLW, VTG,
ZDA
NMEA 08183 Output messages
APB, BWC, BWR, DBT, DPT, GLL, MTW, RMB, RMC, RTE, VHW, VLW, VTG,

ZDA


This allows info to be set to autopilots etc. In your situation you could use both ports one for the GPS and one for the VHF, or infact just use one input port on the A series conncted to an output port on the GPS and and input port on the VHF. Hnce the GPS is the talker and there are two common listeners ( the A series and the VHF).
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Old 13-02-2010, 12:37   #10
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RTCM is a signal format, there is also Raw and SCM (I think) .
RTCM stands for Radio Technical Commission for Maritime Services.
Different signal formats need different baud rates.
DGPS normally uses RTCM format.
From what I have struggled to learn over the years, the bigger the data package being sent, the higher the baud rate needed. Some formats can compress the data package which will allow a lower baud rate to be used. SCM ( I think) stood for Super Compressed ...........?
Have a look at
Fugro Omnistar
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Old 13-02-2010, 12:44   #11
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With the original posters limited knowledge of the NMEA, I am pretty sure that after a few more posts he will be much more confused than he originally was
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Old 13-02-2010, 13:39   #12
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I agree with that! RTCM is not a signal format. It is the Radio Technical Commission for Maritime Services which set standards for communication and navigation equipment. RTCM SC-104 is the standard for the transmission of ground based differential GPS signals. The term RTCM alone without reference to a special committee or technical standard is meaningless. I assume you don't have any equipment on board that receives the land based GPS differential corrections that would have the SC-104 output. If you don't use the SC-104 input to the RS 125, you are supposed to ground that wire to help prevent interference.

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Old 13-02-2010, 14:13   #13
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The Raystar 125 actually uses WAAS (Wide Area Augmentation System) instead of DGPS, which is a satellite based correction system for GPS . The WAAS beacons are primarily at airports to support eventual ALS applications, but the correction is broadcast by satellites and received and applied by the Raystar 125 in real time to reduce the error to 3m or so. Some Raymarine gear actually shows this as SDGPS.
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Old 13-02-2010, 14:31   #14
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I see sded corrected his post, initially stating ground based differential gps corrections via beacons (SC-104) were no longer in operation. They certainly are and the RS 125 can use them if connected but yes, the 125 already uses the satellite based WAAS corrections.

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Old 13-02-2010, 21:17   #15
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With the original posters limited knowledge of the NMEA, I am pretty sure that after a few more posts he will be much more confused than he originally was
Be fair, it was the OP that brought up the RTCM issue, and hes got every right to mention it. The other posters actually all gave correct information
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