NMEA 0183 Talkers and Listeners.

[DJE]

As originally wired the boat had a GPS feeding position information into a Navico Corus instrument network. The Corus system was also feeding information to the radar. The GPS has died but the radar still displays depth information from the Corus system. I jury rigged a connection the other day from the Corus "NMEA out" terminals to a chart plotter and the chart plotter happily displayed depth, boat speed, wind speed, and wind direction. So can I connect the chartplotter's "NMEA out" to the Corus "NMEA in" to replace the feed from the dead GPS set? The chartplotter will then be talking GPS data and listening for speed, depth and wind data (albeit on two seperate cables as it has two data sockets).

In the original set up the Corus system was listening for position data from the GPS and talking position and depth data to the radar. So what exactly does this "One talker, many listeners" criterion really mean?

 

[pvb]

So can I connect the chartplotter's "NMEA out" to the Corus "NMEA in" to replace the feed from the dead GPS set? The chartplotter will then be talking GPS data and listening for speed, depth and wind data (albeit on two seperate cables as it has two data sockets).

Can't see why that shouldn't normally work. If you have more than one talker sending NMEA data to a listener, the messages will clash and data will be lost; that's when multiplexers come into play.

 

[RichardS]

I believe that the "one talker" thing is the ideal situation and guarantees no interference / confusion amongst the listeners. However, you can have more than one talker if they are not interfering with each other by transmitting the same sentences. If your set-up is not overlapping in this way, and works in practice, then go with it.

I think ....

Richard

 

[dom]

Also, I find these "Compact Series Wago" connectors a great way to join and un-join (when I get it wrong!) the tweeny little NMEA wires.

https://www.tlc-direct.co.uk/Main_Index/Boxes_and_Enclosures_Index/Wago/index.html#Wago_Lever_C

 

[bitbaltic]

Different manufacturers give different numbers, but in general you can connect the NMEA inputs of up to four (some say six) devices (the 'listners') to the output of any single NMEA device (a talker). That's what it means.

It does not work the other way around (without blind luck), ie you should not connect multiple talkers to a single listner, because the talking devices will get into party mode and all talk over the top of each other, so the listner won't understand what's going on. This problem is what multiplexers are there to solve.

There's absolutely no reason at all why you can't connect the output of the plotter to the input of the corus, in order to feed it with position data as your GPS used to do. Then you can connect the output of the corus to both the plotter and radar. This way the radar will get position from the plotter via the corus and both plotter and radar will get instrument data.

The closed loop between the plotter and the Corus will mean that the position data sent from Plotter>Corus will be echoed back to the plotter by the Corus system, but it is very unlikely that this will cause any problems.

Depending on what the plotter will transmit, you could also connect the plotter outputs to both the instruments and and the radar, and the instrument outputs to just the plotter. This way the plotter would pass instrument data to the radar (as opposed to the instruments passing position) which has more redundancy in that your radar will still get position data if your corus instruments went down, which might have a safety benefit, but I don't know what kit you have so wont speculate further.

Edited to add: do check baud rates, NMEA versions etc, to ensure compatibility, especially if all this kit is of varying vintages.

 

[prv]

you can have more than one talker if they are not interfering with each other by transmitting the same sentences

That's nonsense, I'm afraid.

Think of it like a telephone line, with extensions in different rooms of the house. If I phone my friend, he can hear me fine. If his girlfriend picks up the extension in another room, she can hear me too, all is well. But if my brother picks up another phone in my house, doesn't listen to what's already going on, and just starts barking out words, my friend and his girlfriend will not be able to properly hear either of us. If my brother and I are only talking intermittently, they may catch bits of it where we happen not to talk over each other, but you can't rely on that and it would be an error to set a system up this way.

NMEA 0183 talkers are like my non-listening brother, they expect to have the line to themselves and they do not check to see whether anyone else is already talking. What sentences they may or may not be sending is irrelevant if you can't even make out the words.

The OP's suggestion should work fine, though, because he's not suggesting multiple talkers on one line.

Pete

 

[DJE]

Thanks all. I was thinking of running some RJ45 cable between the two locations so that I have 4 cores for now and 4 spares. Will that be up to the job or is there something better?
I like the look of the little connectors. Getting my head into the locker to see the Corus terminals and making the connections was a major PITA.

 

[prv]

Thanks all. I was thinking of running some RJ45 cable between the two locations so that I have 4 cores for now and 4 spares. Will that be up to the job or is there something better?

Yes and yes

I've used cat5 cable before (RJ45 is the plugs) and it worked, but on Ariam I used a thicker tinned stranded twisted-pair data cable that complies with the NMEA spec. Afraid I can't remember offhand what that is, but David will probably be along soon to elaborate

Pete

 

[elton]

I believe that the "one talker" thing is the ideal situation and guarantees no interference / confusion amongst the listeners. However, you can have more than one talker if they are not interfering with each other by transmitting the same sentences. If your set-up is not overlapping in this way, and works in practice, then go with it.

I think ....

Richard

No. You definitely can not have multiple NMEA 0183 talkers connected to the same input. They try to pull each other up and down, and the result is garbage. Even if the data is exactly the same, they won't be in sync.

 

[RichardS]

No. You definitely can not have multiple NMEA 0183 talkers connected to the same input. They try to pull each other up and down, and the result is garbage. Even if the data is exactly the same, they won't be in sync.

Except that I wasn't referring to one input. I was referring to the OP's set-up where he said he had two NMEA ports?

Richard

 

[prv]

Except that I wasn't referring to one input. I was referring to the OP's set-up where he said he had two NMEA ports?

Then why bring up "one talker" at all? If you don't mean one talker per channel, what do you mean? Per boat? Per person? Per county?

Pete

 

[elton]

Except that I wasn't referring to one input. I was referring to the OP's set-up where he said he had two NMEA ports?

Richard

I see

 

[ex-Gladys]

The same talker can talk on two ports if it has them, but I can't even have the power/data plug plugged into my standby Garmin 152. The 751 works but no data goes anywhere.

 

[RichardS]

Then why bring up "one talker" at all? If you don't mean one talker per channel, what do you mean? Per boat? Per person? Per county?

Pete

Sarcasm is not only unattractive but unimpressive I'm afraid.

My meaning was that you can have more than one talker in one interconnected system, so that would usually equate to one boat unless the boat has two completely separate systems.

Richard

 

[nigelmercier]

My meaning was that you can have more than one talker in one interconnected system...

I suggest you put your shovel down.

 

[mattnj]

I was thinking of running some RJ45 cable between the two locations so that I have 4 cores for now and 4 spares. Will that be up to the job or is there something better?

Good idea...but be aware there is at least 2 types....the stuff you get in boxes for installed CAT5 is single core, so it can be punched down (google idc tool if you need to know) not designed to be moved around and it breaks if you do....the stuff used in Patch cables (between PCs and routers for example) is multi strand cable to be moved around...make sure you are using the right stuff....also probably a good idea to use the outdoor rated, uv stable and generally more robust stuff.

 

[RichardS]

I suggest you put your shovel down.

Good idea .... except that I'm not digging in the first place. I'm just stating my understanding, which might be wrong, of course, as I suggested in my original post!

Richard