NMEA

[LadyInBed]

Is the NMEA a bi-directional signal on a pair of wires with/without a handshake or does it Tx (out) on one wire and Rx (in) on another wire with a common return.

As I want to hook up the radar to the GPS, and when I looked in the GPS handbook, the wiring diagram just showed a NMEA (in) wire with a common return and no NMEA (out) wire.

 

[HaraldS]

NMEA is neither, but closer to your second guess. In and Out is entirely separate, both have differential on two wires.
Often they are labeled NMEA IN+, NMEA IN-, etc.
Some devices have just NMEA out, (like a GPS), some only NMEA in, (some Radars).

NMEA out + is connected to a NMEA in +, and NMEA out - to a NMEA in -.

NMEA out is called a NMEA sender or talker, NMEA in is called a listener.

You can connect multiple listeners to one NMEA talker.

You cannot directly connect multiple talkers. To join data from multiple NMEA senders, you need a device that is called a NMEA bridge or NMEA multiplexer.

This makes combining multiple information providers more difficult than say with a bus architecture where multiple senders can bid for the wire (ie. SeaTalk).

Usually neither NMEA in/out + nor NMEA in/out - is connected with common or ground. You can connect the NMEA in - and the NMEA out - of the device that has both and get what you described as a common return, you will loose some noise immunity, but it will very likely work unless you have very long cables.

You can connect NMEA to a PC, by conecting NMEA out - with the ground of your RS232 (seriell) port and NMEA out + with data in. Set the interface to 4800 Baud, 8-bit no parity. You can see the NMEA sentences with a terminal program, they are all clear text.

Just ask you you have any questions.

 

[Guest]

The NEMA signal is normally standard RS232 levels (+/- 12v) commonally at 4800 baud though this can often be changed in the system setup. From a GPS receiver only a signal out would be expected as there is no need for any input, the unit will put out whatever 'sentences' it is programmed to give out.

Each sentence has an identifying header showing what it is, where it comes from and is followed by the data in a format that is fixed for each type of sentence. Any receiving device must look for the type of sentences that your GPS is putting out otherwise you may as well not connect the units at all!

Best of luck

John

 

[Chris_Stannard]

I went through this in my last boat with a Philips GPS and a Navico Radar. The conclusion I came to was it is best to have all of the equipment from one manufacturer. Although both were NMEA, the Philips was a different (earlier )version which did not have the same data structure. I had the Philips upgraded but then found the two manufacturers did not use the same data format although both were the same version of NMEA.
If you have a Ray Marine System, as I do now, the GPS will both output data, from anywhere in the system, to NMEA devices, and will accept an NMEA input.

Chris Stannard

 

[philip_stevens]

My AP-Philips GPS has 4 input and 2 output NMEA connections. I had the software updated my Comar to the latest version - before they brought out a new model.

The outputs are to the Autohelm, and Navman wind instrument. The inputs are from dGPS (yes, I still have it), Navman speed, depth and wind. I don't appear to have any conflicts with any of them.

The GPS can steer the boat, though I only use that facilty when motoring. Although I have not yet tried it, I think the Navman wind can steer the boat via the Autohelm.

regards,
Philip

 

[LadyInBed]

Thanks all - Got it! nm

 

[Trevor_swfyc]

Re: NMEA Cable

Harold,

I have a question from your posting. A club member asked me to wire up his GPS to his autohelm 1000 as I have mine connected via a laptop. He unfortunatly had already put in a cable which was not screened computer cable, I was not sure if this was necessary so using his cable I connected the GPS talker to the correct pins on the autohelm, but it did not work. Was this because the cable was not screened? The GPS was set up to send the correct signal to the autohelm.
Thanks for any comments you have.
Trevor

 

[jtwebb]

Re: NMEA Cable

It is not essential to screen except for long runs past electrically noisy places. A good source of cables to connect into a lap top is a dead serial mouse! Make sure the pins are correctly wired up, I can not remember off hand but there are plenty of sites which will give the wiring for a RS232 9 pin Canon D which is what you are after.

J Webb

 

[HaraldS]

Re: NMEA Cable

Trevor,

most likely the cable is not the problem. Unless it is very long. Also the shileding would prevent interference and if missing it might work unrelaible, but not like you say, not at all.

I'm sure you have tried bost polarities. You can reverse it without breaking things.

Problem is more likely that the two are not sending and looking for the same semtences. It could be NMEA standard level, or the choice of sentences.
What GPS and what version of the Autohelm ST1000 are you talking about?

Need to catch a flight, but will look into this again.

Cheers,
Harald

 

[Trevor_swfyc]

Re: NMEA Cable

Harald,

Thanks for your reply I have just got back from Burnham buying gas pipe.
The GPS is a Furuno GP-30 and the autohelm a 1 yr old ST1000+.
As I said mine works perfectly through my laptop using the same GPS and the same autohelm which I wired up, the run to the autohelm that I used was computer cable. The GPS-30 to autohelm only involves two wires but on somebody elses boat and his wires things can be more difficult. Sea/River trials, when you press the two buttons -10 and +10 to confirm the autohelm is getting the signal from the GPS no confirmation.
So if its not the cable maybe I have the wrong polarity, maybe a break in the wire is possibly, I will have another look.
Thanks again.
Trevor

 

[Shanty]

Re: NMEA Cable

Hi Trevor,

Just been looking at the handbook for my Furuno GP 31 - a couple of points that may be relevent:

the GP31 will handle two versions of NMEA - 1.5 and 2.0

the GP31 will output three different collections of NMEA sentences. Only one of these collections (NMEA-AP) contains the APB autopilot sentence.

Don't know if the GP30 is the same - might be worth some experimenting. On the GP31, you change these settings via the I/O Setup menu.

Hope this helps. If you want any more words of wisdom from the GP31 manual, let me know

Regards
Tim

 

[HaraldS]

Re: NMEA Cable

Trevor,
yes, more likely a break or reversed polarity. Check the signal with a multi-meter at the GPS and the gain at the end of the cable. NMEA is run on a serial transmission standard called RS 445, for which the two poles change polarity for mark and space. Mark, is the idle state and has plus at NMEA out +, during space (logical zero) it reverses. Even when sending it is idle most of the time, so that a multimter set to DC-voltage should give you a positive voltage reading, with plus on NMEA out+. Not the full 12V, but something between zero and 12V. You should get the same at the far end of the cable.
Also make sure that one of the wires isn't shorted to ground or so. Since both, NEMA out and in are galvanically isolated it shouldn't matter, but it could make quality worse.
Best cable for NMEA would be the same as is used for 10/100-base-T ethernet, which is unsgielded twisted pair. The two wires are twisted which gives it great comon mode noise rejection and it doesn't need a shield. No shield means less capacitance and less attenuation for higher frequencies. Though having a shield will not affect NMEA at it's rather slow transmission speed.
Still thing that for up to 20 feet any wire should work for NMEA, maybe with an occasional transmission error.
So best seems you check the connection again, and if it still doesn't work we can dig deeper. My internet acces is spotty right now as I'm travelling in Canada, so an answer may take a moment.
Cheers,
Harald