NMEA 0183

[Graham_Wright]

I am confused!

A "reliable source" stated the following whilst discussing the purchase of Raymarine ST60s to interface to one of their plotters.

"Nothing wrong with NASA instruments;- they output in NMEA0183. Just connect them all together (i.e. in parallel on the same bit of electric string) and plug them in to the plotter".

Now burdened with a lifetime in electronics, I have a problem understanding this. If all the "talkers" talk at once, there will be an inevitable corruption of data. My understanding is that a talker does not first listen to see if the line is "busy" before spouting but does so at regular but unsynchronised intervals. Sure, any listener may be able to find data whose sense is commensurate with a single source but Sod will always be lurking in the background to defeat this.

In my experience, any shared data highway has to be synchronised in some way or some priority accorded to the sequences of transmission. If a talker is programmed to wait a listening interval before transmitting and all the talkers are given different wait times, I could see that working but I believe this is not the case.

I cannot see how a multiplexer can be avoided with multiple sources of data.

The NMEA standard according to one interpreter requires a single talker connected to one or more (according to electrical loading) listeners.

Or have I got it all wrong?

 

[tome]

Graham

You are quite correct about NMEA0183. One talker can feed multiple listerners, but 2 or more talkers can only be connected via a multiplexer.

If using NASA instruments with NMEA output, you will need an NMEA bridge to connect them to a Raymarine plotter. Raymarine use a different bus (Seatalk) for their instruments and the NMEA bridge converts it to this standard.

The Seatalk bus is multi-drop and binary. You can connect many talkers and it takes care of collisions within its protocol.

The bridge is expensive (about £120) and an NMEA multiplexer about twice this price so you may want to take this into account when choosing which instruments you need. You'll need neither if you stick to ST60.

Cheers
Tom

 

[temptress]

HI,

not wanting to go into much detail but both you and your source are correct. NEMA is acutally a form of RS232 serial communication. They talk about out and in (ie transmit and recieve).

Technically speaking you can only have one NEMA out on a given bus BUT as the data transmitted is very small and the trasmission rates are (electronically) very short connecting a small mumber of NEMA out and NEMA in togeather will generally work. There will be some data loss, corruption, collision but generally wit say up to 4 instruments they will cope.

However there are 'multiplexers' that will allow the connection of a large number of instruments but these generally run into trouble as well. That's why B&G and Raymarine and many other brands use their own communications protocols (with Raymarine's SEATALK one of thebetter known, this came from Autohelm originally and this is now being replaced by a new technology).

Let us know what you are trying to connect to what and we can have a better discussion.

 

[Graham_Wright]

That's what I thought. Thanks for the confirmation.

To add a little;- I thought that, as Raymarine plotters can display sensor data (depth, compass etc.), I could dispense (if I wanted to) with the display heads themselves (and thus save the readies).

However, Raymarine tell me that the sensor signal processing is carried out in the display head which is thus necessary even if not wanted. The head then outputs the NMEA data and/or Seatalk.

 

[awyatybw]

You do not necessarily need an NMEA<->SeaTalk bridge as several of the Raymarine instruments have NMEA inputs and will typically repeat any thus-received data over the seatalk bus.

 

[tome]

Correct again. The speed, depth and wind sensors are processed by the heads themselves and then converted to Seatalk for output on the bus. If you need NMEA outputs (rather than Seatalk) you'll need the bridge.

 

[tome]

Sorry Alan, but this is a myth except for the autopilot which will put NMEA heading data onto the Seatalk bus.

 

[tome]

Connecting several talkers together is asking for trouble - you should be wary of giving advice like this.

 

[ParaHandy]

st60 multi display and st60 graphics do, indeed, receive nmea ok ....

 

[tome]

Yes, but they don't repeat them on the Seatalk bus

 

[awyatybw]

Well my (old) ST50 Navdata certainly does. It repeats just about everything it gets from the GPS via NMEA.

On a related point, the biggest problem I have had is with interface tolerances. NMEA 0183 is specified to use RS232 electrial characteristics. Many manufacturers choose to use an RS423 driver, presumabably because they can be supplied with just a 5V rail. The trouble is, although there is good overlap between RS232 and RS422, it is quite possible for a pair of such drivers not to talk to each other: the minimum ON output voltage for RS422 is 3.6V (max 6V) and the minimum ON sense voltage for RS232 is 3V. A bit of voltage drop in the wires and comminucation can be unreliable. I had to breadboard up a simple amplifier.

 

[temptress]

Carefull here. Typically the NEMA output (and input) on Raymarine instruments only repeat (or receive) a limited subset AND these vary between instrument heads and versions.

This is how Raymarine justify selling the Seatalk/Nema bridge.

this is a highly speciliased problem and it is very difficult to have a general discussion as the subject is vast and there is no simple set of rules.

No one has added USB to the mix yet.....

 

[tome]

Not sure about the old ST50. It will certainly display any NMEA data from GPS, and can be connected to the autopilot via Seatalk so I'd guess it must generate the autopilot control messages but I think that's it. I'm pretty sure it doesn't put all the NMEA received onto Seatalk.

NMEA actually specifies EIA-422 but you are correct that some use RS 232 levels. EIA-422 is specified at 2 - 6 Volts but I don't think every manufacturer complies. I can well believe that an amplifier would be needed for some applications, especially with longer cable runs.

 

[aluijten]

Actually NMEA does not 'specify' RS232 it just conforms to the serial comunication protocol used by both RS232 and RS422. The difference is that RS422 is current-loop based and RS323 is using a voltage threshold. For long wires generally RS422 is recommended because of signal loss. The sensing part of the inteface relies on a current flow instead of a voltage. The difference between RS232 and RS422 is in the electric specifications, not in the logical build up of the signal.
This is why connecting RS232 and RS422 sometimes work and sometimes it doesn't.
There are special converters to migrate from RS232 to RS422. That will make things more reliable.

Arno

 

[jfkal]

Graham,

some of the advise given is correct, most however is wrong. PM me with exactly what stuff you have and what you want it to do.I will let you now precisely what extra bits you need (if any) to make the whole lot work.

/forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

 

[bendyone]

Have a read of this months Sailing Today - it explains all about Nema in a simply way.

 

[tome]

[ QUOTE ]
Graham,

some of the advise given is correct, most however is wrong. PM me with exactly what stuff you have and what you want it to do.I will let you now precisely what extra bits you need (if any) to make the whole lot work.

/forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

[/ QUOTE ]

Good way of killing a thread, and I feel Graham has enough background experience to filter the replies and make up his own mind

 

[ParaHandy]

willing to bet!!!! how muchos ... let's think now, I know, your cook?

we're probably both right. it doesn't appear on either wind, depth or speed instruments (there is no page to display such data except autopilot) but does on a multi and graphics (I've got both) so it does repeat on the bus ....

 

[tome]

You're not going to get my cook that easily!

I stand corrected, this is what the Raymarine techie told me. Maybe they're just keeping sales going on their NMEA bridge?

 

[Graham_Wright]

The fear that Raymarine are trying to sell me something I don't need is very real.

As with most threads, the original point has been lost.

The suggestion made to me was that I could use cheap and cheerful (and very good) NASA transducers (and presumably display heads if that is where the signal processing is done as with Raymarine) and use the data thus gathered to display on a Raymarine plotter (don't think NASA have got that far yet!). This data could be just dumped willy nilly all on the same cable and the mish-mash of signals thus generated would be sorted out by the listener(s).

I find this hard to believe and, as stated, Sod will contrive to ensure that the most useful sources of data will always transmit simultaneously and the data gathering will be very much a hit and miss affair.

If I were designing such a data system, I would have to go for a multiplexer but then I was working for an aerospace company where cost was much less important than fidelity.

I think the answer has to be to stick to Raymarine displays (which after all have a reputation for excellence as does the support).

Again, drawing on my experience in avionics, if you mix and (try to) match different families, either by technology or manufacture, you open the can of worms labelled "it's not our machine, it must be their's".

Thanks for all the feedback;- once again - fascinating!
If it all comes from one source, you can avoid that.