NMEA - can someone explain some basics to an electronics duffer!

[skyflyer]

Previous owner of my boat has some rather ancient instrumentation (c1996) which has been installed piecemeal and doesn't all interface with the others. I can't afford to replace it all this season - although I am installing DSC radio as my first replacement - but I do want to try and rationalise the miles of (what i think is duplicated) cabling and tidy everything up.

From what I have read on t'internet an NMEA device can talk to several others OR listen to just one other, but ...

The Chartplotter (newest of all the instruments but a discontinued product nevertheless!) has two (+ve and -ve) input wires and two output wires. Does this mean that it will "re-transmit" it's input to the output line, i.e. it can listen AND talk?

Basically the GPS aerial feeds the plotter which will be needed to feed the DSC (brand new). but the plotter would also like to receive depth, heading, and log speed info, so it looks like it wants to be a listener and a talker!

If that is the case does it follow that any other instruments (depth, log, fluxgate heading) that have an In and an OUT pair of wires, can be daisy chained to allow the info to passthrough. Or do I have to decide which talk and which listen?

I see you can buy a multiplexer, but at over £100 that seems a bit silly if I am starting on a steady program of replacing the instruments progressively.

Oh yes, theres an old autopilot that wants to listen to info too!

Thanks for any guidance!

 

[pvb]

Perhaps you should start by revealing the make/model of the various components of the system?

 

[prv]

From what I have read on t'internet an NMEA device can talk to several others OR listen to just one other

This is nearly right, but slightly too simplistic in the modern world. I would replace the word "device" with "channel". Each channel must have exactly one talker, and one or more listeners. Max number of listeners somewhat variable depending on the particular devices, distances, and quality of wiring, but often considered around three or four. With a single talker, it naturally follows that an NMEA channel is a one-way thing. Also worth noting that an NMEA channel can be either low speed (the usual case) or high speed (used mostly for AIS data because there's so much of it). These speeds are also known as baud rates, named after a French telegraph engineer, because it's just a series of pulses on a wire.

The other NMEA fundamental is the "sentence", a particular bundle of maybe half a dozen pieces of data. So for example the RMC sentence contains a lat and long, the time at which that lat/long fix was taken, the speed over ground, the course over ground, the date, and the magnetic variation. The sentences don't seem to be well-planned and several of them overlap to varying degrees, so things like position can be obtained from one of several different sentences. There's a fairly complete list of them here: http://www.catb.org/gpsd/NMEA.html

So, how does a modern plotter or VHF fit into this? Well, it probably has several channels, some incoming and some outgoing. On a plotter they're generally fairly general-purpose, but on a VHF they're more likely to be dedicated to particular data. So there might be an input for position for DSC (which is looking for GLL, RMC, or GGA sentences, all of which have position in, and ignoring any others), an output for DSC alert messages (so a little sinking-ship icon can be drawn on the plotter), and maybe an output for AIS data operating at high speed.

So, as to your specific question of whether a device with both input channels and output channels will copy messages from one to the other - the answer is, it depends. Some do, some don't. There's absolutely nothing in the standard saying that they should, and I would advise making no assumptions on the matter. They will only copy data across if the designer thought that was a useful thing to do, so for example several Digital Yacht AIS receivers have a low-speed input which they don't even need for their own purposes, but they take data from it and combine it with their own AIS data on their high-speed output channel, so that someone with a cheap plotter with only one input channel can have both GPS and AIS data - a considerate little bit of design. Whereas I would not generally expect a plotter to copy any data at all, because they tend to envisage themselves at the centre of the network with everything else existing only to serve them, and why should the minions in the VHF Department be receiving memos from the Directorate of Steering?

A lot of people seem to approach NMEA 0183 wiring with a vague handwavey desire for "all my stuff to talk to each other". Unfortunately it doesn't really work like that (but it's a good description of the original Seatalk) - you need to be quite clear about what information you need where, and ensure that you have all the required channels in place. It's point-to-point wiring, not a general-purpose network.

Pete

 

[skyflyer]

Pete

Thats really useful thanks!

pvb - i didn't list them because they are all so old! But if you want to know:-
Stowe Navigator2, Log
Ctrek depth
Ctrek windspeed and direction
Ctrek compass
Simrad wheelpilot
Standard Horizon DX2100 DSC VHF
Standard Horizon CP175 chartplotter
JRC 3000 Radar

Std Hrzn will link together no problem using fast and slow (38000 and 4800 baud I think) dedicated connections. That will get the GPS signal to the VHF and the AIS info to the plotter (which actually isn't able to make very much use of it).

So i guess the 38000 AIS channel is dedicated so the VHF is not seen as a 'talker' as far as the rest of the kit is concerned.

The wheel pilot is going to want a heading input from the compass

The radar has an NMEA connection and would like course and speed, I guess, to do CPA calculations

But as Pete says - its not necessary that everything talk to everything!

 

[lpdsn]

Don't worry, NMEA is an ancient protocol so it suits ancient instrumentation.

There are also two types, with proprietary variants (i.e. some manufacturers bend the rules). NMEA 0183 is the older standard. NMEA 2000 is marginally more recent. NMEA 0183 messages are called sentences and are often known by a three or four letter acronym.

NMEA 0183 allows one sender to talk to up to three receivers. Irritatingly, you can have two boxes talking to one input without fitting a multiplexer box.

There can be up to five wires, but often only two are necessary. No hard and fast rules about what various instruments understand, relay or transmit. They may well transmit completely different information to what they receive. You need to look at the manuals.


There aren't hard and fast rules on the wire colours either although there are common themes - again you need to look at the manuals. You may find older manuals on-line.

In theory NMEA 0183 should be shielded, but at 4.8K baud it would need a really noisy environment or a long run to cause problems.

There also a 38Kb NMEA 0183 standard used mainly by AIS instruments. Shielding is more important there.

In each case when connecting instrument A to instrument B you need to make sure they have the same baud rate and that they understand the same sentences.

I've found the best way to neatly interconnect NMEA0183 is to fit blocks of ring terminals and to put a 4mm ring terminal on each wire that needs to be connect to another. You can then document it and get away from the spaghetti approach.

NMEA 2000 differs in that each instrument is connected to a common bus. The messages are completely different from 0183 and there's more variation between manufacturers, although the core messages should be the same. The physical connectors also vary between manufacturers, although it is still possible to interconnect with adaptor cables - usually. Generally with the bus system, all instruments listen to all messages and pick out the ones of interest. Anything transmitted is available to all instruments on the bus.

 

[nigelmercier]

... I've found the best way to neatly interconnect NMEA0183 is to fit blocks of ring terminals and to put a 4mm ring terminal on each wire that needs to be connect to another. ...

See this thread:

I've finally got round to replacing the rats nest of wires connecting my various bits of NMEA kit...

 

[lpdsn]

See this thread:

Not so far

 

[nigelmercier]

Not so far

Click the blue "fast forward" arrow to the right of Originally Posted by nigelmercier

 

[lpdsn]

Click the blue "fast forward" arrow to the right of Originally Posted by nigelmercier

We're agreeing then. Always handy getting things back together if you don't have to bung all the wires in and hope the lid will close.

 

[pvb]

Std Hrzn will link together no problem using fast and slow (38000 and 4800 baud I think) dedicated connections. That will get the GPS signal to the VHF and the AIS info to the plotter (which actually isn't able to make very much use of it).

I can't find an up-to-date manual online for the CP175, so won't comment.

 

[ianj99]

I see you can buy a multiplexer, but at over £100 that seems a bit silly if I am starting on a steady program of replacing the instruments progressively.

Oh yes, theres an old autopilot that wants to listen to info too!

Thanks for any guidance!

There is a lot to be said for multiplexers since you can collect nmea data from several sources and send it to several others and/or back to the same ones. This is especially true if you have a mix of old and new - there is often no need to bin older gear which otherwise works just because its nmea functionality is more basic.

I use a Shipmodul USB version which has 4 inputs, one of which can read Seatalk data eg heading from a Raymarine autopilot, and another can read AIS data at 38400 baud, whilst the others are for an old Seafarer speed/log/depth meter and Tacktick wind + gps data.
There is also a wifi version which can send the data to any wifi device eg smart phone/laptop etc. However I am working on a low cost nmea to wifi adaptor which will cost under £40 to build if it works! Watch this space.

Note that just because a device has both inputs and outputs, does not always mean it will combine (multiplex) data on its input with its own data and then transmit it. For example, a Tacktick wifi nmea interface has 2 inputs, one of which is intended for gps data so it can display true wind speed and direction. Its passes this data + the wind data through to its output but ignores some data present on either inputs eg depth. Tacktick's support were hopeless - I never did get a straight answer, very like raymarine's when I asked whther an SPX30 AP will work without their fluxgate compass (using hdg data via Seatalkng).