NMEA multiplexer

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AlexL

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Does anybody know of any websites, or books with a circuit diagram for an NMEA multiplexer. I have made myself an RS422 splitter which will take 1 NMEA data stream and retransmit it on 4 separate isolated channels. What I now need is a multplexer which will take multiple (say 4) NMEA data streams, buffer them and reransmit as a single stream. A quick trawl of the web has found some for sale but they are all a couple of hundred quid.

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You will need more than a circuit diagram to do this. You will need a microprocessor and some software as well. Even if you did find a website freely publishing the circuit diagram and the software, you would need some means of programming your microprocessor or EPROM, which usually means buying something.

The software needs to listen on each input port, and buffer the incoming data ready to send it on when time is available on the output port. If there is too much data coming in to send it all on, then some means of prioritizing needs to be implemented.

This sort of software is easy to develop on a large processor with plenty of resources (ie a PC) but doing this on a cheap restricted processor, like a PIC or an 8051 derivative that would typically be used in this sort of application, is nowhere near as easy.

Do you need a new display as well? Cetrek make a multi-display (ie it can display just about any incoming NMEA data) which incorporates a 3 channel multiplexer. It is not much more than a dedicated multiplexer.

<hr width=100% size=1>One day, I hope to own a real boat.
 
I thought you could just connect them all together and it would sort itself out, like all the nmea stuff from the GPS, to the VHF, radar, computer for chart plotter, or do they all have to be seperate? Oh! Shit!

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You can connect multiple 'listeners' to 1 talker. ie. you can connect the OUTPUT from your GPS to many INPUTS i.e DSC radio, GPS repeater, autopilot etc. But you cannot connect more than 1 Talker together - i.e if you have NMEA data coming out of a GPS and also coming out of a Wind instrument you need a multiplexer to turn these into a single data stream.

I have made a splitter to connect multiple listeners as I was having some trouble with a repeater which had a single ended input, being conected to the same line as a DSC VHF which required a differential RS422 input - but this isn't usually necessary.

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I kind of figured I'd have to use a PIC or similar - I just wondered if anybody had done it already (being lazy!)

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Thank God! I had visions of melted EXPENSIVE instruments! That is all I do, GPS, to other things. Phew!

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I can only offer you a manually switched NMEA selector based on TTL logic components. Since i have only one continuous talker -the GPS- i can get away with this. It provides 5 buffered outputs.
When i need the pc or Yeoman to talk to the lot i simply disable the GPS output and enable the pc or plotter output.
Obviously, if you have more than one continuous talkers this methode is of no use to you.

PM me should you want the schematic.

Happy sailing, Steffen

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Tome's your man for this - I'll wait for him to chip in, some relly good stuff coming soon.

Alternatively if you are in a big hurry, you could start <A target="_blank" HREF=http://www.contechnav.com/>here</A>

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Brookhouse (in NZ) are the cheapest I found on the WEB - still haven't got round to buying one though - a simple DPDT switch works for me at the moment. If you're going to buy from them over the web you'll need to set up a PayPal account first.

Jeff.

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Hi AlexL,
I'm very interested to know how you constructed your nmea amplifier - have you got a copy of how to do it (or is there a website out there with it on?). Where did you get your components and how much did it cost to make?


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I bought a Brookhouse from NZ, if you choose to do so - beware the hefty carriage charges and import duty and VAT. Ended up being very expensive and also illegal as it isn't CE approved and in theory could have been seized by customs

I bought the version with 3 input channels of NMEA & one input of Seatalk, planning to muxing the whole to one NMEA output for use with

Never got the Seatalk input is to work properly - it missed some Seatalk sentences, though the NMEA mux worked ok - disadvantages of buying from NZ was that to get it fixed would have involved shipping it back at great expense.

To their credit they did refund me the extra charge for the Seatalk input option.


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AlexL

As posted below, you will need a microprocessor to do this. The NMEA sentences have to be received in full before they are put into the output stream, otherwise you'll get garbage. This means that the multiplexer has to recognise NMEA sentences and then queue them in the output.

The best 'off the shelf' devices I've come across so far are from <A target="_blank" HREF=http://www.nolandengineering.com/> Noland engineering </A>. They have 2 models available depending on whether or not you need USB.

Raymarine don't publish their SeaTalk spec, but there's a German engineer who has done a lot of work on the format, see <A target="_blank" HREF=http://www.thomasknauf.de/seatalk.htm> Knauf</A>. The problem is that he is unlikely to keep up with changes in the spec, possibly the safest bet is to buy one of Raymarines NMEA bridges.

There's a bit more info <A target="_blank" HREF=http://home.btconnect.com/Amaya/Amaya_Web1.htm> Amaya </A> where I've put together some integration notes.

You may well come to the conclusion that a couple of hundred quid isn't too expensive!

regards
Tom

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Its partly these worries which have kept my hand off the "Buy" button. I'd far rather spend the money in the UK if possible - then I'd always have a fighting chance of sorting things out if it didn't go 100% as described.

The down side (as this thread started out) is how expensive these little gadgets are. In all honesty there cannot be more than a fivers worth of bits in them. Yes, I'm aware that the software side isn't simple - it would be like programming 20 years ago - ah yes, I used to do that for fun.

The real problem is that any product developed would have to recoupe its costs and make a living for the developer over a fairly small prodcution run - I guess that is why they are expensive.

Is anyone game for doing this as a none profit exersize?

Jeff.

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Sadly there's a lot more than just a fivers worth of bits these days.

I've written the code for a 4 way NMEA multiplexer which includes filters to get rid of unwanted sentences (you'd be amazed at how much repeated data and garbage there is). This is part of a much more ambitious project, and once the design is completed and fully tested it will cost in the region of £30k to qualify it to CE standards. These are just some of the hidden costs which have to be recovered in the price. The distributor mark-up is likely to be near 100%.

I wouldn't deliberately do it as a non-profit excercise, but it may well end up that way. At least I will benefit as a user!





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Is that why I get false readings from my plotter if I accidently have two GPS connected to my system, all works well with one or the other GPS is on but with both i get a position error.
Some years ago I was sold or persuaded to by an Interface which I newver could make much sence of, but this appears to take NMEA and turns it into Sea talk. but my system seems to work well without it?
Mike

<hr width=100% size=1>www.sailingforfun.co.uk
 
I was thinking of doing it as a non-profit exercise - however I'm so used to programming in VB these days I'm not sure If I can turn my brain back 20 years! If I get any success I will let you know

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I've yet to test it in anger, but its faily simple. Maplin Electronics sell a chip call the SN75176B which is can either recieve or transmit RS422 they are 99p each. You also need an optoisolator to isolate the input and outputs and a 5v power supply - I guess the whole lot is less than a fiver plus a box to put it in - which is probably the most expensive bit! When i've checked it out i'll send you the circuit diagram and details.

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"In all honesty there cannot be more than a fivers worth of bits in them. "

... I once (partly in jest) complained to an Intel employee how much they charged for what started out as a few grammes of seashore.

He said... "Well, I'll tell you what -- I'll *give* you a bucket full of sand, and lets see how you get on..."

... not my finest hour!

Alan

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