Nmea 0183 wiring for 2 gps "sources"

[alisdair4]

After a fortnight of short-handed and very "interesting" sailing in the Baltic, I have convinced SWMBO that a chartplotter will be an excellent idea. Currently, MD has a fairly old-fashioned set-up, with the instruments, radar, DSC VHF and boat laptop being provided with GPS info from a Garmin 152 at the chart table. This all works fine, and I would like to keep it in place as a backup system when I install a chartplotter. (I am considering the Lowrance HDS 7, but without the fishfinder. Earlier discussion on this forum caused me to discard the Raymarine A50, as it had a smaller screen and there was no advantage in using Seatalk to Seatalk NG).

To do this, it would seem that I need a NMEA "Switch" (if there is such a thing) to allow me to toggle the GPS data from either the Garmin or the Lowrance GPS sources. The sort of thing I am considering is shown in this diagram:

http://www.facebook.com/photo.php?f...45771255.45807.176203555771564&type=1&theater.

The exam questions are:
Am I making things unnecessarily complicated;
Given the number of NMEA "Outs", do I need a NMEA multiplexer (and if so, what would forumites recommend).

For background, there doesn't appear to be a multiplexer currently, and with the exception of an issue transferring waypoints between the laptop and the Garmin 152, works well.

Thanks in anticipation.

 

[rob2]

If I've understood you correctly, there will be no increase in the number of listeners, just a second optional GPS input (talker). Surely just a changeover switch will do the trick (carefully labelled, of course)?

Rob.

 

[elton]

To toggle between two NMEA sources you just need a simple SPDT switch (changeover switch) to select between the Data Out of either source, as Rob2 says. The ground connection from both sources will be common (connected together, and with the destination device), and won't require switching.

 

[alisdair4]

Nmea wiring

Thanks, guys. I get the point about the SPDT switch. For the splitting of the NMEA out, previous owners have used a fairly low-tech solution (choc box conectors). Given this works, do you see any need to go for a more complex solution, such as a multiplexer?

 

[davidej]

Thanks, guys. I get the point about the SPDT switch. For the splitting of the NMEA out, previous owners have used a fairly low-tech solution (choc box conectors). Given this works, do you see any need to go for a more complex solution, such as a multiplexer?

no

 

[yoda]

A simple switch for choosing which GPS source will be fine (did that in my last boat), the only check you will need to do is that the Garmin and new chart plotter output at the same baud rate etc otherwise the laptop my not like what it sees. Shouldn't be an issue just a needs to be set up right.

Yoda

 

[alisdair4]

Thanks, brevity is good. So are choc-block connectors (ie, cheap!)

 

[johnphilip]

My favourite plug

Before buying the Lowrance plotter have a look at the Garmin 750. I see it now reduced to under £700 AND it includes built in charts for GB' Belgium and most of Holland and France channel coast. Do not be put off by Luddites who come along and say touch screen is unreliable at sea. It is far more intuitive to use.
Mine provides NMEA output to DSC, Yeoman Plotter, Nasa AIS and Wheelpilot without blinking. It actually has two separate outputs which can be set for NMEA 0183 or NMEA 2000 to suit newer equipment. If you merely use the older Garmin as a back-up solid wire the outputs but only use one of them at a time should work surely.

 

[laika]

Am I making things unnecessarily complicated;
Given the number of NMEA "Outs", do I need a NMEA multiplexer (and if so, what would forumites recommend).

For outputs, the normal rule is that you can have up to 3 listeners attached to one talker (e.g. wired together at a chocblock). I have an "nmea 0183" chocblock screwed to a piece of ply with sources coming in one side and destinations attached to the other. Where I've got only one listener for a talker this may add in unnecessary resistance ( as opposed to taking the output directly to the listener), but it works, it's neater, and I can add stuff in and re-organise it at will.

For switching GPS sources, my task list this week includes fitting the low tech switch solution, although I'm going to be trying a DPDT switch and using the other pole for power: No point (other than speed of changeover) in having my backup GPS on when I'm not using it. If anyone thinks that's a bad plan do let me know

I'm just switching GPS sources so I can't currently justify the outlay of a multiplexor. However your setup may be rather more complex. Whilst I understand the picture posted is not necessarily your setup, if you want to start feeding GPS, AIS and navtex to a laptop and the chartplotter you may start running out of input ports on laptop or plotter. A multiplexor (e.g. this one which I have no experience of but have considered) would simplify your wiring by combining inputs to a single output, combine all inputs to a single output for your PC, allow for automatic switchover of GPS sources rather than manual, and if your instruments are old raymarine ones as in the picture posted, will potentially convert their output to nmea for viewing on non-raymarine plotter or pc application.

 

[alisdair4]

Multiplexers

Laika,

thanks for that suggestion. I actually have enough ports on the laptop (Í use a USB hub, which after a bit of messing around works really well). The diagram is as complex as it will get (I hope!), so I will have a look a the chocblox option as suggested earlier. If I run out of NMEA "oomph" I will have to multiplex (by that expression, you can see how little I understand about the subject)!

 

[alisdair4]

Garmin 750

Before buying the Lowrance plotter have a look at the Garmin 750. I see it now reduced to under £700 AND it includes built in charts for GB' Belgium and most of Holland and France channel coast. Do not be put off by Luddites who come along and say touch screen is unreliable at sea. It is far more intuitive to use.
Mine provides NMEA output to DSC, Yeoman Plotter, Nasa AIS and Wheelpilot without blinking. It actually has two separate outputs which can be set for NMEA 0183 or NMEA 2000 to suit newer equipment. If you merely use the older Garmin as a back-up solid wire the outputs but only use one of them at a time should work surely.

John, thanks for the heads-up. I had a look at the 750 online, and it seems an impressive bit of kit. The problem is that I need Baltic charting, whcih would add another 250-330 to the price. FWIW, I did consider the smaller model (Garmin 620) but the screen was too small for what I need.

 

[Topcat47]

I have "legacy" electronics on Snark. When trying to connect my GPS to my DSC radio, I discovered it was not "reverse compatible" and required a more modern version of the NMEA output than my plotter (for which no software update was available to enable it to read the later version of NMEA 0183). I'd advise checking compatibility before parting with money. That said, I bought a new Garmin to feed data to the radio and am revelling in the bigger and brighter display so this winter it's going to be re-sited closer to the chart table.

 

[alisdair4]

Legacy nmea

Topcat,

good advice - I have been poring over manuals very carefully!

 

[nigelstgeorge]

Before buying the Lowrance plotter have a look at the Garmin 750. I see it now reduced to under £700 AND it includes built in charts for GB' Belgium and most of Holland and France channel coast. Do not be put off by Luddites who come along and say touch screen is unreliable at sea. It is far more intuitive to use.
Mine provides NMEA output to DSC, Yeoman Plotter, Nasa AIS and Wheelpilot without blinking. It actually has two separate outputs which can be set for NMEA 0183 or NMEA 2000 to suit newer equipment. If you merely use the older Garmin as a back-up solid wire the outputs but only use one of them at a time should work surely.

I have something close to this setup and providing both GPSs are not on at the same time it works perfectly.

 

[nigelmercier]

Thanks, brevity is good. So are choc-block connectors (ie, cheap!)

No, they are ****. Just use a single pole changeover switch.

 

[elton]

Thanks, brevity is good. So are choc-block connectors (ie, cheap!)

No, they are junk. They're only any use for testing and emergency temporary repairs.

 

[alisdair4]

Confused choc blocks

No, they are ****. Just use a single pole changeover switch.

Nigel,

we are at cross purposes! I am quite clear that I need to use a SP switch to change between the GPS. However, I was referring to using chockblocks rather than a NMEA multiplexer. I understand the technical drawbacks which are often listed in this forum. However, said chockblocks have been working perfectly satisfactorily (in a dry, fairly stable area behind the instrument panel) for at least 5 years. I guess that is "temporary"?

 

[Norman_E]

For anyone that has not already seen it, this PDF from Actisense is a very useful guide to NMEA0183. I have printed out a hard copy to take to the boat.

 

[alisdair4]

Scales fall from eyes

Thanks, an excellent document!

 

[nigelmercier]

I was referring to using chockblocks rather than a NMEA multiplexer...

I'm pretty sure that there is more inside a multiplexer than a chocolate block, so they are hardly interchangeable. You can't join the two feeds, you need to switch between them.