NMEA Single/Differential Converter please

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alla_breve

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Hello all,
Be gentle with me, I'm no electronics expert, and have spent all afternoon googling to try and ask the right questions!
I have the following system:

Eagle Intellimap 642C Plotter, NMEA output position, RMC, Yellow is data out, black is gnd.so probably single ended?( also has orange -data in. not used) Spec in book says NMEA v2.0, 4800 baud.

Cobra MR F57B VHF. NMEA input. (reqd for GMDSS). Yellow is +in, Green is -in. So probably differential?

NASA AIS "Radar". NMEA input for position. Blue is +in. Also has a black and red wires to a buzzer. NASA implies that data negative is supply gnd. So probably single ended?

Now the problem, Plotter to just VHF works fine, Plotter to just AIS works fine. Connect AIS AND VHF as two listeners to the single Plotter Talker, and the AIS stops receiving data,

I think its a problem with the single/ differential lines? I've read as much as I can of previous posts, including one with an optocoupler (4N25) which looks within my capability to solder up, but dont know on which lines to put it?

I think the Plotter is 232 as its NMEA v2.0, the AIS is probably 232 as well, whereas the VHF is brand new and looks to be 422,(as its new, so probably NMEA more than v2.0) and as it has the +- data lines and is therefore differential? Maybe a 232 to 422 converter would work? But where does it need to go?

I have tried connecting the plotter data gnd black to the supply gnd- didnt help. Also tried connecting the AIS black wire , buzzer gnd, to supply gnd- didnt help. All three instruments have the same 12v negative connection.

Tried to sketch the connections, failed to insert sketch into post, I'll try again.

Any suggestions, apart from buying a £150 NMEA isolator, much appreciated!

Thanks,
Tim.
 
You should be able to connect an old version 2 single ended talker to a version 3 or later differential listener as the standard requires the differential input circuit to be isolated from everything - it doesn't care about voltage levels relative to the outside world, just the difference. You should also be able to connect a talker to up to 4 listeners. So what you are doing sounds like it should all work, but it doesn't. Difficult to diagnose without getting a scope on the line to see where it's going wrong.

Easiest cheapest solution is as Paul says, get a cheapo separate GPS receiver for you radio. It will should work fine in the cabin of a GRP boat.
 
I've successfully mixed balanced and unbalanced NMEA feeds to listeners and vice versa. Just make sure you connect the yellow data out from the plotter to the data + in on both listeners and then the other 3 (ground/-) wires to each other.
 
Where is the radio green data -ve attached? Try it connected and disconnected from ground. Also try reversing the NMEA wires to radio (again, with non-signal wire connected and disconnected with ground).

Clutching at straws but have you tried with the buzzer -ve disconnected?

If none of those, as Paul says, separate GPS receiver is path of least resistance, and probably cheapest.
 
Thanks to all for replies. I think I'll take the advice of fitting a separate antenna to the VHF, its then "standalone"., and hopefully more reliable in case of emergencies. I can then use the plotter to feed just the AIS.
I might risk the ebay NMEA antenna, or more likely shell out for the Cobra one that will definitely work.
I'll post a reply when its all working!
Thanks again, good forum this, very helpful,
Tim
 
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