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Old 19-11-2014, 19:59   #1
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Problems getting NMEA to Open CPN

hi all,
trying to set up an old windows xp netbook on the boat as better, faster, plotter than the clunky, slow 20yo furuno

opencpn software is on the computer and have loaded it with local maps

BUT, have not been able to get NMEA data from the GPS to computer via serial to usb converter

- NMEA is good as it supplies gps position to the DSC VHF
- serial to usb converter driver installed, device manager shows it com6, baud rate 4800
- opencpn connection created at com6, baud rate 4800

net suggests the 2 NMEA wires should connect to pin 9(gnd) and pin 2(rx)

but have tried all possible wiring connections with no success. opencpn nmea debugging never showing anything received

guess next step is to use device manager to lower the com port buffers...

open to other suggestions...
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Old 19-11-2014, 21:09   #2
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re: Problems getting NMEA to Open CPN

sorry, that should be pin 5 for the gnd not 9
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Old 19-11-2014, 21:44   #3
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re: Problems getting NMEA to Open CPN

Try toggling the connection's "Control checksum" button in OpenCPN.
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Old 20-11-2014, 03:37   #4
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re: Problems getting NMEA to Open CPN

Quote:
Originally Posted by eric e View Post
guess next step is to use device manager to lower the com port buffers...
Probably best to start with a very simple application - on XP a Virtual Terminal, Kermit or something like that. Or use some specialized scanning tool like "SOB Ports" at DigiBOAT Downloads
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Old 20-11-2014, 04:35   #5
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re: Problems getting NMEA to Open CPN

You might also consider to upgrade to 3.3.2316 (or install it in parallel).
Handling of communication has improved a lot since 3.2.2

And you know for sure that just one program can access to a COM port at the same time.
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Old 20-11-2014, 10:09   #6
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re: Problems getting NMEA to Open CPN

I found that the manual had the wiring colors inccorrect or reversed. A friend who is wise about electronics had me find the wire that showed a variation in amperage and then used that as signal wire and the ground to connect. Does any wire show voltage variation?
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Old 21-11-2014, 03:00   #7
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Re: Problems getting NMEA to Open CPN

You've probably considered this but just in case...

Getting GPS *into* OpenCPN you're going to be using the tx wires from the GPS. If you've got differential NMEA (i.e. tx+/tx- as opposed to tx and gnd)...perhaps someone with more electronics knowledge than myself can advise on prudence of connecting the tx- to gnd as opposed to gnd to gnd. I seem to recall that the latter gives you half the signal (which is probably enough) but is the safer option in the absence of opto-isolation (but I could be talking nonsense).

When wiring up the connector remember 2 things:
1: you're *transmitting* data, not receiving
2. there's a back view and a front view to the connector. The pin numbering is different depending on which way you look at it.

On the latter point there is something spooky going on akin to the effect of butter on free falling toast. I seem to get it wrong far more than the 50% of the time which chance would dictate.
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Old 21-11-2014, 03:37   #8
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Re: Problems getting NMEA to Open CPN

Quote:
Originally Posted by muttnik View Post
You've probably considered this but just in case...

Getting GPS *into* OpenCPN you're going to be using the tx wires from the GPS.
The Tx from the sending equipment - AIS or GPS - has to go to the Rx pin of the receiving equipment. (And vice versa)
Also called a crossed over or Null-modem cable.

Modern RS232 chips should deal with differential signals - so Tx- to GND

Good luck...
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