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Old 24-06-2020, 19:36   #1
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Open CPN To NMEA

Hi all,
I use Open CPN on a GPS enabled laptop, it serves my purposes just fine.

I recently installed a radar that accepts NMEA input from other devices.
Is there a way to export the GPS info FROM my laptop to the radar NMEA cables I have?

Thank You
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Old 26-06-2020, 06:13   #2
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Re: Open CPN To NMEA

Yes. OpenCPN allows a serial port to be designated as an “output” using the options->connections settings. Then OpenCPN will send each NMEA message it receives back out that output connection. If you fit your computer with the right kind of adapter it can output NMEA data. There are RS-485 USB serial adapters in the market. Sometimes you can use an RS-232 USB adapter with some success. It depends on the MFD design. RS-232 works with probably 90% of MFDs.
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Old 27-06-2020, 04:04   #3
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Re: Open CPN To NMEA

Thank You!

This is precisely the information I was looking for.
I couldn't find anything online.

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Old 27-06-2020, 05:14   #4
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Re: Open CPN To NMEA

Quote:
Originally Posted by transmitterdan View Post
Yes. OpenCPN allows a serial port to be designated as an “output” using the options->connections settings. Then OpenCPN will send each NMEA message it receives back out that output connection. If you fit your computer with the right kind of adapter it can output NMEA data. There are RS-485 USB serial adapters in the market. Sometimes you can use an RS-232 USB adapter with some success. It depends on the MFD design. RS-232 works with probably 90% of MFDs.
For a good connection the NMEA-send connection should be galvanic isolated as required by specification.

When OpenCPN is sending information over NMEA you should add galvanic isolation, at least if you don't want to risk failure due to electronic discharge caused by ground loops. This means just an RS232 to USB connector is not sufficient while it will work for a few days...

If you are very conservative you will isolate each connection (both send and receive) and the computer itself (power). It's your installation, you decide.

Main difference between a (real) RS485 and a RS232 is as far as I know the isolation.
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Old 27-06-2020, 05:48   #5
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Re: Open CPN To NMEA

It is well known the internet is a very dangerous source for informations. So better never beleave what you read in the internet.
The standard used for NMEA 0183 is RS422 for sending/transmitting data. NMEA 0183 v2 and higher use a "diiferential" method with connections lines "A" and "B" and shield. While NMEA 0183 v1 use a "single ended" method similiar RS232 with one data line and ground as the other line.
To read more about the standard NMEA 0183 go to:
https://www.actisense.com/wp-content...ssue-4-1-1.pdf
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Old 27-06-2020, 11:17   #6
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Re: Open CPN To NMEA

You guys are funny. Many people use RS-232 adapters with simple 2 device NMEA data connections and they work just fine. Plus they are way easier to find and less expensive. The probability of damage is zero. You might need the higher cost differential version if you want to talk to multiple devices.

In case you didn’t know, almost all laptops are already galvanically isolated from mains and boat DC.

If you want to transmit and receive on the same adapter then RS422 allows
that. As does RS232.

I did mistype RS485 so thanks for the catch. I was using that standard the other day. Although RS485 will work as a transmit only device.
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Old 28-06-2020, 06:32   #7
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Re: Open CPN To NMEA

Just go ahead and call me weird. I burnt through too many adapters (sometimes literally) to unconditionally trust them.

The problem with isolation is a non-issue _IF_ the rest of the installation works correctly. In the past many manufacturers isolated both input and output to prevent problems, but since then there have been many cost optimizations. In practice you will still see lines in NMEA networks isolated on both sides. However you will also find non-isolated and non-compliant lines.

What the OP does with his installation is however not my decision and not my problem. And I don't know the devices he uses.
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