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Old 09-02-2017, 07:52   #1
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Question trouble reading nmea from transducer

Hello!

I'm trying to connect an Airmar P79 smart transducer (NMEA 0183) to opencpn on linux mint.(I have already had USB GPS working).
I'm using a USB converter (Same as Sailoog sells), and it looks ok in the usb connections list.
I used the "screen tool" to read the signals from the transducer, and it does transmit, although it does not look like NMEA sentences. Depending on what baud rate I choose to read, I get different combinations of letters and question marks. And on high baud rates I only get the question marks.
It does seem to change when I move the transducer from the bucket where I'm testing it. So there is some kind of signal coming through.

There is no nmea data coming through in opencpn.

Any help is very much appreciated!
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Old 09-02-2017, 10:13   #2
bcn
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Re: trouble reading nmea from transducer

Quote:
Originally Posted by systreif View Post
Hello!

I'm trying to connect an Airmar P79 smart transducer (NMEA 0183) to opencpn on linux mint.(I have already had USB GPS working).
I'm using a USB converter (Same as Sailoog sells), and it looks ok in the usb connections list.
I used the "screen tool" to read the signals from the transducer, and it does transmit, although it does not look like NMEA sentences. Depending on what baud rate I choose to read, I get different combinations of letters and question marks. And on high baud rates I only get the question marks.
It does seem to change when I move the transducer from the bucket where I'm testing it. So there is some kind of signal coming through.

There is no nmea data coming through in opencpn.

Any help is very much appreciated!
The "normal" baud rate for basic NMEA would be 4800.
Which ones are you trying? What does the manual say?
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Old 09-02-2017, 10:28   #3
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Re: trouble reading nmea from transducer

yes, 4800 is what I first tried, sadly I have not been able to find any information about the baud rate from Airmar, but I have tried all baud rates in the dropdown menu in opencpn, and none of them gave readings that looked like NMEA sentences.
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Old 09-02-2017, 10:59   #4
bcn
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Re: trouble reading nmea from transducer

- (Double)Check cables and connectors then. GND well soldered?
- Remember that the Tx from the sensor has to go to Rx of the adapter.
- And/or try a different USB2serial adapter.
- If you have a PC with a real RS232 port try with this first
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Old 09-02-2017, 12:44   #5
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Re: trouble reading nmea from transducer

Quote:
Originally Posted by bcn View Post
- (Double)Check cables and connectors then. GND well soldered?
- Remember that the Tx from the sensor has to go to Rx of the adapter.
- And/or try a different USB2serial adapter.
- If you have a PC with a real RS232 port try with this first
I do not have a RS232 port on the pc, however I will try another adapter.
You ask if GND is well soldered, the way I have it connected now is:

The transducer has 5 cables, naked is shield (unconnected), blue and white is NMEA - and + (Connected to RX - and + on adapter), red is + 12-24 Vdc and black is Ground.

I have red and black connected to a separate 12V battery, should the black one also go to GND on the adapter? I assumed only the blue and white carried signal?

Thank you for your response so far
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Old 09-02-2017, 14:26   #6
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Re: trouble reading nmea from transducer

When a NMEA0183 device has 2 cables it is obviously line A and line B of the RS422. To use it for RS232 you need to take line A (+) only and Ground (not liine B). This is too often misunderstood.

Gerhard
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Old 09-02-2017, 15:22   #7
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Re: trouble reading nmea from transducer

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Originally Posted by CarCode View Post
When a NMEA0183 device has 2 cables it is obviously line A and line B of the RS422. To use it for RS232 you need to take line A (+) only and Ground (not liine B). This is too often misunderstood.

Gerhard
The serial adapters I have are RS422 (and 485), and the cables from the transducer are marked with NMEA + and NMEA -, which means it's NMEA V2 right? RS232 is NMEA V1 where there is only one dedicated signal cable and the ground cable is also used for signal
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Old 09-02-2017, 16:06   #8
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Re: trouble reading nmea from transducer

Problem solved!
It turnes out either the cables from the transducer or all of the converters are mislabeled. I switched polarity on the signal cables and now I get the NMEA data.

Thank you for the suppot guys

Now I will start connecting my 3G radar
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