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NMEA.PY
rmentjens Posted: 9 Jan 2021, 06:01 PM
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Good evening.

I am trying to use the nmea.py script to generate $GPRMC sentences. So far I am partially succesful. However there seems to be a problem with the script.

I run the script on a RPI zero W. I connected a USB UART to the USB port and connected that to a console on my computer. showing the NMEA sentences.

The thing is. The time runs too fast. after every 6 seconds, one minute seems to have passed. every second ten new GPRMC sentences are sent. Is there a way to make the time run slower, and also is there a way to only send the GPRMC string every minute. in stead of ten times a second?

Kind regards, Raymond.

Last edit by rmentjens at 9 Jan 2021, 06:01 PM

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mit Posted: 10 Jan 2021, 05:01 PM
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yeah whatever

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Something has gone wrong if it is outputting ten times a second.

Are there any error messages? What version of python are you running?

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rmentjens Posted: 11 Jan 2021, 05:01 PM
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Evening again.

The Python version is 2.7.16

I added a screenshot Putty screen on the right is the received $GPRMC string. The left and bottom screens are from the RPI WH You can see in the orange circles that there is a time diffirence between the RPI's time and the time that the second computer receives (don't look at the hour diffirence. my timezone is CET) The right screen gets a new $GPRMC line approximately ten times a second. adding one second to the GPRMC string.

(User posted image)


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mit Posted: 11 Jan 2021, 06:01 PM
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yeah whatever

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Strongly recommend using python3, python2 is obsolete.

A quick google suggests the behaviour of datetime.utcnow() may be different in python2 vs python3.

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rmentjens Posted: 14 Jan 2021, 04:01 PM
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Evening.

Using python3 did the trick!

It works like a charm now. This script makes it easy to sync any $GPRMC synched clock. I use it to sync a nixie clock to NTP for cheap.
I use a wireless serial bridge with two 2.4G Wireless Serial Transparent Transceiver Module (OPEN-SMART).

As I said works like a charm. Possibly useable to sync the precision clock via a wireless link.

Thank you for the script and the assistance to get it to work.

Kind regards, Raymond.

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mit Posted: 15 Jan 2021, 05:01 PM
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yeah whatever

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Excellent. At some point I'll update the script to do the checksums properly and periodically resync with the system clock.

I did make a start on a wireless NTP module for the precision clock. Using an ESP8266 and an NTP client, even made a cute PCB for it:

(User posted image)


Never got round to finishing it, but I hope to do so when I get a chance. I really wanted it to reliably synchronize to within a millisecond, which might be an impossible goal with NTP over wifi. I got pretty close though.

I was just at the point of potentially starting over, using PTP instead of NTP, and possibly fitting a TCXO to the PCB, when I got distracted with another project.

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rmentjens Posted: 3 Apr 2021, 09:04 AM
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Good Morning.

I saw you made some changes to the nmea.py script, making it generate the correct checksum.

I had to change one line in the script to make it work.
Line 20:
with serial.Serial('/dev/ttyUSB', 9600) as ser:
to
with serial.Serial('/dev/ttyUSB0', 9600) as ser:

After this script worked like a charm.

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mit Posted: 3 Apr 2021, 06:04 PM
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yeah whatever

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The name of the serial device will depend on what hardware you're using. With enough effort it's possible to auto-detect the device name, but for this little script I didn't bother.

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