Using .parse() on a read line from Serial

[Nilles3D]

Product: ZED-F9R on Sparkfun GPS-21305
MCU: Raspberry Pi 3B+ with bullseye
Python: 3.9.2
Library: pyubx2 v 1.2.51
u-Center: 23.08

I'd like to use the UBXReader.parse() on an incoming byte array from a Serial stream to interpret a UBX message for processing. How should I modify the script below to do that?

I started with the example from the README:

from pyubx2 import UBXReader
msg = UBXReader.parse(b'\xb5b\x05\x01\x02\x00\x06\x01\x0f\x38')
print(msg)
msg = UBXReader.parse(b'\xb5b\x01\x12$\x000D\n\x18\xfd\xff\xff\xff\xf1\xff\xff\xff\xfc\xff\xff\xff\x10\x00\x00\x00\x0f\x00\x00\x00\x83\xf5\x01\x00A\x00\x00\x00\xf0\xdfz\x00\xd0\xa6')
print(msg)

and made:

    import serial
    from pyubx2 import UBXReader
     
    ser=serial.Serial(port='/dev/serial0',
        baudrate=38400,
        bytesize=serial.EIGHTBITS,
        parity=serial.PARITY_NONE,
        stopbits=serial.STOPBITS_ONE,
        timeout=1
    )
     
    cont=True
    print('read_test start')
     
    while cont:
        try:
            if ser.in_waiting>0:
                red=ser.readline()
                print(red[:100])
                print(UBXReader.parse(red))
                ser.flush()
        except KeyboardInterrupt:
                cont=False
                ser.close()
                print('end')

Where did I make the biggest blunder?

[semuadmin]

Hi @Nilles3D

I would STRONGLY recommend you take another look at the README and maybe some of the examples in the \examples folder, as your script is pretty wide of the mark. Some immediate observations...

1. u-blox receivers like the ZED-F9* series only output NMEA sentences by default - if you want UBX messages you have to explicitly enable these on the relevant port(s). pyubx2 will happily parse NMEA, UBX and RTCM data, so it can handle anything the ZED-F9* can output, but you need to configure the ZED-F9* appropriately first.
2. the default navigation interval is 1 second (1 Hz), so setting a serial timeout of 1 second probably isn't going to cut it - try raising this to, say, 3 seconds.
3. serial.readline() only works for protocols which are delimited by CRLF (carriage return, line feed) characters, such as NMEA. UBX is a binary protocol which does not use such delimiters, so readline() won't work.
4. you rarely need to specify the bytesize, parity or stopbits arguments - they can all be left at the defaults.
5. it's generally prudent to use Serial within a "context manager" (i.e. a with statement), as this will ensure the serial port is correctly released after the read loop.
6. pyubx2.UBXReader implements an iterator, so you can use a simple for loop to read messages.

At the most basic level, all you need is something like this - exactly as shown in the README:

from serial import Serial
from pyubx2 import UBXReader

with Serial('/dev/serial0', 38400, timeout=3) as stream:
   ubr = UBXReader(stream)
   for raw_data, parsed_data in ubr:
      print(parsed_data)

This will parse all data output by the receiver. I suggest you start with this first, and then refer to the ZED-F9R Interface Manual for details on how to enable UBX messages via the UBX CFG-VALSET command.

Or you could install the PyGPSClient GUI application, which has 'out of the box' facilities to help you do this.

Hope this helps

[Nilles3D]

Thank you for the detailed response. I will use the Reader example exactly as shown in the README.

Answers to observations:

1. NMEA sentences versus UBX outputs: Yes, I had set the CFG-MSGOUT-UBX_NAV_PVAT_UART1 to 1 and all other message outputs to 0 to avoid sending too much data.
2. Navigation solution intervals: Yes, I had changed the default of 1Hz to 10Hz, but I can put the timeout back from 3 to 1 as suggested.
3. readline(): This was my blunder. I did not know that readline() was the wrong tool to use, so I went down the wrong path by trying to make the snippet from the README do more than it should.
4. Default arguments: Paranoia made me set them for troubleshooting, but I will remove them as suggested.
5. Context manager: This is a good tip that I will implement.

Thank you for the links to the Manual and GUI as well.

[semuadmin]

@Nilles3D

FYI if you don't want to use a GUI, the pyubxutils and pygnssutils libraries offer a number of CLI tools which you might find useful. For example, the pyubxutils ubxsetrate tool can automatically enable a minimum cohort of UBX messages (NAV-PVT, NAV-SAT, NAV-DOP) e.g.

python3 -m pip install --upgrade pyubxutils pygnssutils
ubxsetrate --port /dev/serial0 --baudrate 38400 --msgClass minubx --rate 1

Response:

Opening serial port /dev/serial0 @ 38400 baud ...
Sending configuration message <UBX(CFG-MSG, msgClass=NAV, msgID=NAV-DOP, rateDDC=1, rateUART1=1, rateUART2=1, rateUSB=1, rateSPI=1, reserved=0)>...
Sending configuration message <UBX(CFG-MSG, msgClass=NAV, msgID=NAV-PVT, rateDDC=1, rateUART1=1, rateUART2=1, rateUSB=1, rateSPI=1, reserved=0)>...
Sending configuration message <UBX(CFG-MSG, msgClass=NAV, msgID=NAV-SAT, rateDDC=1, rateUART1=1, rateUART2=1, rateUSB=1, rateSPI=1, reserved=0)>...
Configuration message(s) sent.