How can I send custom PAIR commands?

[harshil21]

Hello,

Thanks for this amazing library. I have a Quectel LC29H (BS) and (DA). I am trying to send certain commands like setting MSM7 output. For my device, this is done using PAIR commands:

I have looked at the configuration file, where you can send PQTM messages, but there is no such example on how to do the same for PAIR messages. Is this a feature which can be supported?

Also, I am confused on what the last field (msgmode) of the user defined command is supposed to mean? Why is it always 1?

NMEA - <description>; <talker>; <message id>; <payload as comma-separated string>; <msgmode>

[semuadmin]

Hi @harshil21

Second question first - the msgmode parameter is usually but not necessarily always 1 (SET). It's needed by the underlying pynmeagps NMEA parsing library and signifies whether the message is 0 GET (i.e. output sent from the receiver), 1 SET (i.e. command sent to the receiver) or 2 POLL (i.e. query to the receiver). In the context of PyGPSClient's NMEA Configuration panel, NMEA sentences will usually be 1 SET or 2 POLL, but in other contexts the user may wish to create a GET message (e.g. a GGA message to send to an NTRIP caster).

First question, the list of presets in the example I provide is by no means exhaustive - the intention is for the user to create their own.

The Quectel L29 and L79 support a series of proprietary $PAIR NMEA messages, where 'P' is the talker and AIRnnn is the 'message id'. Unfortunately, pynmeagps does not currently support the creation of these message types but I've put an action on my backlog to add them in - it's a relatively trivial enhancement (or you could submit your own PR if you feel up to it :-)). Once support is added, you'll be able to create a $PAIR message preset as follows e.g. PAIR050: PAIR_COMMON_SET_FIX_RATE:

"Set common fix rate; P; AIR050; 1000; 1"

FYI pynmeagps is capable of parsing any valid incoming NMEA sentence even if its payload is not defined, but it will be parsed to a 'nominal' format e.g. for $PAIR050, it will currently parse this as:

from pynmeagps import NMEAReader, SET
msg = NMEAReader.parse(b"$PAIR050,1000*12\r\n", SET)
print(msg)
print(msg.serialize()

<NMEA(PAIR050, NOMINAL, field_01=1000)>
b'$PAIR050,1000*12\r\n'

FYI PyGPSClient provides an interactive Python function nmeapreset which converts one or more binary NMEA sentences (e.g. from a datalog file) to NMEA preset string format, which can be copied-and-pasted into the nmeapreset_l list in your json configuration file, e.g. assuming the 'mynmea.log' file contains a single binary PAIR050 message (and assuming pynmeagps has been enhanced to support PAIR message types):

python3
> python3
>>> from pygpsclient import nmea2preset
>>> preset = nmea2preset(mynmea.log, "Set common fix rate")
>>> print(preset)
>>> "Set common fix rate; P; AIR050; 1000; 1"

[semuadmin]

@harshil21

FYI A new version of pynmeagps (1.0.52) is now available which supports all the PAIR (and some additional PQTM) NMEA sentences as used by the Quectel L29H and L79H range. You can update to this latest version via the 'Check for updates' facility on PyGPSClient's About dialog.

Once updated, you should be able to add PAIR preset commands along the lines of the one I illustrated above to your pygpsclient.json file and invoke them via the NMEA Configuration panel, e.g.

Remember - PAIR commands ('Set') will be msgmode = 1, PAIR queries (which the Quectel documentation refers to as 'Get') will be msgmode = 2.

[harshil21]

Awesome, thanks for implementing this feature so quickly! I can test the PAIR messages later this weekend.