Fix to use endpoints MaxPacketSize instead of hardcoded RECV_CHUNK size

[sessl3r]

The issue was found to be not able to download screenshots from a RigolDS1074Z.
From this issue it was seen there is some workaround but not a fix.

As the same issue existed in the linux-usbtmc driver module it was easy to refix it here.

The main point is that those scopes have a limited MaxPacketSize which is reported by the USB device correctly and the TMC protocol is relying on using this MaxPacketSize.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 24.92%. Comparing base (13bda1d) to head (6bcbaa3).

❗ Current head 6bcbaa3 differs from pull request most recent head 6b3c851. Consider uploading reports for the commit 6b3c851 to get more accurate results

Files	Patch %	Lines
pyvisa_py/protocols/usbtmc.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #417   +/-   ##
=======================================
  Coverage   24.91%   24.92%           
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  Files          25       25           
  Lines        3516     3515    -1     
  Branches      489      489           
=======================================
  Hits          876      876           
+ Misses       2623     2622    -1     
  Partials       17       17           

Flag	Coverage Δ
unittests	24.92% <0.00%> (+<0.01%)	⬆️

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[MatthieuDartiailh]

Wow a fix that helps with Rigol devices is definitively welcome ! The formatting failure is unrelated to your changes so do not worry about it. Could you add a changelog entry ? Once it is done I will merge.

@Fellypao it may be worth checking if this fixes your issue.

[sessl3r]

added changelog (hope 0.7.2 is ok for the version)
at least on my ds1054z the :display:data? on,off,png now works without monkeypatching pyvisa_py - hope also for others :)

[MatthieuDartiailh]

Perfect ! Thanks a ton !

[Fellypao]

@Fellypao it may be worth checking if this fixes your issue.

@MatthieuDartiailh Yes, this definitely fixes my issue (#414). Thank you. And many thanks to @sessl3r 👍

[MatthieuDartiailh]

Amazing !!! I will try to cut a release ASAP

[sessl3r]

Only as a little late FYI: From the usbtmc PR I got the info that some Agilent scopes will suffer performance from this as they report a smaller MaxPacketSize then they can do (1k vs 4k).
I totally dislike the way usbtmc has solved this - however eventually - if someone reports problems a parameter could be added to overwrite it.

[MatthieuDartiailh]

Could you open an issue so that the information is less likely to be lost ?

Honestly I take the win of people having Rigol scope working out of the box over maximal performance for Agilent scope.

[MatthieuDartiailh]

Thanks @sessl3r