Use fewest transfers possible in read_binary_values

[aWZHY0yQH81uOYvH]

See pyvisa/pyvisa-py#472 for more details. Essentially, I have an instrument that will lose data when too many small reads are issued. This PR makes it so read_binary_values, when not passed an explicit chunk_size, uses one transfer of default length to read the data header, then if there is more data to read, it reads it all in one more transfer. This should be more efficient and solves a problem with my instrument. Behavior for users that specify chunk_size is unaffected.

In order for this to work with the pyvisa-py backend, pyvisa/pyvisa-py#470 needs to be merged since it currently further breaks up reads.

• Executed ruff check . && ruff format -c . --check with no errors
• The change is fully covered by automated unit tests
• Documented in docs/as appropriate
• Added an entry to the CHANGES file

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.54%. Comparing base (2c8677f) to head (0bbbf1c).

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  Lines        5378     5378           
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  Misses       1077     1077           
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[MatthieuDartiailh]

LGTM.

Could you update the release notes with the PR number ? Also could you add a note in the docs about the new behavior ?

[aWZHY0yQH81uOYvH]

Sorry, didn't notice the other items in the changelog had PR numbers. I did update the docstring for the read_binary_values function to describe the new default behavior, which shows up in the docs. Would you like it mentioned somewhere else also?

[MatthieuDartiailh]

You need to adjust the section Reading binary values in docs/source/introduction/rvalues.rst

[aWZHY0yQH81uOYvH]

Done.

[MatthieuDartiailh]

Thanks, it looks good. I am wondering if this could break when transferring very long responses in which case it may be better to cap the maximum chunck_size. I will try to test either tomorrow or next week before merging.

[aWZHY0yQH81uOYvH]

I wonder that as well.

Looks like the length field in the USBTMC header is a 4-byte value (up to 4GiB), while the IEEE header represents the length with up to 9 decimal digits (up to 999MB), so it should be impossible to overflow the length field of the USB header. Similarly, looks like the HP header is a 2-byte length value, so that should also be fine.

I have tested this with 1MiB transfers, but I don't have other equipment to test this with, so further testing would be great to see if there are issues below these limits.