Fix `common.iter_bytes`

[dougthor42]

Fixes #80.

See that Issue for more detail.

• Adjust iter_bytes function signature to use data_bits instead of mask. Calculate the mask base on those number of bits.
• Update docstring
• Update tests

[codecov-commenter]

Codecov Report

Merging #81 (7644d02) into main (dccb9ba) will increase coverage by 0.17%. The diff coverage is 95.23%.

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@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   84.80%   84.97%   +0.17%     
==========================================
  Files          18       18              
  Lines        1033     1058      +25     
  Branches      152      160       +8     
==========================================
+ Hits          876      899      +23     
- Misses        120      121       +1     
- Partials       37       38       +1     

Flag	Coverage Δ
unittests	84.97% <95.23%> (+0.17%)	:arrow_up:

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Impacted Files	Coverage Δ
pyvisa_sim/testsuite/test_serial.py	100.00% <ø> (ø)
pyvisa_sim/common.py	91.66% <92.85%> (-2.78%)	:arrow_down:
pyvisa_sim/sessions/serial.py	68.75% <100.00%> (ø)
pyvisa_sim/testsuite/test_common.py	100.00% <100.00%> (ø)

[MatthieuDartiailh]

I have a slightly different understanding of send_end.

The way I read the specs if we have 7 data bits and send_end is True, then the bit 7 should be true only for the last bytes. In your case it seems you encode send_end in an extra bit compared to the number of allowed data bits.

I checked on my phone (little time this days) so I may have missed something.

[dougthor42]

Hmm... So you're interpreting "transmit the last character with the eighth bit set" as "8th bit == 1", not "8th bit == whatever value it would have been for that character"

Like this? (7 data bits)

Input	send_end=True	send_end=False
N/A	7th bit set to 1 for last byte no matter what the original value was. 8th bit always removed b/c data_bits=7.	7th bit set to 0 for last byte no matter what the original value was. 8th bit always removed b/c data_bits=7.
0b1111_1111	0b0111_1111	0b0011_1111
0b0000_1111	0b0100_1111	0b0000_1111
0b1111_1100, 0b1011_1100, 0b1011_1100	0b0111_1100, 0b0011_1100, 0b0111_1100	0b0111_1100, 0b0011_1100, 0b0011_1100

Do I have that correct?

(The below is for my own future reference, emphasis and additions mine)

If it is set to VI_ASRL_END_LAST_BIT (pyvisa.constants.SerialTermination.last_bit), and VI_ATTR_SEND_END_EN (pyvisa.constants.ResourceAttribute.send_end_enabled which is the send_end arg) is set to VI_TRUE (True), the write will send all but the last character with the highest bit clear, then transmit the last character with the highest bit set. For example, if VI_ATTR_ASRL_DATA_BITS is set to 8, the write will clear the eighth bit for all but the last character, then transmit the last character with the eighth bit set. If VI_ATTR_SEND_END_EN is set to VI_FALSE, the write will send all the characters with the highest bit clear.

[MatthieuDartiailh]

Rereading https://www.ni.com/docs/en-US/bundle/ni-visa/page/ni-visa/vi_attr_asrl_end_out.html, I realize I missed one case which calls for taking bool | None as argument:

• None: serial termination does not depend on send end, so we simply apply the mask
• False: serial termination depends on send end and send end enabled is False, we always clear (set to zero) the highest of the data bits.
• True: serial termination depends on send end and send end is enabled, we clear the highest of the data bits for all bytes except the last for which it is set to 1.

Does that make sense ?

[dougthor42]

Yeah I think so. Check out the new test suite and LMK.

[MatthieuDartiailh]

Thanks !

Could you make a similar PR againt PyVISA-py (I have no good solution to avoid the duplication) ?