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Subject: Re: Non-POSIX parity (mark/space) with Python-Serial on Linux.
From: David Riley <fraveydank@gmail.com>
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Date: Mon, 21 Nov 2011 12:22:28 -0500
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To: Matthew Lenz <matthew@nocturnal.org>, python-list@python.org
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On Nov 21, 2011, at 11:52 AM, Matthew Lenz wrote:

> Ahh. Ok.  So how would I go about doing that with python?  I think in =
perl (sorry for the naughty word) I could use the tr// (translate) but =
is there a quick way to do so with python?  Is it going to be necessary =
to convert commands I SEND to the device or only convert what I receive?

The high-level overview is that you'll want to OR in 0x80 on transmit, =
and AND 0x7F on receive (thus inserting the high bit when you send it =
out and removing it when you receive).  If you wanted to be extra-sure =
you're receiving things correctly, you should also check to see if your =
value ANDed with 0x80 is not zero.

In Python 2.x, as mentioned, when you iterate over a string, you get a =
bunch of tiny one-character strings, which you then need to convert into =
numbers with ord() and back into strings with chr() when you =
re-concatenate it.  ord() and chr() may be familiar to you from Perl.  =
For example, you could do this on reception:

----

# However you get your data out of serial
received_str =3D receive_my_string()

ord_str =3D [ord(x) for x in received_str]

# An exception may be extreme in this case, but hey.  Note that
# the filter() function actually returns a list of all the
# characters that don't have the high bit set, so you could
# actually use that if you wanted to.
if filter((lambda x: x < 0x80), ord_str):
   raise IOError("Received character without mark parity")

return "".join([chr(x & 0x7F) for x in received_str])

----

In Python 3.x, iterating over a bytes array (which is, hopefully, what =
your serial interface returns) will give you a bunch of ints, so you =
shouldn't need to do the conversions:


----

# However you get your data out of serial
received_bytes =3D receive_my_string()

# In Python 3.x, filter() returns an iterator, which is generally a
# better thing to return, but doesn't test as nicely for a bool.
for b in received_bytes:
   if b < 0x80:
       raise IOError("Received character without mark parity")

# None of this "".join() nonsense with byte arrays!
return bytes([(x & 0x7F) for x in received_bytes])


----


There are surely more efficient ways to do what I've typed up there, but =
those should work pretty well for whatever you're looking to do.  For =
sending, you pretty much want to swap (x & 0x7F) with (x | 0x80) to =
insert that high bit.


- Dave