[pywin32-bugs] [ pywin32-Bugs-2132639 ] win32file.GetCommState incorrect handling of DCB

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Bugs item #2132639, was opened at 2008-09-27 14:56
Message generated for change (Comment added) made by jiaailun
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Category: win32
Group: None
>Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Alan Gardner (jiaailun)
Assigned to: Nobody/Anonymous (nobody)
Summary: win32file.GetCommState incorrect handling of DCB

Initial Comment:
I believe I have found a bug in win32file.GetCommState and win32file.SetCommState.  I have seen it in Python 2.4 and Python 2.5, running an older version of pywin32, as well as the current (212) version.  It exists in Win2k and WinXP.  I use pyserial 2.4 as a wrapper for the comm ports, though I believe the problem is in win32file.

The problem manifests itself when I try to open a com port in Python after having that port open in Procomm (an old, no longer supported terminal program).  I have only seen it happen with a particular brand of USB to serial converters (Edgeport, made by Digi).  In these conditions the python script will hang at the win32file.SetCommState() command, usually timing out after 5-30 seconds, and quitting with no error message.

After having done some probing I think I basically understand the problem, and have a workaround, but I lack the know-how to fix it where it should be fixed, in win32file.  When Procomm opens the port, it sets XoffLim to 16128 and the XonLim to 15872.  When my script (actually, serialwin32.py from pyserial) tries to open the port it first reads the DCB with GetCommState(), changes baud rate, etc in the local PyDCB object, and then tries to reconfigure the port with SetCommState().  

>From what I can tell, SetCommState won't accept a value for XoffLim or XonLim >4096.  For example, if I try to execute the following:
DCB.XoffLim = 16128
SetCommState(porthandle,DCB)

I get an exception: (87, 'SetCommState', 'The parameter is incorrect.')

However, if GetCommState gets a DCB which has XoffLim = 16128, and then that DCB is passed to SetCommState, the script hangs for several seconds before exiting without opening the port and with no error message.  For example, after having opened and closed the port in Procomm:

>>> DCB = GetCommState(porthandle)
>>> DCB.XoffLim
16128L
>>> SetCommState(porthandle,DCB)
(...... hangs here for several seconds ......)

If I set XoffLim and XonLim to values from 0-4096 before calling SetCommState, then it works fine.  For example, after having opened and closed the port in Procomm:

>>> DCB = GetCommState(porthandle)
>>> DCB.XoffLim = 128
>>> DCB.XonLim = 512
>>> SetCommState(porthandle,DCB)
>>>
This works fine, and I can go on to open the port normally.  I have incorporated this fix into serialwin32.py which is working for me now, but it's not an elegant solution.  I am still baffled by why
SetCommState limits Xon/XoffLim to 4096, and by what GetCommState does to the PyDCB to make it cause SetCommState to fail so dramatically.  I also don't understand why I only see this problem with the Edgeport USB converters, and not with several other USB converters or hardware serial ports.  Maybe someone with a more intimate understanding of Python and Win32 can come up with some answers.

Thank you
-Alan Gardner
Woods Hole Oceanographic Institution
Woods Hole, MA 02543

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>Comment By: Alan Gardner (jiaailun)
Date: 2008-10-02 09:11

Message:
Gentlemen,
I appreciate your time helping me work this out.  I have been in contact
with the vendor, and after trying several things they had started to point
the finger at Python.  I am feeling pretty confident now that the problem
is not related to Python, and I have a reasonably good handle on what's
going on.  I will follow up with the vendor.  Unfortunately Procomm is
unsupported now, but I really don't think that it is doing anything
"wrong", it just leaves the port in a state that the driver reacts badly
to.  I will follow up here if I learn anything more.
Thank you.
-Alan

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Comment By: Roger Upole (rupole)
Date: 2008-10-01 17:23

Message:
The code to call SetCommState does not do any retries.
If you're seeing multiple attempts to set the configuration
parameters, they're probably coming from the driver itself.

Since this problem only happens for a particular device, and
only after a completely separate program has accessed it,
it's most likely due to the device driver, and/or the
state in which the other program leaves the port.  I really
don't think there's anything that can be done in win32file
to solve it.

 

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Comment By: Mark Hammond (mhammond)
Date: 2008-09-30 00:52

Message:
If you think pywin32 is doing something incorrect with its DCB handling,
the only real way to demonstrate that is a snippet of C code that behaves
differently.  Given our arg handling looks OK, we will otherwise need to
assume the behaviour is just a characteristic of the windows API itself.

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Comment By: Alan Gardner (jiaailun)
Date: 2008-09-30 00:41

Message:
OK, since I realize this may be a difficult problem to recreate I have
annotated some screen captures from the interpreter.  In the process I
realized that the XonLim and XoffLim are obviously going to be limited by
the buffer size, and yes, pyserial does set the buffer to 4096.  OK, so now
I have another workaround, increasing the buffer size so that the XonLim
and XoffLim values are no longer to large.  But I still think that there is
an underlying bug.  When a port is opened with CreateFile it appears to
assign a default buffer size, and this can lead to XonLim and XoffLim
values which are out of range.  If SetCommState then tries to set an out of
range value to one of these parameters, it hangs.  I have some captures
from DebugView and PortMon which show Python.exe repeatedly getting and
setting parameters on the serial port in a nearly endless loop.  I can
upload these if it would be helpful, but they are moderately large and
arcane.  For now I will upload the annotated screen captures showing what I
see at this end.
File Added: win32file.SetCommState_interactive.txt

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Comment By: Roger Upole (rupole)
Date: 2008-09-27 16:13

Message:
I don't see any problem with the way the DCB is passed to/from
SetCommState and GetCommState.  The XonLim and XoffLim members
consistently have the correct value.

If I recall from looking at pyserial a while back, it sets the
buffer sizes to 4096.  Have you tried increasing them in the
call to SetupComm ?


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