When USB communication device class devices are installed in Windows they appear as COMxxx ports, just like legacy serial ports. They are different in that many RS-232 device parameters don't apply (such as baud rate, handshaking, etc). Also, since they are USB devices they may be suspended while remaining connected (as when the computer goes into sleep mode), or may be unplugged at any time.
I am creating a simple Python communications library to support USB-CDC devices, ignoring the unused legacy features but adding suspend and disconnect support. I have found some problems with exception processing that is making this problematic. win32file.WriteFile works well. If the device is suspended I get pywintypes.error (31, 'WriteFile', 'A device attached to the system is not functioning.'). If the device is unplugged I get pywintypes.error (1167, 'WriteFile', 'The device is not connected.'). I can find out if there are any characters in the input buffer using win32file.ClearCommError. Its exception reporting half-way works. If the device is suspended I get pywintypes.error (31, 'ClearCommError', 'A device attached to the system is not functioning.'). But if the device is unplugged I get no error reported at all. win32file.ReadFile provides neither of these errors. Either suspended or unplugged just causes ReadFile to hang. I can get ReadFile to return if I set a timeout, but no error is reported so I don't know if it returned because the device hasn't sent the data yet, or because the device is suspended or disconnected. Is there some way to get ReadFile to report errors 31 and 1167 like WriteFile does? Or is there some other way to determine port status as "attached but not functioning" or "not connected"? Thanks for the help! -- Ron _______________________________________________ python-win32 mailing list python-win32@python.org http://mail.python.org/mailman/listinfo/python-win32
