pyserial and com port interrupts

Discussion in 'Python' started by engsol, Jan 12, 2005.

  1. engsol

    engsol Guest

    Has anyone done a script that will rspond to the serial com port(s)
    receive buffer interrupt, as opposed to polling and timeouts?
    Win2000 is the main interest right now.
    Thanks
    Norm B
     
    engsol, Jan 12, 2005
    #1
    1. Advertisements

  2. engsol

    Peter Hansen Guest

    What problem do you hope to avoid by not using "polling
    and timeouts"? (Note that if you specify a sizable
    read timeout, you're closer to being interrupt-driven
    than you are to what is traditionally called "polling".)

    -Peter
     
    Peter Hansen, Jan 12, 2005
    #2
    1. Advertisements

  3. engsol

    engsol Guest

    Peter,
    Thanks for the reply. I'm working on a s/w test program using python
    code. Com1 and com2 play a part. The problem is that the python code
    has a lot of work to do...and the results from the hardware under test can
    come from either com1 or com2...at any time. It may be a few milliseconds,
    or several seconds, sometimes minutes, before a response is expected.
    I'm not sure what timeout value I'd use. Using threads, and re-loading the
    timeout values on the fly may be a solution, but I'm not experienced with
    threads....and was hoping to avoid them.
    Norm B
     
    engsol, Jan 13, 2005
    #3
  4. Have you looked into letting the OS do it? I.e., reading from COM4:
    or whatever port in a thread maybe one byte at a time? Maybe you can
    get your end functionality without writing low levels stuff, depending ;-)

    I haven't done this, but it seems an easy thing to try a few experiments with.
    The control panel should let you set baud rates and handshaking etc. I would think.

    Regards,
    Bengt Richter
     
    Bengt Richter, Jan 13, 2005
    #4
  5. PySerial[1] is more than capable of handling all the low level
    operations with little more than 's.baud = 9600'. The OP was more
    interested in how to write his program so he could react to com port
    input in a timely manner in the face of having blocking procedures
    elsewhere in his code.

    Regards,
    Stephen Thorne

    [1] http://pyserial.sourceforge.net/
     
    Stephen Thorne, Jan 13, 2005
    #5
  6. Using threads in Python is really, really painless.
     
    Grant Edwards, Jan 13, 2005
    #6
  7. engsol

    Neil Benn Guest

    Hello,

    I came across this problem as when I first used PySerial, I
    came from a java background which has the ability to register listeners
    to a serial comm instance and receive interrupts (although some
    implementations use polling behind the scenes anyway). I don't think
    that pyserial has this kind of thing so I used the following :

    def __checkSerial(self):

    self.__objLock.acquire()
    try:
    try:
    intNoChars = self.__objSerialPort.inWaiting()
    if (intNoChars > 0):
    strReceivedString =
    self.__objSerialPort.read(intNoChars)
    #or you could fire an event
    self.newMessage(strReceivedString)
    except:
    #put any clean up code in here.
    raise
    finally:
    self.__objLock.release()

    You can then wrap this in a thread which runs the method every x
    milliseconds (the code to ensure that I have all the information is
    elsewhere in a superclass which deals with this use case across
    communication implementations). However you will have to be aware that
    you will need to lock around groups of calls to ensure you are receiving
    the correct information (as above).
    Therefore, although threading in Python is 'easy' you would still
    have to think about this issue, it is unavoidable in communications I'm
    afraid. Even if you could receive interrupts then you will likely be
    receiving the interrupt on a different thread of execution than the main
    anyways so you'll still have threading problems and need to lock around
    calls. This is the problem which I usually express as '/RS232C is not
    really a standard, more of a rumour/'.

    Interestingly, if we take the 'python in threading is easy'
    discussion (partially due to the GIL), then for this use case we could
    also say that threading in VB6 is even easier than python - I'll leave
    you to guess why!!!

    Cheers,

    Neil

    --

    Neil Benn
    Senior Automation Engineer
    Cenix BioScience
    BioInnovations Zentrum
    Tatzberg 47
    D-01307
    Dresden
    Germany

    Tel : +49 (0)351 4173 154
    e-mail :
    Cenix Website : http://www.cenix-bioscience.com
     
    Neil Benn, Jan 13, 2005
    #7
  8. engsol

    Neil Benn Guest

    <snip>

    Tabs got screwed up here is teh code again :

    def __checkSerial(self):

    self.__objLock.acquire()
    try:
    try:
    intNoChars = self.__objSerialPort.inWaiting()
    if (intNoChars > 0):
    strReceivedString =
    self.__objSerialPort.read(intNoChars)
    self.newMessage(strReceivedString)
    except:
    raise
    finally:
    self.__objLock.release()

    Cheers,

    Neil

    --

    Neil Benn
    Senior Automation Engineer
    Cenix BioScience
    BioInnovations Zentrum
    Tatzberg 47
    D-01307
    Dresden
    Germany

    Tel : +49 (0)351 4173 154
    e-mail :
    Cenix Website : http://www.cenix-bioscience.com
     
    Neil Benn, Jan 13, 2005
    #8
  9. engsol

    Peter Hansen Guest

    I'm with Grant on this: threads in Python are really easy. If
    you can describe a little more about what you need/would like
    to do with the serial data when it *does* arrive, and how that
    relates to the "lot of work" that the rest of the code is doing,
    I'm sure there are a number of helpful replies just waiting to
    be written to convince you how easy it really is.

    For example, if you need to retrieve the data from the serial
    ports right away, so the sending device doesn't block or lose
    data, but can afford to just store it for later processing
    when the main thread has time, that's only a few lines of
    code.

    If you actually want to reply immediately, then the only
    real question is how you come up with the reply. If it needs
    data from the main thread, we can show you several easy and
    appropriate ways to do that with no thread safety issues.

    If you're not experienced with threads, there's almost no place
    better to start learning than with Python and c.l.p. :)

    -Peter
     
    Peter Hansen, Jan 13, 2005
    #9
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.