pyserial and com port interrupts

[Neil Benn]

engsol wrote:

>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
>  
>
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 : benn at cenix-bioscience.com
Cenix Website : http://www.cenix-bioscience.com