[Twisted-Python] asyncronous access to serial ports under windows

Tom Brown brown at esteem.com
Fri Oct 26 23:10:53 EDT 2007

On Fri, 2007-10-26 at 15:42 -0700, Tom Brown wrote:
> Hi,
> I've been able to create an application in Linux that reads/writes
> multiple serial ports asyncronously. The setup code that does this looks
> like this:
> ...
> from twisted.internet.qtreactor import install
> a = QApplication(argv)
> install(a)
> from twisted.internet import reactor
> from twisted.internet.serialport import SerialPort
> ...
>   ports, badPorts = getGoodPorts()
>   if not ports:
>     exit(1)
>   data = ConfigData(join(sep, 'etc', 'qa.conf'))
>   dbInfo = copy(data['qadata'])
>   getLogin(dbInfo)
>   w = MainWindow(data, dbInfo, ports)
>   w.show()
>   reactor.addSystemEventTrigger('after', 'shutdown', a.quit)
>   a.connect(a, SIGNAL('lastWindowClosed()'), reactor.stop)
>   for portObj in w.portObjs:
>     SerialPort(portObj.scanner, portObj.port, reactor, baudrate=38400)
>     portObj.sendLine()
>   reactor.run()
> Where the portObj.scanner is an instance of a descendent of Protocol.
> Like I said, the above code works under Linux. Then I tried porting this
> to Windows. The first problem I came across is that the qtreactor.py
> would not work. I had to subclass QTReactor from Win32Reactor. It runs
> without errors. However, I am not reading anything off of the serial
> port. I can see the lights blink on the port when the portObj.sendLine()
> is called, so I believe I am writing to it ok and data is coming back.
> The data is just not read by the application. I think it must have
> something to do with the SerialPort instance not getting an even that
> data is ready. I suspect this is a Windows issue in that Windows is not
> signaling an event when data is ready to read on the serial port.
> Does anybody have any experience with this? Is there a work around? Am I
> doing something wrong?

Well, I found out that it has something to do with the qtreactor. If I
use just a Win32Reactor, it will read/write the serial port just fine. I
played around with writing to the serial port using
scanner.transport.write('\n') and found that
Win32Reactor.doWaitForMultipleEvents() is called when a Win32Reactor is
used. It is not called when a QTReactor(Win32Reactor) is used. The
question is why is this the case?


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