[Tutor] COM Ports

[Michael P. Reilly]

kromag@nsacom.net wrote
> I am using (at this moment) linux for this particular job, but would have no 
> problem switching to Free/NetBSD (I don't think PostGreSQL will run on 
> OpenBSD yet...). As I understand it, one has to create a C module to read 
> from the serial port in python under unix. I have read the Linux-Serial-
> Programming-HOWTO and the C Extentions Overview in Programming Python and am 
> beginning to get some idea of how creating such a mawnstah might work, but 
> would really like to see some example code that is a little less generic. 

You don't need to create a new C module just to access the serial lines,
open the device file for the serial line (eg. /dev/ttyS0), configure the
line as needed with the termios/TERMIOS modules and read and write the
data as needed.  The select module can help you for when data is ready.

  >>> import termios, TERMIOS
  >>> f= open('/dev/ttyS2', 'r+')
  >>> settings = termios.tcgetattr(f.fileno())
  >>> settings[3] = settings[3] & ~TERMIOS.ICANON
  >>> settings[4] = settings[5] = TERMIOS.B9600
  >>> termios.tcsetattr(f.fileno(), settings, TERMIOS.TCSAFLUSH)
  >>> f.write('AT\r')  # Hayes modem protocol
  >>> f.read(10)
  'AT\n'  # this is contrived, it might return 'AT\r\n'
  >>>

There are other packages out there that will do this for you as well.
[Shameless self-promotion] Such as ExpectPy, which is a C module interface
to the Expect library (usually associated with Tcl); it doesn't sound
like you need ExpectPy though.  Definately look into termios.

Myself, as a developer of FreeBSD products, I'd stay with Linux.
There are a lot of problems that FreeBSD creates (and if you were to
use FreeBSD, do NOT use the canned Python 1.5.2 in the distributions,
build it yourself).

  -Arcege

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| Michael P. Reilly                | arcege@speakeasy.net              |