Is it possible to use python to get True Full Duplex on a Serial port?
Hendrik van Rooyen
hendrik at microcorp.co.za
Fri Aug 14 11:04:07 CEST 2009
In the past, on this group, I have made statements that said that on Linux,
the serial port handling somehow does not allow transmitting and receiving at
the same time, and nobody contradicted me.
I am running into the self same issue again.
What I normally do is to open the port like this:
port = open("/dev/ttyS0","r+b",0)
and then I unblock it with:
"""Given file 'f', sets its unblock flag to true."""
fcntl.fcntl(f.fileno(), fcntl.F_SETFL, os.O_NONBLOCK)
Then I can write a loop that uses a try-except to see if there are characters
available, and that examines a queue to see if there is something to
transmit, to give the appearance of full duplex functionality.
What I would really like is to have two threads - one that does blocking input
waiting for a character, and one that examines an output queue and transmits
the stuff it finds.
When I try to do this, it does not seem to work - as far as I can see, it is
as if the underlying implementation is somehow single threaded - if it is
waiting for a received character, it waits until something comes in before it
will transmit anything. So if you are talking to a device that does not
respond, the whole thing freezes up waiting for a character that never comes,
and nothing is transmitted either, despite the call to
port.write(somestring). The write blocks, and everything stops, waiting for
the receipt to finish.
Is there a way to get full duplex, so that the transmit and receive are
independent of each other?
Or are we stuck with a disk-like model that forces a sequence on reads and
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