Is it possible to use python to get True Full Duplex on a Serial port?

Grant Edwards invalid at invalid
Fri Aug 14 16:17:35 CEST 2009


On 2009-08-14, Hendrik van Rooyen <hendrik at microcorp.co.za> wrote:

> 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,

That's not true.  Linux/Unix does and always has supported
full-duplex communications on serial ports.

> and nobody contradicted me.

Um, sorry, I guess.

> 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.

That's the traditional way of doing full-duplex serial IO on
Unix back in the day before the select/poll system calls were
available.

> 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.

Nope.   I'll try to dig up an example, but that approach has
always worked for me.

> 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.

I've never observed that behavior.

> Is there a way to get full duplex, so that the transmit and
> receive are independent of each other?

That's the way serial ports work on Unix.

> Or are we stuck with a disk-like model that forces a sequence
> on reads and writes?

No.

-- 
Grant Edwards                   grante             Yow! Like I always say
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