The way we handle end of transmission flags is that
we preset an interrupt timer, or one serviced periodically,
with a value that times out after the last character has sent its
stop bits, PLUS a stop bit. We then turnaround the RS485
channel to receive when that timer downcounts to 0.

It is only necessary to set the timer for the LAST character
of the message.

--Bob
EDTec LLC

----- Original Message -----
From: "Geo" <lintech@BLUEYONDER.CO.UK>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Friday, November 22, 2002 8:44 AM
Subject: Re: [EE]: Rs232 - RS485 converter circuit


> On 22 Nov 2002, at 6:25, Dave Tweed wrote:
>
> > On an 8051 system with RS-485 that I built, I had a set of software
timers
> > that were driven by an interrupt from the same hardware timer that
> > generated the UART bit rate. When I put the last byte into the UART
> > transmit buffer, I just started a software timer for 20 bit times, and
when
> > that timer expired, I turned off the transmitter. 20 bit times allows
> > enough time for the last byte to go out, plus one full byte time of
> > enforced idle time between packets.
>
> Hmm - well with the AVR 2313 etc there is also a UART Transmit Complete
> interrupt "when the entire character (including the stop bit) in the
Transmit
> Shift register has been shifted out and no new data has been written to
the
> UDR. This flag is especially useful in half-duplex communications
interfaces,
> where a transmitting application must enter receive mode and free the
> communications bus immediately after completing the transmission".
>
> I used this for a 485 controlled remote device. Is it true that the PICs
don't
> have this facilty? (I've only used 16f84s).
>
> George Smith
>
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>
>

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