All the RS485 half duplex stuff I've done has always used the TXIF to
determine when to put another byte into the transmit buffer.  I use the
TXSTA.TRM bit to determine when the shift register is empty so that I
can disable the RX485 driver.  I don't bother looking at this flag until
I know I've put the last byte into the TXREG.  Then when TXIF is true
(TXREG is now empty) I test TXSTA.TRM.

John Dammeyer

Automation Artisans Inc.
http://www.autoartisans.com
Ph. 1 250 544 4950


> -----Original Message-----
> From: piclist-bounces@mit.edu 
> [mailto:piclist-bounces@mit.edu] On Behalf Of Dario Greggio
> Sent: Thursday, June 14, 2007 1:45 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [PIC] 2 Wire RS-485 question
> 
> 
> Robert Ammerman wrote:
> 
> > In the transmitter:
> > 
> > (1) Send an additional byte at the end of each 'packet' (in 
> response to 
> > TXIF)
> > (2) One BIT (not byte!) time later disable the transmitter
> 
> (and related to other answers - just my opinion).
> I'd avoid sending another "byte", i.e. another start bit etc. I 
> preferred just keeping the Bus "high" for some bit-time.
> 
> I also find sooo "oldish" :) a method, to use an external 555 
> or other 
> switch to recover from TX mode into RX mode...
> Having a MCU should allow for easier handling.
> 
> -- 
> Ciao, Dario
> -- 
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