OneShotTriggersDtr485

Category:Sasecurity
http://groups.google.com/group/comp.os.linux.development.system/browse_frm/thread/d2f563a07cad5f28/762f2fe665b9cf6d?lnk=gst&q=RS485&rnum=7#762f2fe665b9cf6d
=== One shot rs232 falling edge triggers 485 ===
Trigger RS485
Olav.....I went thru the very same thought process you are going thru. I also
use a rs485 network
in the half duplex mode (on 2 wires) communicating with some 8051 based
controllers....after much
work the solution i used was a VERY SIMPLE hardware solution....turns out there
are times when
a few parts replace alot of software and result in a much more reliable
system....since i had built a
simple rs232 to rs485 converter (worked the same as yours...DTR enabled the
rs485 tranmit) i just
added a single one shot that was triggered off the falling edge of the rs232
signal and whose output
enabled the rs485 transmit....set the one shot for the byte time and your
DONE!!!! now i fought this
solution for a few weeks as i am a software guy at heart but when i saw how
simple it was in the
hardware domain and versus how complex it was in the software domain i switched.
This approach
allowed me to use standard drivers and also the standard linux time slice
scheduler rather then the
SCHED_FIFO mod to be able to get the execution resoltion i needed to control the
DTR lead from
user space.
 
=== Digital pot ===
Use a RC combination to set the length of the one shot. There probably are IC's that can be clocked for variable 
one shot time lengths. 
=== asdf ===
More so than you have realized yet. I'm not familiar with the Linux
implementation, but most serial drivers consider the buffer flushed,
and done as soon as the last byte to be transmitted as been sent to
the uart. The problem is, it takes about 1mS (9600 baud) for the uart
to finish transmitting the character. If your using a buffered uart,
the problem gets much worse!
If you manage to get a fast DTR shut off routine working, you WILL cut
off the last few bits of your last character. Been there, done that.
The easiest solution is to not control the 485 transmitter in
software. Use a one shot circuit in the RS-232 to 485 converter to do
the dirty work for you.
B&B has just such an animal, and it works great! We use about 20-30
485TBLED units per year. DB25 for RS-232 side, Screw terminals for the
485 side. 
=== Use four wires to Rx the Tx data ===
=== Links ===
RS485