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'RS485 comms problem'
1999\02\24@204757 by Stuart O'Reilly

Hello everyone, I have some problems with my RS485 network and I'm
hoping someone could shed some light on them. My setup at the moment is
a simstick with a 16c84 running at 4Mhz as a slave, I have another setup
as the master. The master receives commands from my PC via the RS232
port and then proceeds to send them down the RS485 network to the slave,
the slave receives the commands and does what it should and then sends
back some data as a reply. Around 70% of the time the data that is
returned as a reply is corrupt, but the slave executes the commands sent
to it ok. I'm using the LT1487 chips that were used in the Nuts n Volts
example and I'm running it at 9600baud, Ive just discovered if i lower
the speed right down to 300 baud it functions fine , Any suggestions ?
(if it was the crystal you would expect it to have errors on receiving
as well as txing, wouldn't you?)
The other problem i am having with this network is if i put the 120r
terminating resistors on, the network it sends no data at all (put a cro
on it and you get a flat line) but without the resistors you get
beautiful square waves

Thanks for your help

P.S. I'm using Pic Basic Pro

1999\02\24@213426 by Gerhard Fiedler

picon face
At 12:39 02/25/99 +1100, Stuart O'Reilly wrote:
>(if it was the crystal you would expect it to have errors on receiving
>as well as txing, wouldn't you?)

don't know much about the rest of your problems, but this is not
necessarily true (even if i don't think that it's the matter in your case,
if 300bps is the fastest you can make it go). if one crystal is faster than
the other, one receiver sees too high a speed and the other too low a
speed, and the tolerances of the algorithm are not necessarily the same
with over- and under-speed. so it might be that one signal is within the
tolerance of the receiver (even if both use the same algorithm or uart) but
not the other.


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