I work with rs422 similar projects almost every day.
The way you need to do is to send let say 10 byte of data
the last one is modulo 256 checksum. When receiver gets the data
based on checksum value can detect if there is an error. 
As far as what can you send there is no limit you can send
any info you want. Here is basic frame looks like.



     byte1,byte2,...,byte10_________delay

the way modulo 256 works is 

byte10= byte1+byte2+...+byte9

I am not sure if this is what you are asking.


Andre

 

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]On Behalf
Of Neil Baylis
Sent: Thursday, June 14, 2007 8:16 AM
To: Microcontroller discussion list - Public.
Subject: [PIC] 2 Wire RS-485 question


I'm running a network of instruments using a 4 wire RS-485 scheme,
where there is a single master and multiple slaves. All the slave
receivers connect to the master transmitter, and all the slave
transmitters connect to the master receiver. I have logic that
automatically switches all the slaves into receive mode whenever the
master drives it's transmit signal active.

All the slaves contain PICs (18fxxxx)  and use the UART to implement the RS-485.

I Now want to make a different version that operates as a party line,
with all transmitters and receivers connected together. To do so, each
node has to know how to turn off its transmitter as soon as it has
finished a transmission.

What's the cleanest way, in a PIC, to do this? I think I can't use the
UART Transmit interrupt, because it does not tell you when the last
byte has gone out on the wire. I know I could use a timer but consider
that my solution of last resort. I'm looking for any other ideas that
would let me turn off the transmitter at the earliest possible moment,
as soon as the last transmitted byte is on the wire.
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http://mailman.mit.edu/mailman/listinfo/piclist