> i need to comunicate with PLC
> through modbus rtu protocol.

The protocol itself is simple. Look
at specs:

www.modicon.com/techpubs/toc7.html

RTU protocol was used in my last
project (12 AT89C2051's as PLCs, one
AT89C52 as data acquisitor). Since it
is commercial, I can not send code
pieces, but some notes will be useful.
Do calculate CRC while receiving /
transmitting data, byte by byte. This
will save a lot of time and delays will
be always the same.
Take a serious care for byte timeouts.
Let make them configurable, say, with 1
ms accuracy. This can be the only way
to deal with line noise.
Any more detailed questions?

--
Tomas