siemens s7-200

Hi friends
I am doing a Project with S7-200 Plc. All are digital input and dihital outputs only.
But my project spec reads a line as follows:
" The PLC system should be communicated with the DCS system thru RS485 communication"
I havent done this before. Please help me for the instruction.My CPU model is CPU-226.
Do we nee to use modbus lib instruction for this?
Thanks

MPI and Profibus are also RS485.
To specify a connection to a DCS as just "RS485" is not detailed enough.
It is most important that the protocol is exactly specified. Dont just assume a certain protocol, get it confirmed.

Hi
The protocol is MODBUS.
The PLC is programmed to be the MODBUS Slave.
The Slave is the Client's DCS. The DCS is supposed to be in the site in Qatar.But we need to test or scan the outputs thru the MODBUS in the Factory during FAT.HOw do we do it?
We tried to use MODSCAN in our PC nd connect to the PLC , But we are unable to connect. Is there anyother way to check the outputs thru the MODBUS?
chitra

Sorry
The MASTER is the client's DCS.Typo error in the third sentence
chitra