writing status to PLC

Hi,
I was able to associate status of DO on my PLC via OPC connection. I could use your help on how to write (i mean change) status to my plc. Should I write a script or can i somehow do this through OPC server? Could you give me an example of how to achieve this?
Thanxs in advance!

Connecting to MODBUS PLC

MODBUS Plugin Configuration and test

Introduction

MODBUS is an application layer messaging protocol, positioned at level 7 of the OSI model, that provides client/server communication between devices connected on different types of buses or networks.
It is currently implemented using:
TCP/IP over Ethernet at a reserved system port 502 on the TCP/IP stack
Asynchronous serial transmission
MODBUS is a request/reply protocol and offers services specified by function codes. MODBUS function codes are elements of MODBUS request/reply PDUs.


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