Mr.Russell,
Thanks for your kind information.It will help me a lot.As you said,I already have acquired the softwares MODSIM & MODSCAN & my modbus communication is going to be finished for the digital ouput card.And i have the MODBUS specification from modbus.org.The PLC controller is using ATMega128 but for the modbus compatible input/output i am using 8051.Also for the keyboard scanning & LCD display driving i am using 8051.

I want to know is PLC and its I/O cards(modbus compatible) & PC are on the same modbus.If so the PLC MUST be the master as it has to read the input & set the output.Then in this case how the PC can be a master.And also if the PC need to read from a particular IO card then it will have to get the data from the PLC only NOT directly from the IO card.

I am having all the IO's(both digital and analog) on the modbus.

Thanks,

Regards,

S.Sheik mohamed