Hello,

A PLC is used to acquire data from its I/O cards. This data acquisition is not on Modbus protocol but is done in a continuous poll. The data acquired is stored in the PLC memory which has a modbus address.
Using this modbus address, the PC (which acts as a Modbus Master) requests for the data with the appropriate Modbus function code. Response is to be given by the PLC as per proper Modbus protocol.

Digital input and outputs are represented in the internal memory of the PLC and for reading/writing modbus protocol is not used. Modbus protocol will be used by the PC to communicate with the PLC.

Regards
Kunal