Accessing Modbus register Modicon Pentium


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I am using Modicon Pentium PLC. I want to map IO status Modbus list to holding register. I am using Unity Pro XL programming. How can I access this Modbus register address in programming?

As an example:
20001 (digital input memory address) maps to 40001 (holding register address).
How does your PLC communicate with Modbus device? Is your PLC connected directly to the Modbus device or through gateway device?

40001 holding register is a word, 16 bits cannot map to digital input address. And 2001 is not digital input address.