But since the same starting address can refer to 2 data types (or more, if there are Coils and holding Registers, too), I'll also feel a little guiltyīesides my preferences, I have a 'real case scenario' that support the offset approach.

offset approach: if the user sends a read Input register function code for address 0 I will answer with the data but what if he wanted to read a Discrete Input but sent the wrong function code? Dumb him, ok.

absolute address approach: if the user sends a read Input register function code for address 10000 I will answer with an error code maybe the user wanted to read an Input register, but sent the wrong address, or maybe he wanted to read a Discrete Input but sent the wrong function code better notify him with an error than send a data that he'll probably misuse.

The first approach seems more reasonable for me, since it avoids registers access errors

offset approach: if the user wants to read the first Input register he'll send 0 as starting register from firmware side, I know it is an Input register from the function code, and I will look for the first Input register since I received a 0 offset.

absolute address approach: if the user wants to read the first Input register he'll send 30000 in the starting address field from firmware side, I know it is an Input register both from the function code and from the register address itself, and I know it's the first one since I received exactly the address of the first one.

Now, how to access these registers? What will be passed in the Modbus message starting address field? This is my firmware and can do whatever I want, but what is the standard? (this is what I don't understand) Reading here and there, it looks like these will translate to register addresses 10000, 1000, 30001.

From Modicon standard, the Discrete Inputs will have register number 1002, while the Input registers have 3002.

Let's say I have 2 Discrete Inputs and 2 Input registers. I'm developing a Modbus Server device, and I'm having a little trouble understanding how to name or address my registers.

Modbus tcp 400000