Why do we need Input Registers, Coil Bits and Input Bits in Modbus

Question

In modbus there are 4 object types which are Input registers (R), Holding registers (R/W), Input Bits (R) and Coil Bits (R/W).

What's stopping someone just using holding registers? Why are the other three data types needed?

Answer 1

This protocol was originally developed for serial connection devices. Each of the 4 types can be classified as proprietary equipment.

Input - discrete input (button, limit switch, etc.)

Register input - analog input (temperature, pressure, etc.)

Coil - discrete output (relay, etc.)

Holding register - analogue output (typically 0-10 V, 4-20 mA) (setting speed or temperature, etc.)

Accordingly, we have no right to write something or somehow change the input or input of the register.