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modbusmodbus-tcp

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

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?

Solution

  • 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.