Modbus data format (1)

Modbus has four kinds of data, DI, DO, AI, AO
DI: digital input, discrete input, an address and a data bit, the user can only read its status, can not be modified. For example, the state of the buttons and switches on the panel, and the fault state of the motor.
DO: digital output,Coil output, one address and one data bit, the user can set, reset, and read back the status, such as relay output, motor start and stop control signals.
AI: Analog input, input register , a 16-bit address data, the user can only read, not modify, such as a voltage reading.
AO: Analog output, holding register, an address 16-bit data, the user can write or read back, such as a current value to control the inverter.

Users who have no input signal can only look at it and cannot change it. The output signal is controlled by the user and can be read back. There is only one bit for discrete data and 16 bits for simulated data.