For the Modbus master and servers to correctly communicate you need to pair the correct registers with the appropriate function makes sure that they are set as per the following guidelines
Function
|
Modbus Function
|
PLC Address
|
Coils
|
1, 5, 15
|
0x0001
|
Inputs
|
2
|
1x0001
|
Input registers
|
4
|
3x0001
|
Holding registers
|
3, 6, 16
|
4x0001
|
Different Modbus clients (Masters) would support some or all, the functions are:
1 - Read coils.
2 - Read discrete inputs.
3 - Read holding registers.
4 - Read input registers.
5 - Write single coil.
6 - Write single register.
15 - Write multiple coils.
16 - Write multiple registers.