Modbus Functions

In the Modbus protocol, the functions determine whether data is to be read or written, and what kind of data is involved.

Function	Code	Description
Read coil status	1	Read digital outputs
Read input status	2	Read digital inputs
Read holding register	3	Read analog outputs and inputs / GPR
Read input register	4	Read analog inputs / GPR
Force single coil	5	Writing a digital output
Preset single register	6	Write one analog output / GPR
Diagnostics	8	Diagnostic
Force multiple coils	15	Write a number of digital outputs
Preset multiple register	16	Write a number of analog outputs / GPRs
Read / write registers	23	Write and read a number of process data outputs / GPRs

GPR (General Preset Register) - register structure of the Modbus interface (see Appendix)