From OpenSCADAWiki
Module of the protocol implementation
The ModBus protocol module contains the code implementing of the protocol part of ModBus, that is specifics of variants of the protocols ModBus/TCP, ModBus/RTU and ModBus/ASCII. The protocol module in conjunction with the selected transport is actively used by the data acquisition module for implementation the direct queries. Due for the protocol module is independent, using it you can create additional modules for data acquisition by non-standard functions of the ModBus expansion of various automation equipment.