The Modbus Protocol was initially developed by Modicon in 1979, and today it is an open standard, maintained by the Modbus Organization (modbus.org), and implemented by hundreds of manufacturers in thousands of devices. Schneider Electric, current controller of Modicon, transferred protocol rights to the Modbus Organization in April 2004, and committed to keep Modbus as an open protocol. Its specification can be downloaded for free at Organization's website, www.modbus.org, and protocol's usage is free of licensing fees.

This protocol is based on command and response messages, positioned at layer 7 (seven) of the OSI (Open Systems Interconnection) model, the application layer, which provides Client and Server communication among devices connected to different types of networks. It offers services with functions defined by a code with 8 (eight) bits. The existing categories of function codes are the following:

•Public function codes: Functions well-defined by this protocol, guaranteed to be unique, validated by the Modbus community, and publicly documented in MB IETF RFC. They can assume values ranging from 1 to 64, from 73 to 99, and from 111 to 127

•User-defined function codes: Non-standard functions, which do not need Modbus.org approval, without any guarantee of being unique, and freely implementable. They can assume values ranging from 65 to 72 and from 100 to 110

•Reserved function codes: Codes with values inside the range of public functions, currently used by some manufacturers for legacy products, and not publicly available anymore. Examples are 9, 10, 13, 14, 41, 42, 90, 91, 125, 126, and 127 codes. For more information, please check Annex A of the specification of this protocol, version 1.1b, which is available at protocol's official website

This Driver currently implements 11 of all 19 public functions defined on the current version of the specification of this protocol, 1.1b, as well as some specific functions of manufacturers or related to specific features of this Driver, known as Special Functions. All public functions implemented are described on topic Supported Functions.

This Driver supports secure communication in compliance with the Modbus/TCP Security specification, maintained by the Modbus Organization, which adds a TLS 1.2 security layer on top of traditional Modbus TCP transport, including real certificate chain validation, remote device identity verification, and restriction to the cipher suites required by this specification. To enable it, select the Use SSL option on the Ethernet tab of the properties window of this Driver and, on the SSL Settings option, provide the Certificate Authority (CA) certificate used to validate the identity of a remote device. When a remote device requires mutual authentication (mTLS), also provide a proper certificate and private key in the Certificate Path and Private Key fields.