Which protocol is characterized as being open standard and royalty-free?

Modbus is recognized as an open standard and royalty-free protocol. This means that it is publicly available for use without the need for licensing fees or royalties, which encourages widespread adoption and implementation in both industrial and commercial environments. The protocol was originally developed by Modicon (now a part of Schneider Electric) for use in PLC communications, but its open nature has led to its use in various devices and applications. This accessibility has made it a popular choice for integrating and connecting various types of automation equipment, ensuring interoperability among different manufacturers’ devices.

In contrast, other protocols like Profibus are proprietary to certain organizations and typically require licensing or adherence to specific standards established by governing bodies. DNP3, while also an open standard, primarily serves the utility sector, focusing on functionalities that cater to specific needs, and may not have the same level of general adoption as Modbus. ISA-95 refers to a standard for integrating enterprise and control systems, which is more about defining models and terminology rather than serving as a direct communication protocol like Modbus does. This distinction underscores Modbus's unique position in its open and royalty-free nature, allowing developers and engineers easier access to utilize and implement the protocol in multiple applications.