Which PLC communications protocol is in the public domain?

MODBUS RTU is the correct answer because it is an open-standard communication protocol widely used in industrial environments for connecting electronic devices. Since it is in the public domain, manufacturers can implement MODBUS RTU in their products without needing licensing fees or proprietary constraints. This encourages interoperability between different devices from various manufacturers, promoting easy integration in control and automation systems.

The protocol operates in a master/slave configuration, providing a straightforward way to manage communication over serial lines. MODBUS RTU is recognized not only for its widespread use but also for its simplicity, making it accessible for many applications in automation and control systems.

In contrast, protocols like DH+, Modicon+, and Honeywell LCN are proprietary and not available for public use without specific licensing agreements. This limits the ability of manufacturers to freely implement these protocols, which could restrict their application in different systems or devices. Thus, MODBUS RTU stands out for its accessibility and broad acceptance in the industry.