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An an application layer messaging protocol at Level 7 of the OSI Model that provides client/server communication between devices connected on different types of buses or networks. The Modbus messaging structure was developed by Modicon in 1979. Different versions of Mopdbus used today include Modbus RTU (based on serial communication like RS485 and RS232), Modbus ASCII and Modbus TCP, which is the Modbus RTU protocol embedded into TCIP packets. It’s an open protocol, meaning the specification is available free of charge for download, and there are no licensing fees required for using Modbus or Modbus TCP/IP protocols.
