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Modbus is an application layer data protocol present at Level 7 of the Open System Interconnection (OSI) model of communication. It provides a connection between servers and clients located on different devices and networks. This serial communication protocol was introduced in 1979 by Modicon as a messaging structure between devices connected through buses or networks.
Modbus is an open-source protocol, which means the application is available online free of charge for downloading and no license fee is changed for using it. A number of versions of Modbus are available, including Modbus RTU (based on serial communication like RS485 and RS232), Modbus ASCII and Modbus TCP, which is the Modbus RTU protocol embedded into TCP/IP packets. A number of devices connected serially in a network can communicate via the Modbus application protocol. A device requesting information is called a Modbus master, whereas a device providing data is a Modbus slave. A typical network has only one master and up to 247 slave devices.