![]() The simplest Modbus codes read and write single bits of data in the device’s memory, for example the status of a PLC input channel, PLC output channel, or status bit within a PLC program. ![]() A Modbus-compliant industrial device has been programmed to understand these codes and respond to them appropriately when received. Modbus consists of a set of standardized digital codes intended to read data from and write data to industrial devices. The Modbus standard primarily defines the meaning of various Modbus commands, the addressing scheme used to place data within devices, and the formatting of the data. In other words, Modbus primarily falls within layer 7 of the OSI Reference Model (the so-called “Application Layer”) and therefore is compatible 445 with any lower-level communication protocols including EIA/TIA-232, EIA/TIA-485, Ethernet (the latter via TCP/IP), and a special token-passing network also developed by Modicon called Modbus Plus. The Modbus standard does not specify any details of physical networking, and thus may be deployed on many different types of physical networks. ![]() Developed by the Modicon company (the original manufacturer of the Programmable Logic Controller, or PLC) in 1979 for use in its industrial control products, Modbus is a protocol designed specifically for exchanging process data between industrial control devices. ![]()
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