Modbus is an industrial control bus protocol invented by Modicon (now Schneider Electric) in 1979. It is the world’s first bus protocol that is actually used in industrial fields. Modbus has become a general-purpose communication protocol because of its simplicity, robustness, openness, rs232 to ethernet adapter and no need for licensing. In order to adapt to the Ethernet environment, Modbus is encapsulated in TCP packets, and by default it is transmitted through port 502 of the TCP protocol.
The system based on the Modbus protocol is composed of a programmable logic controller with an intelligent terminal and a computer connected through a public line or a local dedicated line. Its system structure includes both hardware and software. It can be applied to various data acquisition and process monitoring.
The Modbus protocol adopts a master-slave structure to provide a communication connection between the client and the server.
The Modbus protocol defines a protocol data unit (Protocol Description Unit, PDU) that has nothing to do with basic communication, and describes the basic functions of the protocol. industrial iot gateway The PDU is part of the Application Data Unit (ADU). In addition, the ADU also includes an additional address field, an error check field, and the actual transmitted data. This data may be business data, or it may be an instruction or response. Information or alarm information, etc.
The Modbus protocol includes ASCII, RTU, and TCP three message types, and the serial port can be used to transmit data and instructions.