Detailed explanation of Modbus protocol

Modbus is an application layer message transmission protocol on the seventh layer of the OSI model. It provides client / server communication between devices connected to different types of buses or networks. At present, Modbus communication can be achieved in the following three ways:

TCP / IP on Ethernet;

Asynchronous serial transmission on various media (wired: EIA / TIA-232-F, EIA-422, EIA / TIA-485-A; optical fiber, wireless, etc.);

Modbus PLUS, a high-speed token delivery network.

As a national standard of China, the "Industrial Automation Network Specification Based on Modbus Protocol" provides a guide for the implementation of Modbus application protocol on serial links and TCP / IP based on the description of Modbus application protocol.

Modbus communication line

Modbus data unit The Modbus protocol defines a simple protocol data unit (PDU) that has nothing to do with the basic communication layer. Modbus protocol mapping on a specific bus or network can introduce some additional fields on the application data unit (ADU). The client that initiates Modbus transaction processing creates a Modbus PDU. The function code in it indicates to the server which operation will be performed. The function code is followed by a data field containing request and response parameters.

Universal Modbus frame

When Modbus is applied on a serial link, the address field of Modbus ADU contains only the slave address, and the error check code is the result of performing a "redundancy check" calculation based on the content of the message, according to the transmission mode used (RTU Or ASCII) using different calculation methods.

Modbus frame on serial link

When Modbus is applied on TCP / IP, a special header—MBAP header (Modbus application protocol header) will be used to identify the Modbus application data unit.

Modbus frame on TCP / IP

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