| With development of science and technology, modern industrial systems become more complex, and testing and control requirements of the process of industrial production is also rising. The traditional detection and control can no longer meet the requirements of the modern industrial. The emergence and development of fieldbus technology causes a revolution in the field of industrial automation. Distributed control system is gradually replaced by fieldbus control system. And the fieldbus control system becomes the main force in the field of industrial control.At present, there are many fieldbus types in the international. Modbus protocol which is the first protocol used in the industrial, with its high reliability, real-time and easy maintenance, has been widely used in industrial production automation control.In this paper, we analyze the Modbus protocol, then develop a set of Modbus slave protocol stack based on real-time operating system RT-Thread with C language.Firstly, we research the architecture of the Modbus protocol,including the message frame format, data transfer mode, transaction processing, and abnormal response, then analyze the Modbus serial link protocol and Modbus TCP/IP protocol carefully. On this basis, we design a set of Modbus protocol stack with C language.Secondly, in order to test and verify the Modbus protocol stack,we design a of development board which is called LM3S6911based on the ARM (?) Cortex-M3, and use the board as the test system of Modbus protocol stack. The evelopment board can use the RS-485interface and Ethernet interface to connecte to the master system in order to communicate with the master system.Finally, we set up the testing platform of Modbus protocol stack in the lab.We use the Modbus Polltool which is installed on the computer to simulate the master system,and use the development board as the slave system. After a series of communication test, the results show that the Modbus protocol stack can works stably. |
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