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UDP / IP protocol under the Research and Implementation of Modbus protocol

In Electronic Infomation Category: U | on April 12,2011

Abstract: Industrial Ethernet M odbus / TCP has become the de facto industry standard Ethernet communication. This depth of the research communication standards and HFA1113IB datasheet and make improvements, proposed in the UDP / IP protocol on the implementation of the M odbus agreement, followed by real-time and HFA1113IB price and reliability from the two aspects of the M odbus / UDP protocol stack of the advantages of the laboratory and HFA1113IB suppliers and then the development of a monitoring terminal test system set up experimental testing of the protocol stack. Theory and practice show that: M odbus / UDP protocol can achieve interoperability with great difficulty, real time and high reliability, Zu Wangfang it flexible.

With Ethernet technology and the rapid development of Internet-based TCP / IP protocol for Industrial Ethernet Fieldbus research has become the highlight, in particular, Modbus / TCP protocol agreement with its simple, open, easy to understand and stable performance to become the de facto standard Ethernet communications industry. At present, many domestic and foreign manufacturers of PLC, monitoring and control terminals, the configuration software and other products are used Modbus / RTU and Modbus / TCP protocol, but along with the further development of industrial Ethernet technology and a breakthrough on key issues, more and more industrial manufacturers use UDP as a transport layer protocol to meet the real time and deterministic communication needs, such as: Siemens ProF ibnet, times Calais Pow erlink, Modbus-IDA, have real-time communication in industrial use of Ethernet UDP protocol. This paper describes a new communication protocol Modbus / UDP, the transport layer using UDP protocol, and Modbus / TCP to further improve the system compared to real-time and networking flexibility.

1 Modbus / UDP Protocol

1.1 Modbus protocol description

M odbus protocol uses a request / response approach to client and server application layer communication, the agreement provides an independent basis for a simple communication layer protocol data unit PDU, PDU frame structure including the function code, data fields and error school experience domain; specific field bus based on the PDU header information to introduce some form of application data units ADU. Figure 1 shows a generic Modbus frame.

First implemented on different physical media of the Modbus protocol, the PDU unit is uniform, while the additional address field and the error checking field bus or network to be based on different specific requirements and format of the organization and implementation. Second, the definition of the three types of Modbus function code: public function code, user-defined function code and function code reserved.


Figure 1 M odbus frame

1.2 UDP Modbus protocol on the introduction

And M odbus / TCP protocol similar, M odbus / UDP in the TCP / IP application layer protocol based on the use of Modbus data, the transport layer using UDP protocol, the network layer using IP protocol. UDP / IP protocol on the application data unit MBAP using Modbus and TCP / IP the same structure, including 2-byte transaction identifier, 2 bytes of protocol identifier, 2 bytes and 1 byte length byte unit identifier. UDP protocol is not connection-oriented, communication is not required to establish a connection with each other, for not very high reliability requirements of the occasion, real-time high, effective communication. Use UDP to send a Modbus message frame, it is simple to use UDP protocol and the efficient communication effects. The practical application of the encapsulated Modbus message frame as part of UDP data sent directly to the server 502 port, the server parses the incoming Modbus / UDP datagrams, the M odbus response to the request data frame rotation client.

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