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Firstly multiplexing system cut off the trunk line of the remote process control RS485,and pided which into two domains.Then TCP / IP data with different RS485 protocols formats is transmitted over Ethernet [4] between regions withoutdamaging the raw data, while other RS485 applications can also be added. Principle structure over Ethernet is shown in Fig.2, the ID number begin with # code in the Fig.2 represents the existing different RS485 system in industrial and mining enterprises, dotted lines represent virtual circuits after different RS485 transmit over Ethernet, which is similar to virtual communication between peer-to-peer layer based on TCP/IP.As can be seen from the figure, RS485 system can access to the Internet architecture by this structure,by this way the kind of communication based on IP without domain is achieved.The advantage of this architecture is no limitation to communication distance, communication speed and protocol types.
It’s necessary to design application layer protocol format because multiple different RS485 can make peer-to-peer communication using TCP/IP data over Ethernet. Application layer protocol format is shown in Fig.3, the code format can complete the RS485 communication between corresponding entities [5].In the protocol format,command, parameters and priority level each occupy one byte, the contents occupy 1024 bytes.About the specific definition of protocol format is shown in Tab.1. The command is 0x22 when communication between the RS485 entities defined in Tab.1.The command changed accordingly if need to set the interface parameter value. Parameters bytes can represent 256 different interface number, priority level bytes can represent 256 priority, higher the number, the higher the priority, the content bytes represent the raw data transmitted between RS485 entities or the value when configure the interface parameter. In the design of the data protocol format, by selecting the appropriate content size and packaging UDP,maximum
response time is set.At the same time multiplexing system deal with each I/O buffer data by descending priority sequence to meet the communication structural requirements.
V. SYSTEM DESIGN OVER ETHERNET Design over Ethernet adopts two modes co-existing of client and server to deal with multiple RS485 serial port raw data.The client mode receives the raw data of the RS485 serial port, packages data, sends packaged UDP data; the server mode receives UDP data, unpacks protocol data, sends uncompressed data to the serial port specified by the protocol number. UDP headers contain fewer bytes and its response speed is very fast but consume less load and resources than the TCP, so it is very adapted to the transport layer protocol above IP. Multiplexing system requires fast and efficient multiple I/O handling, which commonly adopts the blocking and nonblocking I /O operation mode [6-7].But for more complex I/O operation, the two models are often inadequate. I/O multiplexing is adapted to multiple I/O efficient access.The two I/O modes are on the contrary, non-blocking I/O mode allows multiplexing hung itself up, and which can know it immediately , correctly and efficiently deal with the I/O once one or more I/O conditions occurred, which simultaneously avoiding multithreading resources is took up too much as blocking and round robin resources. Multiplexing and demultiplexing flow chart based on I/O is shown in Figure 4. Multi-thread multiplexing and demultiplexing structure based on the blocking I/O type is also designed, whose flow chart is shown in Fig.5. Contrast with I/O multiplexing,multi-thread multiplexing based on blocking I/O whose performance and efficiency are not more than I/O multiplexing,especially in many I/O case ,in which more thread need to be opened and more system resources are took up.
Aim at RS485 communication protocol are nonstandardized and transmission line can not be multiplexed, this paper achieve the technology of RS485 over Ethernet which allows multiple different RS485 transparently transmit TCP/IP data by Ethernet and multiplexed to one line. In this paper, start with line multiplexing, design framework model of the
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