Realization of Ethernet Communication between OMRON PLC and Host Computer Based on FINS Protocol

May 30, 2020

Realization of Ethernet Communication between OMRON PLC and Host Computer Based on FINS Protocol

0 Introduction In the modern industrial automation control system, programmable logic controller (PLC) as a commonly used field control device, the host computer as a data acquisition and a human-machine interface has been widely used. In the past, in the development of engineering projects, the communication between the PLC and the host computer often used RS-232C or RS-485 serial mode. This method was difficult to meet the control system with large data volume, long communication distance, and high real-time requirements. With the development, popularization and promotion of Internet technology, Ethernet technology has been rapidly developed. The increase in transmission rate and the application of switching technology have solved the non-deterministic problems of Ethernet communication, making industrial Ethernet widely applicable. The field of industrial information control is also the future development trend of industrial information control.
FINS (factory interface network service) communication protocol is a command / response system developed by Omron for industrial automation control network. The FINS instruction can achieve seamless communication between various networks, including Ethernet (Ethernet) for information networks, Controller Link and SYSMAC LINK for control networks. By sending FINS commands through programming, the host computer or PLC can read and write the contents of another PLC data area, and even control its running status, thereby simplifying the user program. The FINS protocol supports industrial Ethernet, which provides the possibility for OMRON PLC to communicate with the host Ethernet.

1 OMRON PLC and host computer communication method At present, in the Omron PLC network composition, the host computer and PLC communication can adopt RS232C / 485 serial communication, Controller Link communication and industrial Ethernet communication. Their main performance parameters are shown in Table 1.

Literature [3] introduces the scheme of using RS232C / 485 serial communication, its communication rate is only 9600b / s, the rate is slow, it is difficult to adapt to modern control systems with large data volume, long communication distance, and high real-time requirements .
Reference [2] gives the design scheme of Controller Link communication based on FINS protocol. Its maximum rate can reach 2Mb / s, the maximum transmission distance of the entire network is 500m, and the CLK support card needs to be installed on the host computer on the hardware. Performance and application flexibility are not as good as industrial Ethernet.
Among the three communication methods, the advantages of Industrial Ethernet are quite obvious. The transmission rate can reach 10 / 100Mb / s (depending on the actual network environment); the transmission distance between the two nodes can reach 100m, there is no limit to the transmission distance of the entire network; the maximum number of nodes in the network can reach 254, can Realize 1 (PLC): N (PC), N: N, N: 1 and other network forms. All of these provide favorable conditions for constructing industrial network information systems of various sizes, with good scalability, practicality, and flexibility. The host computers currently used are equipped with Ethernet cards, so no additional hardware equipment is required on the host computer side. Therefore, the use of industrial Ethernet has great practical significance for improving the level of factory automation information control.

2 Ethernet FINS communication In Ethernet FINS communication, various data information is sent and received on the Ethernet in the form of UDP / IP packets or TCP / IP packets. Among them, the IP address is used by the remote device at the Internet layer, and the FINS node address is used at the application layer. The local UDP or TCP port number is defined in the transport layer. It provides a communication port for the application layer (that is, FINS communication). The default setting is 9600. The user can modify it according to the actual situation, but in the same network, the communication port number of each device must be consistent. Figure 1 FINS Ethernet communication protocol model

Generally, the IP address is used for Ethernet communication. In FINS communication, the network number, node number, and unit number are used to define addresses for different devices (including PLC, PC, and other field devices). This provides a unified addressing method for the communication of various devices between different networks. In Ethernet FINS communication, an important function of Omron's Ethernet unit is to be able to convert between IP addresses and Fins node addresses. The conversion methods include automatic conversion, IP address table and composite address table The number of address conversions is also limited depending on the module model and conversion method. Ethernet FINS communication service is a communication method based on UDP / IP, which is called FINS / UDP method, and all Ethernet products related to Omron support this method. In addition, the CS1W-ETN21 and CJ1W-ETN21 Ethernet communication modules also support TCP / IP protocol, called FINS / TCP mode.
2.1 FI NS / UDP mode FINS / UDP mode is a FINS communication method using UDP / IP protocol. UDP / IP is a connectionless communication protocol. When a message is sent from one node to another, the two nodes have no clearly connected peer relationship. The UDP protocol has a faster transmission speed, but the reliability of data communication is not as high as the TCP protocol.

As shown in Figure 2, it is a UDP frame structure used for sending and receiving in Ethernet. FINS / UDP uses a nested format of data packets, namely Ethernet header, IP header, UDP header and FINS frame. A UDP data segment (FINS frame) exceeding 1472 bytes will be divided into several data packets for transmission. Separate UDP data will be automatically combined at the UDP / IP protocol layer. Normally, there is no need to pay attention to the data segmentation of the application layer, but in a multi-layer IP network, 1427 byte UDP packets may not be sent. In this system, you need to use FINS / TCP.

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