Programmable Logic Controller is short for Programmable Logic Controller in Chinese. With the rapid development of information technology and hardware performance of programmable logic controller in recent years, the requirement of real-time communication between upper equipment and PLC has been constantly improved. Besides the continuous improvement of hardware configuration performance of PLC, what communication mode or protocol communication is adopted.
Especially important, for example, direct protocol communication is three to four times faster than third-party plug-ins. Nowadays most programmable controllers are based on the communication protocol of industrial Ethernet for data communication. Industrial Ethernet is an extension of traditional ethernet. It meets the specific requirements of industrial production process by strengthening the real-time and security of traditional ethernet. Programmable controller is a new type of industrial control device which integrates computer technology, automatic control technology and communication technology. PLC has been widely used in the field of industrial automation control because of its high reliability, powerful function and convenient use. Light weight and small size: Super-miniature programmable controllers under current technical conditions have a weight of less than 150 g, a bottom size of less than 100 mm, and a function of less than 10 W, which is of key significance for the realization of mechatronics; Anti-interference: programmable controllers are upgraded on the basis of large-scale integrated circuit application technology. The anti-jamming characteristic of the whole control system is studied. At the same time, the automatic fault detection function of the programmable controller itself can also realize the safe and effective operation of the whole control system under the automatic alarm processing; operability: Firstly, from the application point of view, the switching logic control instructions supported by the programmable controller can turn the complex circuit diagram design operation into operation.
It becomes a simple programming language, which makes it easier to use [1]. Periodic I/O communication is often used in remote I/O links of PLC. The remote I/O link works in the master-slave mode.
The main unit of PLC remote I/O is the master station, while the other remote I/O units are all the slave stations.
A “remote I/O buffer” is set up in the main station, which is divided into several sub-boxes corresponding to each slave station one by one.
Each sub-box is divided into two sub-boxes, one tube sending and one tube receiving. In the master station, the communication processor exchanges data with each slave station in order by periodic scanning. The data sent in the sub-box corresponding to the master station is sent to the slave station, and the data read from the slave station is put into the receiving sub-box corresponding to the slave station. In this way, the “remote I/O buffer” in the main station is periodically refreshed. Global I/O communication mode is a serial shared memory area communication mode, which is mainly used for communication between PLCs with link area. In the I/O area of each PLC in the PLC network, a block is designated as the link area, and each link area adopts the mailbox structure. The sending area with the same number is the same size as the receiving area, occupying the same address segment, one is the sending area, the other is the receiving area. Communicate by broadcasting. PLC1 broadcasts the data of 1 # sending area on the PLC network. After listening to it, PLC2 and PLC3 receive it and store it in their respective 1 # receiving area. PLC2 broadcasts the data of 2 # sending area on the PLC network. PLC1 and PLC3 receive it and store it in their respective 2 # receiving area. PLC3 broadcasts the data of 3 # sending area on the PLC network. PLC1 and PLC2 receive it and store it in their respective 3 # receiving area. Obviously, through the above broadcasting communication process, the data in each link area of PLC1, PLC2 and PLC3 are the same. This process is called the equivalence process. Through equivalent communication, the data in the link area of each PLC in the PLC network is consistent. Master-slave bus communication mode, also known as 1:N communication mode, refers to the bus structure of the PLC sub-network has N stations, of which only one master station, the rest are slave stations. 1:N communication mode uses centralized access control technology to distribute bus usage rights, usually using polling table method. The so-called polling table is a sequence table of slave numbers, which is configured in the master station. The master station inquires the slave station according to the order of the polling table to see if it wants to use the bus, so as to achieve the purpose of distributing the right to use the bus.
For stations with high real-time requirements, the slave number can appear several times in the polling table, which gives the station higher priority in communication. Token bus communication mode, also known as N:N communication mode, refers to the bus structure of the PLC sub-network has N stations, their equal status is not divided between master station and slave station, it can also be said that N stations are the master station. The token bus access control technology is adopted in N:N communication mode.
A logical ring is formed on the physical bus to allow a token to flow in a certain direction in the logical ring.
The station that obtains the token obtains the right to use the bus. The token bus access control mode limits the token holding time of each station, guarantees that each station has the opportunity to obtain the bus usage right and provides priority services during the token cycle, so the token bus access control mode has better real-time performance. CSMA/CD communication mode is a random communication mode. It is suitable for PLC network with bus structure. Each station on the bus has equal status and has no master-slave. It adopts CSMA/CD access control mode, that is, “Listen first, speak while listen”. CSMA/CD access control mode can not guarantee that every station in the PLC network can obtain the right to use the bus within a certain period of time, so it is a kind of access control mode that can not guarantee real-time performance. But it uses random mode, simple method, and seam insertion needle, as long as the bus is idle, it rushes to the Internet, thermostatic element the utilization of communication resources is high, so CSMA/CD communication method in the PLC network is applicable to the upper production management subnet [2]. In order to effectively manage and monitor the information of the control system and realize the information integration and integrated automation of the whole enterprise, it is necessary to design a reliable and inexpensive communication system which can operate in the industrial field environment, form the bottom network of the field, and complete the multi-point digital communication between the field automation equipment. In order to realize the information exchange between the underlying equipment and between the production site and the outside world, the PLC network control system will be more widely used.