In the field of automation control, PLC is an important control equipment. At present, there are more than 200 manufacturers in the world producing more than 300 kinds of PLC products, which are applied to automobiles (23%), grain processing (16.4%), chemistry/pharmacy (14.6%) and metals/mines (10/0.5%). In order to make it easier for beginners to understand PLC, this paper briefly introduces the development, basic structure, configuration and application of PLC to help netizens. I. The development process of PLC In the industrial production process, a large number of switches are controlled in sequence, acting in sequence according to logical conditions, controlling interlock protection actions according to logical relations, and collecting a large number of discrete data. Traditionally, these functions are realized by pneumatic or electric control systems. 1968 American GM (general motors) company proposed to replace the electronic control device. In the second year, Digital Company of America developed a control device based on integrated circuit and electronic technology, which was applied to electrical control by programming for the first time. This is the first generation of programmable controller, called programmable controller (PC). After the development of personal computer (PC for short), it was named as programmable logic controller (PLC) in order to facilitate and embody the functional characteristics of programmable controller. Now, PLC is usually referred to as PC for short. There are many definitions of PLC. The definition of PLC by the International Electrotechnical Commission (IEC) is that a programmable controller is an electronic system with digital operation, specially designed for industrial applications. It uses programmable memory to store instructions for performing logical operations, sequence control, timing, counting and arithmetic operations, and controls various types of machines or production processes through digital and analog input and output. Programmable controller and its related equipment should be designed according to the principle that it is easy to form a whole with industrial control system and expand its functions. From the 1980s to the mid-1990s, PLC developed the fastest, with an annual growth rate of 30~40%. During this period, the analog processing ability, digital computing ability, man-machine interface ability and network ability of PLC have been greatly improved, and PLC has gradually entered the field of process control, replacing DCS system, which is dominant in the field of process control, in some applications. PLC has the characteristics of strong universality, convenient use, wide adaptability, high reliability, strong anti-interference ability and simple programming. The position of PLC in industrial automation control, especially in sequence control, is irreplaceable in the foreseeable future. Second, the composition of PLC is divided into fixed and modular. Fixed PLC includes CPU board, I/O board, display panel, memory block, power supply, etc. These elements are combined into an inseparable whole. Modular PLC includes CPU module, I/O module, memory, power module, backplane or rack, which can be combined and configured according to certain rules. Third, the composition of CPU is the core of PLC and plays the role of nerve center. Each PLC has at least one CPU, which receives and stores user programs and data according to the functions given by PLC system programs, and collects the status or data sent by field input devices through scanning and stores them in designated registers. At the same time, the working state of the power supply and the internal circuit of PLC and the grammatical errors in the programming process are diagnosed. After running, the instructions are read from the user program memory one by one, and after analysis, corresponding control signals are generated according to the tasks specified in the instructions to command the relevant control circuits. CPU is mainly composed of arithmetic unit, controller, register and data, control and status bus to realize the connection between them. The CPU unit also includes peripheral chips, bus interfaces and related circuits. Memory is mainly used to store programs and data, and it is an indispensable unit of PLC. In the eyes of users, it is not necessary to analyze the internal circuit of CPU in detail, but it is necessary to fully understand the working mechanism of each part. The controller of CPU controls the work of CPU, which reads, interprets and executes instructions. But the working rhythm is controlled by the oscillation signal. The arithmetic unit is used for digital or logical operations and works under the command of the controller. The register participates in the operation and stores the intermediate result of the operation, and it also works under the command of the controller. CPU speed and memory capacity are important parameters of PLC, which determine the working speed, IO number and software capacity of PLC, thus limiting the control scale. 4. The interface between the input-output module PLC and the electric circuit is completed by the input-output part (I/O). I/O module integrates the I/O circuit of PLC, its input register reflects the state of input signal, and its output point reflects the state of output latch. The input module converts electrical signals into digital signals and enters the PLC system, while the output module does the opposite. I/O is divided into switch input (DI), switch output (DO), analog input (AI), analog output (AO) and other modules. On-off value refers to a signal with only two states (or 1 and 0), and analog value refers to a continuously changing value. Commonly used I/O is classified as follows: On/Off value: 220VAC, 1 10VAC and 24VDC according to voltage level, and relay isolation and transistor isolation according to isolation mode. Analog quantity: according to signal type, there are current type (4-20mA, 0-20mA), voltage type (0- 10V, 0-5V,-10v) and so on, and according to accuracy, there are 12bit, 65438+. In addition to the above general IO, there are special IO modules, such as thermal resistance, thermocouple, pulse and so on. The specification and quantity of modules are determined according to the number of I/O points. I/O modules can be more or less, but the maximum number is limited by the basic configuration capacity that the CPU can manage, that is, by the maximum number of backplane or rack slots. V. Power supply module PLC power supply is used to provide working power for the integrated circuit of PLC module. At the same time, some also provide 24V working power for the input circuit. The power input types are: AC power (220VAC or 1 10VAC) and DC power (usually 24VAC). Intransitive verb baseboard or rack Most modular PLCs use baseboards or racks, whose functions are: electrically, to realize the connection between modules, so that CPU can access all modules on the baseboard, and mechanically, to realize the connection between modules, so that the modules can form a whole. Seven. Other equipment of PLC system 1. Programming equipment: Programmer is an indispensable equipment for PLC development, application, monitoring and maintenance. It is used to program, set up the system and monitor the working state of PLC and its controlled system, but it does not directly participate in on-site control and operation. Small programmer PLC generally has a handheld programmer. At present, a computer (running programming software) generally acts as a programmer. 2. Man-machine interface: The simplest man-machine interface is indicator light and button. At present, the integrated operator terminal with LCD screen (or touch screen) is widely used, and the computer (running configuration software) is very popular as a man-machine interface. 3. Input/output devices: used to permanently store user data, such as EPROM, EEPROM writers, bar code readers, analog input potentiometers, printers, etc. Eight, PLC communication networking can quickly and effectively collect and transmit production and management data by relying on advanced industrial network technology. Therefore, the importance of network in automation system integration engineering is becoming more and more obvious, and some people even put forward the view that "network is controller". PLC has communication networking function, which enables information exchange between PLC and PLC, between PLC and upper computer and other intelligent devices, forming a unified whole and realizing decentralized centralized control. Most PLCs have RS-232 interfaces, and some have built-in interfaces supporting their respective communication protocols. The communication of PLC has not yet achieved interoperability. IEC has stipulated various fieldbus standards, which have been adopted by PLC manufacturers. For an automation project (especially a large-scale control system), the choice of network is very important. First of all, the network must be open to facilitate the integration of different devices and the expansion of future system scale; Secondly, according to the transmission performance requirements of different network levels, the choice of network form must be based on a deep understanding of the protocols and mechanisms of network standards; Thirdly, considering the system cost, equipment compatibility, site environment applicability and other specific issues, determine the network standards used at different levels.