About PLC

1　The development course of PLC
　　In the industrial production process, a large number of on-off sequence control, it performs sequential operations according to logical conditions, and controls the chain protection action according to the logical relationship, and a large number of discrete data collection. Traditionally, these functions are achieved through pneumatic or electrical control systems. In 1968, the US GM (General Motors) company proposed to replace the requirements of the electrical control device. In the second year, the US digital company developed a control device based on integrated circuits and electronic technology, which was first applied to electrical control using programmed means. It is the first generation of programmable controller, called Programmable Controller (PC).
After the development of personal computers (referred to as PCs), the programmable controllers were named Programmable Logic Controller (PLC) for convenience and to reflect the functional characteristics of programmable controllers.
From the 1980s to the mid-1990s, it was the fastest-growing period of PLC, with an annual growth rate of 30-40%. During this period, PLC has greatly improved its ability to handle analog quantities, digital operations, human-machine interfaces, and networks. PLC has gradually entered the field of process control, replacing DCS, which is dominant in the field of process control, in some applications system.　
　　PLC has the characteristics of strong versatility, easy to use, wide adaptability, high reliability, strong anti-interference ability, simple programming and so on. The status of PLC in industrial automation control, especially sequential control, cannot be replaced in the foreseeable future.
2　PLC structure 　
　　 From the structure point of view, PLC is divided into fixed type and combined type (module type). Fixed PLC includes CPU board, I/O board, display panel, memory block, power supply, etc. These elements are combined into a non-removable whole. Modular PLC includes CPU module, I/O module, memory, power supply module, backplane or rack. These modules can be combined and configured according to certain rules.
3　CPU configuration
The CPU is the core of the PLC and plays the role of the nerve center. Each PLC has at least one CPU. It receives and stores user programs and data according to the functions given by the PLC system program, and collects the data sent by the field input device by scanning. The state or data is stored in the specified register, and at the same time, the working state of the power supply and the internal circuit of the PLC and the syntax errors in the programming process are diagnosed. After entering the operation, read the instructions one by one from the user program memory, after analysis, then generate the corresponding control signals according to the tasks specified by the instructions, to command the relevant control circuits.
　　 The CPU is mainly composed of operators, controllers, registers, and data, control, and status buses that implement the connection between them. The CPU unit also includes peripheral chips, bus interfaces, and related circuits. The memory is mainly used to store programs and data, and is an indispensable component unit of PLC.
　　 From the user's point of view, it is not necessary to analyze the internal circuit of the CPU in detail, but there should be enough understanding of the working mechanism of each part. The controller of the CPU controls the CPU to read instructions, interpret instructions, and execute instructions. But the work rhythm is controlled by the shock signal. The arithmetic unit is used for digital or logical operation, and works under the command of the controller. The register participates in the operation and stores the intermediate result of the operation. It also works under the direction of the controller.
　　CPU speed and memory capacity are important parameters of PLC. They determine the working speed of PLC, the number of IO and software capacity, etc., so the control scale is limited.
4　I/O module
　　 The interface between PLC and electrical circuit is completed by input/output part (I/O). The I/O module integrates the PLC's I/O circuit, its input temporary register reflects the state of the input signal, and the output point reflects the state of the output latch. The input module transforms the electrical signal into a digital signal and enters the PLC system, the output module is the opposite. I/O is divided into digital input (DI), digital output (DO), analog input (AI), analog output (AO) and other modules.
　　 Commonly used I/O classification is as follows:
　　 Switching quantity: according to the voltage level, there are 220VAC, 110VAC, 24VDC, according to the isolation mode, there are relay isolation and transistor isolation.
Analog quantity: divided by signal type, there are current type (4-20mA, 0-20mA), voltage type (0-10V, 0-5V, -10-10V), etc., according to accuracy, there are 12bit, 14bit, 16bit, etc. .
　　In addition to the above general IO, there are special IO modules, such as thermal resistance, thermocouple, pulse and other modules.
　　Identify the module specifications and quantity according to the number of I/O points. The number of I/O modules can be more or less, but the maximum number is limited by the basic configuration capability that the CPU can manage, that is, the maximum number of backplanes or rack slots.
5　power module
　　PLC power supply is used to provide working power for the integrated circuits of PLC modules. At the same time, some also provide 24V working power for the input circuit. Power input types are: AC power supply (220VAC or 110VAC), DC power supply (24VDC is commonly used).
6 Backplane or rack
Most modular PLCs use a backplane or rack. Their function is to electrically connect each module so that the CPU can access all the modules on the backplane. Mechanically, connect each module to make each module form one. overall.
7 Other equipment of PLC system 7.1 Programming equipment:
The programmer is an indispensable device for PLC development and application, monitoring operation, inspection and maintenance. It is used for programming, making some settings for the system, monitoring the working status of the PLC and the system controlled by the PLC, but it does not directly participate in the on-site control operation. The small programmer PLC generally has a handheld programmer, and currently a computer (running programming software) is generally used as the programmer. That is the upper computer of our system.
7.2　 Human-machine interface:
The simplest man-machine interface is indicator light and button. At present, the integrated operator terminal of LCD (or touch screen) type is more and more widely used. It is very popular to use computer (operation configuration software) as the man-machine interface.
8　PLC communication networking
　　Rely on advanced industrial network technology to quickly and effectively collect and transmit production and management data. Therefore, the importance of the network in automation system integration engineering is becoming more and more significant, and some people even put forward the view that "the network is the controller".
PLC has the function of communication and networking. It enables the exchange of information between PLC and PLC, PLC and host computer and other intelligent devices, forming a unified whole and achieving decentralized and centralized control. Most PLCs have RS-232 interfaces, and some have built-in interfaces that support their respective communication protocols. PLC communication now mainly uses multi-point interface (MPI) data communication, PROFIBUS or industrial Ethernet for networking