Tutorial explanation of the internal structure and basic structure of a PLC

　　PLC tutorial has become a hot word in Baidu recently because a good PLC tutorial is often hard to find. For this reason, the editor has specially established a series of PLC tutorial articles to try to reduce the difficulty of searching for PLC tutorials. In this PLC tutorial, the editor will introduce what PLC is, the internal structure of PLC, and the basic structure of PLC. If you are interested in the content related to PLC tutorials, you may wish to continue reading.

　　1. What is PLC

　　It is a real-time system that is different from the traditional motor control system based on relays in personal computers. Whenever the design is changed, the entire system almost has to be remade, which is not only time-consuming and labor-intensive; at the same time, because relays have the disadvantages of poor contact, wear and tear, and large size, it causes problems such as high cost, low reliability, and difficulty in maintenance. In order to improve these shortcomings, DEC of the United States first published the Programmable Controller in 1969.

　　In the early days of its release, the programmable controller was called (Programmable Logic -Controller), or PLC for short. Its original purpose was to replace relays and perform sequential control of relay logic and other functions such as timing or counting, so it is also called a sequential controller. Its structure is also like a microcomputer, so it can also be called a microcomputer programmable controller (MCPC). It was not until 1976 that the American Electrical Manufacturers Association officially named it Programmable Controller, or programmable controller, or PC for short. Since personal computers are extremely common nowadays, and they are often used in conjunction with programmable controllers, in order to distinguish the two, programmable controllers are generally called PLCs to distinguish them. There are many types of PLCs on the market, which vary according to manufacturers and applicable places. However, each brand can be divided into large, medium and small according to the complexity of the unit; and general factories and schools usually use small PLCs. Among them, the Japanese MITSUBISHI F series and the A series PLC produced by Shihlin Electric in Taiwan are more popular among Chinese people. This CAI will mainly introduce Mitsubishi FX2 PLC, hoping that users can have a deeper understanding of PLC and be more comfortable using PLC. The basic structure of the programmable controller can be represented by the figure below. Its internal processing unit includes three major parts: CPU, input module, and output module. The CPU of the PLC will obtain the signal generated by the input component through the input module, and then take out the control instructions originally input in the programmer from the memory one by one. After the logical calculation of the operation department, the result will be passed through the output module to drive the external output component.

　　2. PLC internal structure

　　Program input device: Responsible for providing the operator with the functions of inputting, modifying and monitoring the program.

　　Central Processing Unit (CPU): Responsible for PLC management, execution, calculation, control and other functions.

　　Program memory: Responsible for storing the sequence program parameters and comments designed by the user.

　　Data memory: Responsible for storing the status of input and output devices and the conversion data of sequential programs.

　　System memory: Stores the system programs required for PLC to execute sequential control.

　　Input circuit: responsible for receiving external input component signals.

　　Output circuit: responsible for receiving external output component signals.

　　It has a wide range of industrial uses, such as various automation equipment in semiconductor wafer factories, control building elevators, mechanical parking equipment, and roadside traffic light changes to control automated production lines.

　　3. Detailed explanation of the basic structure of PLC

　　PLC is essentially a computer dedicated to industrial control, and its hardware structure is basically the same as that of a microcomputer.

　　a. Central Processing Unit (CPU)

　　The central processing unit (CPU) is the control center of the PLC. It receives and stores the user program and data typed from the programmer according to the functions assigned by the PLC system program; checks the status of the power supply, memory, I/O and warning timer, and can diagnose syntax errors in the user program. When the PLC is put into operation, it first receives the status and data of each input device on site in a scanning manner, and stores them in the I/O image area respectively, and then reads the user program from the user program memory one by one, and after the command is interpreted, the results of the logical or arithmetic operation are sent to the I/O image area or data register according to the instructions. After all user programs are executed, the output status of the I/O image area or the data in the output register are finally transmitted to the corresponding output device, and the cycle runs until it stops.

　　In order to further improve the reliability of PLC, in recent years, large PLCs have also adopted dual CPUs to form a redundant system, or a three-CPU voting system. In this way, even if a CPU fails, the entire system can still operate normally.

　　b. Memory

　　The memory that stores system software is called system program memory.

　　The memory that stores application software is called user program memory.

　　C. Power supply

　　The power supply of PLC plays a very important role in the whole system. Without a good and available power supply system, it cannot work properly. Therefore, PLC manufacturers also attach great importance to the design and manufacture of power supply. Generally, if the AC voltage fluctuation is within the range of +10% (+15%), PLC can be directly connected to the AC power grid without taking other measures.