The full English name of PLC is: Programmable Logic Controller, a digital computing electronic system specifically designed for industrial environment applications. It uses a type of programmable memory for its internal storage of programs, executing user-oriented instructions such as logic operations, sequential control, timing, counting, and arithmetic operations, and controlling various types of machinery or production processes through digital or analog input/output. It is the core component of industrial control. PLC controllers mainly refer to the programmable logic controllers of digital computing electronic systems used to control the production processes of machinery. It is also an abbreviation for Public Limited Company, Power Line Carrier, etc. Major current brands of PLC include: South Korea's LG, USA's AB, ABB, Panasonic, Siemens, Mitsubishi,
Omron, Delta, Fuji, Schneider, Xinjie, etc. Among them, LG PLC is a transistor-output PLC provided by the LS Electric division of South Korea's LG Group, and Shenzhen Hongyiwei Automation Technology Co., Ltd. is its general agent in the Guangdong region. The basic structure of PLC is essentially a computer for industrial control, with a hardware structure largely similar to that of a microcomputer. The basic components are: a. Power Supply The power supply of a PLC plays a crucial role in the entire system. Without a good and reliable power supply system, it cannot function properly, so PLC manufacturers place great emphasis on the design and manufacturing of the power supply. Generally, if the AC voltage fluctuation is within the range of 10% to 15%, the PLC can be directly connected to the AC power grid without other measures. b. Processing Unit (CPU) The processing unit (CPU) is the control center of the PLC. It receives and stores user programs and data input from the programmer according to the functions assigned by the PLC system program; checks the status of the power supply, memory, I/O, and watchdog timer, and can diagnose syntax errors in the user program. When the PLC is put into operation, it first scans and receives the status and data of various input devices on-site, storing them in the I/O image area. Then, it reads the user program from the user program memory one by one, interprets the commands, and performs logical or arithmetic operations as instructed, sending the results to the I/O image area or data registers. After all user programs are executed, it finally transfers the output status from the I/O image area or the data from the output registers to the corresponding output devices, cycling this process until operation stops. To further enhance the reliability of PLCs, in recent years, large-scale PLCs have adopted dual-CPU redundant systems or triple-CPU voting systems. This way, even if one CPU fails, the entire system can still operate normally. c. Memory The memory that stores system software is called the system program memory. The memory that stores application software is called the user program memory. d. Input/Output Interface Circuit 1. On-site input interface circuit consists of an optocoupler circuit and the input interface circuit of the microcomputer, serving as the input channel for the interface between the PLC and on-site control. 2. On-site output interface circuit integrates output data registers, strobe circuits, and interrupt request circuits, enabling the PLC to output corresponding control signals to on-site execution components through the on-site output interface circuit. e. Functional Modules Such as counting and other functional modules. f. Communication Modules Such as Ethernet, RS485, Profibus-DP communication modules, etc.