The English full name of PLC is: Programmable Logic Controller, a digital electronic operating 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 controls 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 electronic operating 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 an industrial control computer, and its hardware structure is basically the same as that of a microcomputer. The basic components are: a. Power supply The power supply of the PLC plays a very important role in the entire system. Without a good and reliable power supply system, it cannot work properly, so PLC manufacturers also pay great attention to the design and manufacturing of the power supply. Generally, if the AC voltage fluctuation is within the range of 10% (15%), the PLC can be directly connected to the AC power grid without taking 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 each input device on site, and stores 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 according to the instructions, sending the results to the I/O image area or data registers. After all user programs are executed, the output status in the I/O image area or the data in the output registers are finally transmitted to the corresponding output devices. This cycle runs repeatedly until operation is stopped. To further improve the reliability of the PLC, in recent years, large-scale PLCs also use dual CPUs to form a redundant system or a triple CPU voting system. 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 circuits 1. The field input interface circuit consists of an optocoupler circuit and the input interface circuit of the microcomputer, and its function is the input channel for the interface between the PLC and the field control. 2. The field output interface circuit is integrated with the output data register, strobe circuit, and interrupt request circuit. Its function is that the PLC outputs corresponding control signals to the field execution components through the field 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.