A Programmable Logic Controller (PLC) is a microprocessor-based digital controller used for automation control. It can load control instructions into memory for storage and execution. A PLC consists of functional units such as a CPU, instruction and data memory, input/output interfaces, power supply, and digital-to-analog converters.
A programmable logic controller (PLC) is essentially a computer specifically designed for industrial control. Its hardware structure is basically the same as that of a microcomputer, and its basic components are shown in the figure below. A detailed description of its basic structure follows:
Power supply: The power supply is used to convert AC power into DC power required by the PLC. Currently, most PLCs use switching power supplies.
Central Processing Unit: The central processing unit (CPU) is the control center of the PLC and its core component. Its performance determines the performance of the PLC.
The central processing unit (CPU) consists of a controller, an arithmetic logic unit (ALU), and registers. These circuits are all integrated onto a single chip and connected to the memory's input/output interface circuits via address and control buses. The CPU's function is to process and run user programs, perform logical and mathematical operations, and control the entire system to ensure its smooth operation.
Memory: Memory is a semiconductor circuit with memory function. Its purpose is to store system programs, user programs, logic variables, and other information. The system program is the program that controls the PLC to perform various functions. It is written by the PLC manufacturer and stored in read-only memory (ROM), which cannot be accessed by the user.
Input Unit: The input unit is the input interface connecting the PLC to the controlled device. It acts as a bridge for signals to enter the PLC, and its function is to receive signals from the master control element and the detection element. Input types include DC input, AC input, and AC/DC input.
Output Unit: The output unit is also the connection component between the PLC and the controlled device. Its function is to transmit the PLC's output signals to the controlled device, that is, to convert the weak electrical signals sent by the central processing unit into level signals to drive the actuators of the controlled device. Output types include relay outputs, transistor outputs, and thyristor outputs.
In addition to the components mentioned above, PLCs, depending on the model, also have various external devices. These devices assist in programming, monitoring, and network communication. Commonly used external devices include programmers, printers, cassette recorders, and computers.
