PLC is short for "Programmable Logic Controller". Originally, Programmable Logic Controller was abbreviated as PC, but to avoid confusion with the abbreviation PC for Personal Computer, it is now called PLC (Programmable Logic Controller). This article focuses on the components of a PLC and how it works.
What are the components of a PLC?
1. CPU. The CPU of a PLC is actually the central processing unit, which can perform various data operations and processing, convert various input signals into input registers, and then perform logical operations, timing, counting, arithmetic operations, data processing and transmission, communication networking, and various other operations. It compiles and executes the programmed instructions and sends the results to the output terminal to respond to various external devices.
2. Memory. The memory in a PLC system is mainly used to store system programs, user programs, and operating status data. The PLC memory includes system program memory and user memory.
3. Digital Input/Output Interfaces (I/O). Digital input/output interfaces are used to connect to control electrical appliances in industrial production sites. These interfaces employ opto-isolation and RC filtering to achieve electrical isolation between the PLC's internal and external circuits, reduce electromagnetic interference, and simultaneously meet the matching requirements of various signals in industrial settings.
4. Power Supply Module. The PLC's power supply module processes external input power and converts it into DC power required for the PLC's CPU, memory, input/output interfaces, and other internal circuits to operate.
PLC working principle
The working process of PLC scanning is mainly divided into three stages: the input sampling stage, the user program execution stage, and the output refresh stage.
1. Input Sampling Phase. During the input sampling phase, the PLC sequentially reads all input states and data in a scanning manner and stores them in the corresponding units of the I/O image area. After input sampling is completed, the program execution and output refresh phases begin. During these two phases, even if the input states and data change, the states and data in the corresponding units of the I/O image area will not change.
2. User Program Execution Phase. During the user program execution phase, the PLC always scans the user program (ladder diagram) sequentially from top to bottom. When scanning each ladder diagram, it always first scans the control circuit composed of each contact on the left side of the ladder diagram, and performs logical operations on the control circuit composed of contacts in the order of left to right and top to bottom; then, based on the result of the logical operation, it refreshes the state of the corresponding bit in the system RAM storage area of the logic coil, or refreshes the state of the corresponding bit in the I/O image area of the output coil, or determines whether to execute the special function instruction specified by the ladder diagram.
3. Output Refresh Phase. After the user program scan is complete, the PLC enters the output refresh phase. During this period, the CPU refreshes all output latch circuits according to the corresponding status and data in the I/O image area, and then drives the corresponding peripherals through the output circuits. This is when the PLC actually outputs.
Now you should know the components of a PLC. A typical PLC consists of four key parts that work together to complete a task. This article has briefly touched on the working principle of a PLC. To learn more about PLCs, you can consult more resources.
