What is the basic working principle of PLC?

1. Core: Cyclic scanning. PLC is like an indefatigable “check – think – execute” robot, always repeating a fixed working cycle (scanning cycle).
2. ** Step 1: Check the input (input sampling). The PLC quickly reads the current status (on/off, on/off) of all input devices connected to it (such as buttons, sensors, switches), and stores them in its own “input image area” (a dedicated block of memory).
3. ** Step 2: Execute the program (program execution). The “brain” (CPU) of the PLC performs logical instructions one by one based on the control program (usually written in languages such as ladder diagrams) pre-written by the user, in combination with the latest input status in the input image area. It only cares about the input status read at the moment the scan begins, and the actual input changes during this period are temporarily ignored.
4. Step 3: Update the output (output refresh). After the program operation is completed, the PLC writes the final result obtained from the operation (which output points should be on and which should be off) into the “output image area”. Then, it immediately sends the status of this “output image area” simultaneously to all output devices connected to it (such as relays, contactors, indicator lights, motor drivers). The output device acts according to these signals, thereby controlling the machine or process.
5. ** Step 4: Handle miscellaneous tasks (internal handling, communication, etc.). During the cycle intervals or specific times, the PLC will perform some internal tasks, such as checking whether it is functioning properly (self-diagnosis), communicating and exchanging data with computer programming software or other PLCS, etc.
6. Repeat! After completing the above steps, the PLC immediately starts the next scanning cycle, reads the input again, executes the program, updates the output… This high-speed cycle repeats itself (the scanning period is usually a few milliseconds to tens of milliseconds).

** Key Point summary: **

The input status is only “photographed” and recorded once at the beginning of the scan. **
The program execution is based on this “input snapshot” for logical operations. **
The output update is sent to all output points at one time and simultaneously at the end of the scan. **
The entire process is high-speed, automatic and loops infinitely. **

This is the foundation for a PLC to reliably control industrial equipment: it constantly perceives the external world (input), makes decisions according to preset rules (programs), and then drives the equipment to act (output), repeating this cycle.