PLCs employ a cyclic scanning working mode. In the PLC, the user program is stored sequentially. The CPU starts executing the program from the first instruction and returns to the first instruction upon encountering the end-of-line character, repeating this cycle continuously. The PLC's scanning process is divided into several stages: internal processing, communication operation, program input processing, program execution, and program output. The time required for one complete scan is called the scan cycle. When the PLC is in a stopped state, it only performs internal processing and communication operations. When the PLC is running, it continuously cycles through internal processing, communication operation, program input, program execution, and program output.
1. Input Processing
Input processing, also known as input sampling, involves sequentially reading the on/off status of all input terminals and storing the information in the corresponding image register in memory. The input image register is then refreshed. Next, the program execution phase begins. During program execution, the input image register is isolated from the outside world; even if the input signal changes, its contents remain unchanged until the input processing phase of the next scan cycle.
2. Program Execution
According to the PLC ladder diagram program scanning principle, the program is scanned line by line in a left-to-right and top-to-bottom sequence. When a program jump instruction is encountered, the jump address is determined based on whether the jump condition is met. When the user program involves input/output states, the PLC reads the corresponding input terminal state from the input image register, reads the corresponding image register from the output image register, performs logical operations according to the user program, and stores the results in the relevant device registers. For each device, the contents stored in the device image register change as the program executes.
3. Output Processing
After the program finishes execution, the state of the Y register in the output image register (i.e., the device image register) is transferred to the output latch during the output processing stage. Through the isolation circuit, this latch drives the power amplifier circuit, causing the output terminal to output control signals to the outside world, driving the external load. (Edited by Industrial Products Mall Editor)
