The cyclic scanning and execution process of the user program can be divided into three stages: input sampling, program execution, and output refresh.
1. Input sampling stage
During the input sampling phase, the PLC scans all input signal states (ON/OFF states) into the input image register and stores them; this process is called input signal sampling. Then, the program execution phase begins. During program execution, even if the input states change, the contents of the input image register remain unchanged. Changes in the input states are only re-read during the input sampling phase of the next work cycle.
2. Program execution phase
During program execution, the PLC scans the program sequentially. If the program is represented by a ladder diagram, it is always scanned in the order of top to bottom and left to right. When a new instruction is encountered, the required input state or other element state is read from the input image register or output image register, and then the corresponding logical or arithmetic operations are performed. The result is then stored in a dedicated register. If a program output instruction is executed, the corresponding result is stored in the output image register.
3. Output refresh phase
After all instructions have been executed, the state in the output image register is the desired output state. During the output refresh phase, this state is transferred to the output latch circuit, and then a signal is output through the output terminal to drive the user's output device; this is the actual output of the PLC. The PLC repeatedly executes the above three stages, and each repetition constitutes one work cycle (or scan cycle). The length of the work cycle is related to the length of the program.
Due to factors such as the time constant of the input/output module filter, the mechanical lag of the output relay, and the work cycle execution of the program, input/output responses may exhibit lag. For general industrial control equipment, this lag is acceptable. However, some equipment requires rapid responses to certain signals. Therefore, some PLCs employ high-speed response input/output modules, while others divide the sequential program into two categories: a high-level program with fast response and a low-level program with moderate response speed. For example, the FANUC-BESK PLC specifies that the high-level program is scanned every 8ms, while the low-level program is automatically divided into segments. When program execution begins, the high-level sequential program is executed first, followed by segment 1 of the low-level program, then the high-level program is executed again, and then segment 2 of the low-level program is executed. This process involves re-scanning and executing the high-level program after each segment of the low-level program is completed to ensure the rapid signal response in the high-level program.
