Characteristics of PLC scanning operation mode:
1. Features: Centralized sampling, centralized output, cyclic scanning
1) Centralized sampling: The scanning of the input state is only performed during the input sampling phase. That is, even if the input state changes during the program execution or output phase, the contents of the input image register will not change, and it can only be read in the input processing phase of the next scan cycle (response lag).
2) Centralized output: Within a scan cycle, the status in the element image register is only output during the output processing stage. In other stages, the output value is always stored in the element image register.
Note: In user programs, if the output is assigned a value multiple times, only the last one is valid; therefore, double-coil outputs should be avoided.
2. Advantages: Improves the system's anti-interference capability.
The centralized sampling and output scanning mode isolates the PLC from peripherals for most of the operating time, fundamentally improving the system's anti-interference capability and enhancing its reliability.
3. Disadvantages: delayed response, reducing the system's response speed.
Input/output lag time is also known as system response time.
1) Input module lag time: The time constant of the RC filter circuit of the input module, typically around 10ms.
2) Output module lag time
Relay-type output: approximately 10ms;
Thyristor-type output: power-on lag time is approximately 1ms, and the maximum power-off lag time is 10ms;
Transistor-type output: less than 1ms.
3) The lag time caused by the scanning mode can be as long as two scanning cycles.
The total response delay time of a PLC is typically tens of milliseconds.
However, since the PLC's scanning speed is extremely fast, this lag in response is completely acceptable for general industrial control.
Note: In medium and large-sized PLCs, there are many I/O points to be processed and the user program is long. Therefore, time-sharing and batch scanning methods or interrupt-based working methods are adopted to shorten the cycle of cyclic scanning and improve real-time control.
