Glossary · PLC and control programming
PLC scan
Also known as: Scan cycle, PLC cycle, Scan
German: SPS-Zyklus
In PLC technology, the PLC scan is the repeated sequence in which a controller reads its inputs into the process image, executes the user program, writes the output image to the outputs and performs system tasks such as communication and self-tests.
- PLC programming
In one sentence
The PLC scan is the repeated cycle of reading inputs, executing the program, writing outputs and running system tasks.
Example
With a scan time of 8 ms, a sensor change can take up to about two scans plus filter and output delays before the valve reacts.
How it applies
- Engineering: The scan model makes PLC behavior predictable: within one scan, inputs do not change, and outputs change only at the end. Program structure, task configuration and communication load all influence the scan time.
- Timing: Reaction time is not one scan but the sum of input delay, waiting for the next image update, program execution and output delay. Worst-case values matter for stopping distances and process timing, see Latency.
- Commissioning: Measure minimum, typical and maximum scan times under full load, including HMI and network communication. A Watchdog timer stops the controller when the scan exceeds its limit.
- Documentation: Record measured scan times and the configured watchdog in the commissioning report. Explain in the software documentation which functions are time-critical and which task they run in.
PLC scan vs. program cycle
The PLC scan is the whole cycle, including I/O update and system tasks. The Program cycle is the execution of the cyclic user program within it. Controllers with several tasks run several program cycles with different periods, but the idea of a scan remains.