Glossary · Automation fundamentals, platforms and components
Cycle (PLC)
Also known as: Scan cycle, PLC scan
German: Zyklus
In PLC programming, a cycle is one complete pass of a cyclic task: reading the inputs into the process image, executing the program and writing the outputs. The time this takes is the cycle time.
- Automation components
In one sentence
A PLC cycle is one pass of reading inputs, executing the program and writing outputs; its duration is the cycle time.
Example
With a cycle time of 8 ms, a short pulse from a sensor lasting only 3 ms may be missed unless it is read by an interrupt or a fast counter.
How it applies
- Engineering: Program size and structure determine cycle time; long loops can exceed the watchdog time and stop the CPU. Time-critical parts run in cyclic interrupt tasks with a fixed interval.
- Commissioning: Minimum, maximum and average cycle times are checked under full load.
- Documentation: Specify cycle and response times where they matter, for example for safety functions or fast signals. In German, "Zyklus" is also used for machine cycles, so translations should distinguish PLC cycle and machine cycle.
PLC cycle vs. machine cycle
The PLC cycle is a software execution loop lasting milliseconds. A machine cycle is one complete working sequence, such as one stroke of a press or one package, lasting from fractions of a second to minutes. Output rates like "cycles per minute" refer to machine cycles.