Glossary · Automation fundamentals, platforms and components
Scan cycle
Also known as: PLC scan, Program cycle, Cycle time
German: Zykluszeit
In PLC technology, the scan cycle is the repeating sequence in which a controller reads its inputs into the process image, executes the user program, writes the outputs and performs system tasks. The cycle time (German: Zykluszeit) is the duration of one such cycle.
- Automation components
In one sentence
The scan cycle is the PLC's repeating loop of reading inputs, executing the program and writing outputs; cycle time is its duration.
Example
The main task has a cycle time of 8 ms; a new visualization routine raised it to 14 ms and delayed the response to a sensor.
How it applies
- Engineering: Response time from input to output is roughly up to two cycles plus input and output delays. Cyclic tasks run at a fixed interval, free-running cycles as fast as possible.
- Operation: A Watchdog monitors the maximum cycle time; if exceeded, the controller typically goes to STOP or runs an error routine.
- Documentation: State typical and maximum cycle times in the software documentation, and include them in response time calculations for safety functions. Note that the German list term "Zykluszeit" refers to the time, while "scan cycle" names the loop itself.
Scan cycle vs. machine cycle
The scan cycle is a controller's program loop, measured in milliseconds. A machine cycle or Stroke (machine cycle) is one production cycle of the machine, measured in seconds, and spans many scan cycles.