Glossary · Scan
What is Scan cycle?
The loop a PLC repeats thousands of times a minute: read the inputs, run the program top to bottom, write the outputs.
A modern controller scans in 1 to 20 ms depending on program size and task settings. Because the program runs in order, the last assignment to a variable is the one that reaches the output.
Understanding the scan explains most surprising PLC behaviour: why a short button press is missed, why a timer has to be called every scan, and why an edge needs an R_TRIG rather than a comparison.
Related terms
- Process image
- A copy in controller memory of every input and output, refreshed once at the start and once at the end of each scan.
- Watchdog timer
- A timer in the controller firmware that stops execution and raises a fault if one scan takes longer than allowed.
- Programmable logic controller (PLC)
- An industrial computer built to control machines and processes: it reads sensors and switches, runs the control program, and drives motors and valves, repeatedly and reliably.