Academy · Basics

What is the PLC scan cycle?

A PLC repeats three steps forever: (1) read inputs → (2) run the program → (3) write outputs. This cycle typically takes a few milliseconds and runs hundreds of times per second.

Why it matters

While the program runs, inputs are "frozen": it uses the image read at the start of the scan. This is why behaviours like an output staying on "for one scan" appear.