Industrial Engineer · phase 3 of 16
Scan Cycle
Repeated controller operation that services inputs, executes scheduled logic, updates outputs, communicates, and performs diagnostics according to the platform design.
The Explain card
- Plain English
- The scan cycle is the controller's heartbeat. Read the inputs, run the logic, write the outputs, service the network and the diagnostics, then start again.
- Example
- A sensor changes state halfway through the logic. The program does not see it until the next input read, which is the start of the next cycle.
- Why it matters
- Everything about how a PLC behaves, from why an output lags a button by a few milliseconds to why an input glitch shorter than the scan can vanish, comes from this loop. Understanding it explains most of the mysteries that get blamed on the hardware.
- Hook
- Read, think, act, repeat.
Where it sits in the deck
Phase 3: PLC Program Organization and Execution
How the logic is arranged and how the scan actually runs it, which is the difference between code that works and code that works every time.
