Industrial Engineer · phase 3 of 16 · noun
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.
Word knowledge
How the word is built, where it came from, and what it sits beside in memory.
In a sentence
The technician checked the scan cycle after a photoelectric sensor changed state too briefly for the PLC to detect.
Why these words
- scan Late Latin scandere, to scan verse specifies the controller's systematic pass through its work
- cycle Greek kyklos, circle or wheel, through Latin cyclus marks the pass as recurring
Where it came from
- Origin
- English technical compound formed from scan and cycle
- Entered the language
- 1960s
- What changed
- Scan broadened from analyzing poetic meter to systematic examination, while cycle broadened from circular motion to a recurring sequence of operations.
How it is spelled
- Pattern
- The first c in cycle has the soft /s/ sound before y.
- Pattern
- Scan doubles its final n before vowel-starting suffixes, as in scanned and scanning.
Spelled like
- cylinder
- cymbal
- planned
- running
Broken into chunks
-
scan
- scanner
- scanning
-
cycle
- cyclic
- bicycle
What it sits beside
Same subject
- scan time
- input image
- output image
- watchdog timer
- task period
Same shape
- duty cycle
- machine cycle
- instruction cycle
PLC timing terms
- scan cycle
- scan time
- task period
- watchdog timeout
controller scan stages
- input scan
- program scan
- output scan
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.
