Industrial Engineer · phase 3 of 16 · noun
First Scan
Controller status that is true during the initial logic pass after entering run mode and is used for initialization.
The Explain card
- Plain English
- First scan is a status bit that is true for exactly one pass after the controller enters run mode. It exists so you can set things up once before normal logic takes over.
- Example
- On first scan the program clears the alarm buffer, loads default setpoints and resets the sequence to Idle. Every scan after that, the bit is false.
- Why it matters
- Without initialization a controller wakes up with whatever was left in memory, which might be a half finished sequence from before the power went out. First scan is the one safe place to decide what a fresh start means.
- Hook
- One chance to set the table before dinner starts.
Word knowledge
How the word is built, where it came from, and what it sits beside in memory.
In a sentence
On First Scan, the PLC clears the batch counters and loads the initial speed reference for the conveyor drive.
Why these words
- first Old English fyrst, earliest marks the opening member of the sequence
- scan Latin scandere, to climb, through the scanning of verse names the controller's ordered pass through logic
Where it came from
- Origin
- English industrial-control compound formed from first and scan
- Entered the language
- 20th century
- What changed
- Scan moved from analyzing poetic meter to systematic examination and electronic traversal, then joined first in PLC terminology.
How it is spelled
- Pattern
- Scan doubles its final n before vowel-led suffixes, as in scanned and scanning.
- Pattern
- The ir in first follows the same spelling pattern found in bird and third.
- Breaks the pattern
- First is historically related to fore rather than being regularly formed from one.
Spelled like
- planned
- running
- bird
- third
Broken into chunks
-
first
- firstborn
- firstly
-
scan
- scanner
- scanning
What it sits beside
Same subject
- program scan
- startup routine
- run mode
- one-shot instruction
Same shape
- last scan
- next scan
- initial scan
PLC execution-cycle terms
- First Scan
- program scan
- scan time
- watchdog timer
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.
