Industrial Engineer · phase 3 of 16
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.
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.
