Beckhoff First Scan Bit
Beckhoff First Scan Bit — Comprehensive Treatise
⚙️ How to Implement First Scan in TwinCAT 3
Common Pitfalls (and How to Avoid Them)
| Mistake | Consequence | Fix |
|---------|-------------|-----|
| Using FirstScan inside a function block | The FB’s FirstScan is local to that instance – may trigger multiple times. | Use a global g_FirstScanDone flag in the main PLC cycle. |
| Assuming FirstScan runs before I/O update | I/O is typically updated before the first PLC cycle, so outputs may glitch. | Explicitly write safe values in FirstScan even if I/O was read. |
| Forgetting FirstScan in interrupt tasks | Fast tasks or alarms may execute before MAIN’s FirstScan clears. | Add FirstScan logic to every task, or use a global semaphore. |
| Relying on FirstScan after online change | Online change (warm restart) also triggers FirstScan. | If you need cold start only, check bInit_Cold in SysLibCallback. |
What is the First Scan Bit?
It’s a system flag that is TRUE only during the first PLC cycle after: beckhoff first scan bit
- Cold start (reboot of PLC runtime)
- Warm start (program download without reboot)
- Login to the running controller
It automatically resets to FALSE after that first scan. Beckhoff First Scan Bit — Comprehensive Treatise ⚙️
What to Initialize on First Scan
- Outputs to safe states (digital outputs, drive enable).
- Internal state machines and setpoints.
- Communication interfaces (EtherCAT master state checks, fieldbus links).
- Persistent variables: load defaults from non-volatile storage if CRC or version mismatch.
- Diagnostics and logging setup.
- Security checks and operator/user prompts if needed.