Beckhoff First Scan Bit Review

IF fbFirstScan.bFirstScan THEN // First cycle only bResetDrives := TRUE; bStartupComplete := FALSE; tStartupTimer(IN:=FALSE); END_IF

For complex architectures where initialization requires checking hardware topology, EtherCAT master states, or waiting for external databases to link, a simple bit might not suffice. In these scenarios, you can leverage the system PLC task variables or PLC status functions.

In TwinCAT 2, it is typically FirstCycle or FirstScan . In TwinCAT 3, it is available via Tc2_System function block or directly as FirstScan in some contexts. beckhoff first scan bit

The First Scan bit is a simple but essential mechanism in Beckhoff/TwinCAT systems to ensure deterministic, safe startup behavior. Implement it consistently, coordinate initialization ordering, protect hardware and retained data, and provide clear diagnostics to maintain reliable system startup.

I can provide more specialized code to fit your exact setup. PlcTaskSystemInfo - Beckhoff Information System - English IF fbFirstScan

| Test Scenario | Method | What to Verify | |---|---|---| | | Power cycle the controller | First scan code runs exactly once | | Warm Start | STOP → RUN from development environment | Custom flags execute if needed | | Program Download | Download without resetting retain variables | Retain-based initialization behavior | | Online Change | Modify code while running | New instances get proper initialization | | Multiple Power Cycles | Cycle power repeatedly | Counters increment correctly |

Do you need an example written in instead of Structured Text? In TwinCAT 3, it is available via Tc2_System

Note: exact symbol names can vary by TwinCAT version and project conventions.