Beckhoff First Scan Bit May 2026

Here’s a complete, ready-to-use post explaining the Beckhoff First Scan Bit in TwinCAT PLC.

PROGRAM MAIN VAR bFirstScan : BOOL := TRUE; // Initializes to TRUE on startup END_VAR beckhoff first scan bit

• Cold start (reboot of PLC runtime)
• Warm start (program download without reboot)
• Login to the running controller

You can use this bit in Structured Text to set default values at startup: Cold start (reboot of PLC runtime) Warm start

Caveat: This method is cycle-dependent. If your cycle time is 10ms, set PT to at least 1ms — but ensure it's longer than one cycle but shorter than two. You can use this bit in Structured Text

IF fbFirstScan() AND (nEtherCATState = 8) THEN  // 8 = OP
    EnableOutputs();
END_IF

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