Beckhoff First Scan Bit | INSTANT |

Understanding the Beckhoff First Scan Bit: Implementation, Use Cases, and Best Practices

In this snippet, the fbTerminalConfig function block (likely a custom or library FB) is triggered only on the first cycle, ensuring the hardware is configured once and not repeatedly during normal operation.

If you are using Object-Oriented Programming (OOP) with Function Blocks, you should generally use the method for hardware checks or setup, rather than a "First Scan" bit inside the body logic. This runs before the first cyclic call and is cleaner for object initialization. beckhoff first scan bit

:Place this line at the very bottom of your MAIN program or the last task to execute :

⚠️ Note that in TwinCAT 3, _TaskInfo[].FirstCycle behaves strictly as the first cycle after the TwinCAT Runtime starts up (moving to Green mode). Toggling the PLC project between STOP and RUN alone does not always re-trigger it unless the system is restarted. :Place this line at the very bottom of

BEGIN IF FirstScan THEN // Execute initialization code here // e.g., set default values, initialize variables FirstScan := FALSE; END_IF

Are you handling during this first scan? set default values

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Implementation Method 2: System Control Variable (TwinCAT 2 Legacy)