Use TwinCAT's Watch Windows and Scope functionality to monitor your first scan variables in real-time, ensuring they behave as expected across all start-up scenarios.
The First Scan Bit, also known as the "First Cycle Bit" or "Initialization Bit," is a special bit in Beckhoff PLCs that indicates when the PLC is executing its first scan cycle. In other words, it signals that the PLC is starting up and executing its program for the first time.
Use the "Restart TwinCAT System" button (red square then green triangle) to truly test your first scan logic. 2. Time-Based Tasks beckhoff first scan bit
While the core concept is universal, the naming and implementation of the first scan differ across platforms.
⚠️ 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. Use TwinCAT's Watch Windows and Scope functionality to
Does not waste execution time checking an IF statement during normal cyclic operation. Critical Use Cases for First Scan Logic
Use the first scan to ensure all physical outputs are in a "Safe/Off" state before the logic takes over. Use the "Restart TwinCAT System" button (red square
As noted in industry forums, the first scan bit in Beckhoff is tied to the Runtime Started state, rather than just the "RUN" mode switch. 3. Best Practices for First Scan Logic