: Declare a global or local boolean variable initialized to TRUE .
If the initialization only affects one Program Organization Unit (POU), keep bFirstScan inside that specific VAR block.
Encapsulate initialization logic inside Function Blocks ( FB_Init ) to keep MAIN clean. beckhoff first scan bit
TwinCAT provides a global structure called TwinCAT_SystemInfoVarList which contains runtime metrics about the PLC. Within this structure, you can access the current cycle counter of the task executing the code.
VAR_GLOBAL RETAIN firstScanFlag : BOOL := TRUE; // Initialize to TRUE END_VAR : Declare a global or local boolean variable
: The official Tc3_Standard provides a clean helper:
: Starting handshake sequences with external devices or HMIs. TwinCAT controllers have distinct boot phases
TwinCAT controllers have distinct boot phases. The first scan bit is set during the transition from to RUN mode. Understanding this can help diagnose why certain initialization tasks might not execute as expected. The SystemTaskInfoArr[1].firstCycle variable will be TRUE during this transition, but not during a simple program restart from the development environment.
Unlike legacy PLCs that feature a global system bit in an overhead memory map, TwinCAT links the first scan status directly to its respective execution task. Because TwinCAT is a multi-tasking, deterministic real-time environment, each separate task runs its own independent cycle tracking.
To detect the first scan in Beckhoff TwinCAT, you can read the system task info or create a classic initialization variable.
The First Scan Bit is a that is TRUE only for a single PLC cycle immediately after the runtime system starts executing the application. On the second cycle, it becomes FALSE and remains FALSE until the next controller reboot or program download.