Then, in your main program or task, you reset this flag after the first cycle:
Telling other machines or HMI that the PLC has just powered on. 2. Implementing the First Scan Bit in TwinCAT 3 (ST) beckhoff first scan bit
: Use bInit in FB_Init – it respects online changes differently. Or explicitly handle a "reinit" via a variable that you toggle manually. Then, in your main program or task, you
Use a timer or a counter after the first scan to delay initialization by a few cycles. 3. Missing Initializations in Function Blocks Or explicitly handle a "reinit" via a variable
PROGRAM MAIN VAR bFirstScan : BOOL := TRUE; (* Initialize as TRUE *) nCounter : INT; END_VAR
If you are using programming, TwinCAT provides implicit variables called SFC Flags .
The array index _TaskInfo[1] points to your primary PLC task. The property .CycleCount increments automatically on every cycle. On the very first pass, it evaluates to 1 , rendering bFirstScan true. On cycle two and all subsequent cycles, bFirstScan becomes false automatically. Method 2: The Classic IEC 61131-3 "Inverted Flag" Approach