Mastering the first scan bit is not just about writing code; it is about adopting a design philosophy that prioritizes safety, reliability, and predictability from the very first moment of operation. Whether you use the robust _TaskInfo[].FirstCycle or a simple manual boolean flag, correctly implementing startup logic is a fundamental requirement for any professional TwinCAT project. By understanding the methods and best practices outlined in this guide, you can ensure your applications always start in a safe, controlled, and predictable state.
IF _TaskInfo[fbGetCurTaskIndex.index].FirstCycle THEN // Your initialization code here END_IF Use code with caution. Copied to clipboard
Are you developing inside a standard or a reusable Function Block (FB) ?
Note: exact symbol names can vary by TwinCAT version and project conventions. beckhoff first scan bit
// Main cyclic code myOutput := TRUE; // Normal logic
: Triggering a one-time read from a CSV file or database.
: Triggering a TP (Timer Pulse) or R_TRIG that needs to fire immediately upon startup. Mastering the first scan bit is not just
PROGRAM MAIN VAR fbSystemFirstScan : FB_FirstScan; bGlobalFirstScan : BOOL; END_VAR
// Reset the flag after first cycle IF bFirstScanDone THEN bFirstScan := FALSE; END_IF
Do you need the logic to survive an , or only a full PLC Restart ? Share public link IF _TaskInfo[fbGetCurTaskIndex
Ensure that the POU (Program Organization Unit) containing your first scan initialization logic is placed at the very top of your task execution list. Initializing data after other code has already processed it for one cycle can lead to erratic machine behavior. To help tailor this to your current project, let me know:
END_IF