Beckhoff First Scan Bit Jun 2026

In Beckhoff TwinCAT, the First Scan Bit is accessed via the _TaskInfo system array using the FirstCycle property. This bit is uniquely TRUE during the first execution cycle of a task, allowing for precise system initialization. First Scan Bit - OpenPLC Forum

The most robust and recommended method is to use the firstCycle variable from the TcSystem library. This variable is part of the SYSTEMTASKINFOTYPE structure, which is accessible via the SystemTaskInfoArr array. The index in this array (typically 1) corresponds to the PLC task ID. beckhoff first scan bit

Pre-loading recipe parameters, positioning targets, or setting default calibration values before the main control loops execute. In Beckhoff TwinCAT, the First Scan Bit is

| Variable Type | Behavior on Power Cycle | Behavior on Program Download | Interaction with First Scan | |---|---|---|---| | | Re-initializes to declared start value | Re-initializes | First scan can set custom initial values | | RETAIN | Retains last value from before power loss | Retains last value (by default) | Useful for detecting first scan without losing value | | PERSISTENT | Retains value across program changes | Retains value across program downloads | Often used with first scan to determine if first run after major update | This variable is part of the SYSTEMTASKINFOTYPE structure,

It acts as a gatekeeper for initialization code. Without a first scan bit, logic intended to run only once would execute continuously, overwriting live operational data or wasting processing power. Why TwinCAT Doesn't Have a Global "S:FS"

PROGRAM MAIN VAR // Variable initializes to FALSE automatically upon startup bInitialized : BOOL := FALSE; // Your operational variables nTargetVelocity : INT; sSystemStatus : STRING; END_VAR // The First Scan Logic IF NOT bInitialized THEN // This code runs EXACTLY ONCE on the very first PLC cycle nTargetVelocity := 100; sSystemStatus := 'System Initialized'; // Set the flag to TRUE so this block is skipped in the next cycle bInitialized := TRUE; END_IF; // --- Your regular PLC program starts here --- Use code with caution. Why this works: