// 3. Request axes to home (if needed) fbAxis1.bHomeRequest := TRUE;
// This runs once when the FB is created (first scan of the FB) METHOD FB_Init : BOOL VAR_INPUT bInitRetains : BOOL; // TRUE if retain variables are restored bInCopyCode : BOOL; // TRUE if FB is copied END_VAR fSpeed := 0.0; // Initialize internal variable bReady := FALSE; END_METHOD
By utilizing the standard initialization behavior of variables in TwinCAT, you can create a self-resetting flag. beckhoff first scan bit
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IF bFirstScan THEN // Execute one-time startup logic here Variables.lrTargetVelocity := 1500.0; Variables.diCycleCounter := 0; Variables.bSystemReady := FALSE; // Reset the bit so this block is skipped in subsequent cycles bFirstScan := FALSE; END_IF; // Normal cyclic logic continues below Use code with caution. Why This Works This link or copies made by others cannot be deleted
// -- First scan detection -- fbFirstScan(CLK := bInit); IF fbFirstScan.Q THEN bFirstScanDone := FALSE;
For scenarios where you need the flag to be TRUE on every program start (e.g., after a STOP→RUN transition), you can create your own. This method offers more control but is less standardized. Try again later
Remember that bFirstScan runs when the PLC transitions from STOP to RUN, which is not always the same as a power-cycle. Conclusion
Initiating communication handshakes with external fieldbus devices or vision systems. Best Practices & Pitfalls to Avoid 1. Beware of Online Changes