Beckhoff First Scan Bit Portable Jun 2026

If you are used to pure IEC 61131-3 logic or want something simple that does not require checking task arrays, you can use variable initialization.

Keeps initialization logic tightly coupled with the object.

If you have nested function blocks, they won't automatically know a "first scan" occurred in the main program.

Conclusion The Beckhoff first scan bit is a simple but essential tool for controlled startup in TwinCAT PLC programs. Properly used, it ensures variables and outputs are initialized predictably, prevents unsafe transient actions, and supports reliable commissioning and diagnostics. Adhering to concise, documented first-scan patterns and combining them with broader safety practices produces safer, more maintainable control software. 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.

// -- First scan detection -- fbFirstScan(CLK := bInit); IF fbFirstScan.Q THEN bFirstScanDone := FALSE;

FirstScan gives you a deterministic moment to reset, preset, or home everything before normal operation begins. If you are used to pure IEC 61131-3

: This function block grabs the internal index of the task currently running the MAIN program.

Because TwinCAT initializes variables to their default values upon a transition to Run mode, we can exploit this behavior to capture the first execution cycle. Code Implementation

PROGRAM MAIN VAR fbGetCurTaskIndex : GETCURTASKINDEX; bFirstScan : BOOL; END_VAR // Fetch the index of the currently running task fbGetCurTaskIndex(); // Read the first cycle flag for this specific task bFirstScan := _TaskInfo[fbGetCurTaskIndex.index].FirstCycle; // Use the first scan bit to initialize logic IF bFirstScan THEN // Execute one-time configuration routines rTargetVelocity := 100.0; sMachineState := 'INITIALIZING'; END_IF Use code with caution. Why This Works Conclusion The Beckhoff first scan bit is a

Name it something like GVL_System : VAR_GLOBAL bFirstScan : BOOL := TRUE; END_VAR Use code with caution. 2. Evaluate it at the start of your main code

IF TwinCAT_SystemInfoVarList._FirstScan THEN // One-time actions END_IF

If you click the "Stop PLC" button inside Visual Studio and hit "Start PLC" again, . This is because the underlying TwinCAT real-time operating system kernel was never restarted—only the execution engine of that specific sub-program was halted. If you explicitly require an initialization pulse whenever the application logic restarts independently of the kernel, use Method 3 (The Custom Software Flag) . Practical Use Cases for First Scan Bits

END_IF