Beckhoff First Scan Bit Best Jun 2026

:Place this line at the very bottom of your MAIN program or the last task to execute :

It’s a system flag that is after:

Call it at the beginning of your main cycle: beckhoff first scan bit

: In industrial settings, a first-scan bit is considered essential for resetting retentive memory and ensuring equipment starts in a safe, predictable state. Alternative for Advanced Users

Another method, often considered more accurate for truly catching the first PLC tick after the system is fully booted and ready, involves using the TwinCAT system library functions if needed, but the method above is generally robust for most applications. :Place this line at the very bottom of

The first scan bit is a fundamental concept for writing robust and predictable PLC programs in Beckhoff TwinCAT. Whether you choose the system variable SystemTaskInfoArr[1].firstCycle for standard cold-start initialization or a custom RETAIN flag for more control, understanding this feature will help you build more reliable automation systems.

VAR bInitialScan : BOOL := TRUE; // Initialized to TRUE on startup END_VAR IF bInitialScan THEN // Startup logic bInitialScan := FALSE; // Permanent reset after first run END_IF Use code with caution. Copied to clipboard 3. Applications and Best Practices The first scan bit is essential for: Whether you choose the system variable SystemTaskInfoArr[1]

There are two primary methods used in TwinCAT to achieve "first scan" functionality: System Variable Method : The most robust way is using the FirstCycle member of the PlcTaskSystemInfo structure. How it works : Every PLC task has a system variable which contains a boolean FirstCycle . This bit is only during the very first cycle of that specific task.

This is particularly useful for initializing complex data structures, allocating memory with __NEW , or passing parameters to function blocks before they are used in the program.

// Reset the flag after first cycle IF bFirstScanDone THEN bFirstScan := FALSE; END_IF

Method 3: Using the FB_init Method (TwinCAT 3 Object-Oriented Approach)