Beckhoff First Scan Bit <FAST ⇒>

Always place your first-scan detection logic at the of your application entry point (usually MAIN ). If you calculate bFirstScan at the bottom of the program, blocks higher up in the execution order will miss the initialization event on the first cycle. Summary Matrix: Choosing the Right First Scan Method Target Use Case _TaskInfo.CycleCount Standard TwinCAT 3 Applications Native, highly reliable, zero manual flags to maintain. Beckhoff system architecture dependent. Inverted Flag ( BOOL ) Code portability across CoDeSys / IEC platforms Simple to understand, works on any PLC hardware.

If you are maintaining legacy TwinCAT 2 systems or want to utilize structural system flags, you can monitor the state of the PLC task control flags. TwinCAT provides standard libraries containing system structures that reflect the state of the execution engine.

Unlike legacy PLCs that feature a global system bit in an overhead memory map, TwinCAT links the first scan status directly to its respective execution task. Because TwinCAT is a multi-tasking, deterministic real-time environment, each separate task runs its own independent cycle tracking.

This simple pattern relies on the fact that static variables ( VAR declared in a program, POU, or function block) are initialized exactly once at program start. While functional, this method is generally discouraged in favor of the system-provided firstCycle because it adds an extra boolean variable and a constant check to every cycle. However, it is a useful fallback and helps illustrate the fundamental concept.

No manual coding is required to reset the bit; it is inherently tied to the task execution. 2. Manual Logic (The Classic Way) beckhoff first scan bit

Unlike a physical switch, this "bit" is a logical pulse that remains TRUE for exactly one task cycle. The Role of Initialization

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When you instantiate a function block, TwinCAT automatically calls its FB_Init method once.

For more detailed technical documentation, you can always refer to the Beckhoff Information System which provides comprehensive insights into TwinCAT programming standards. Always place your first-scan detection logic at the

| TwinCAT Version | Variable Name | Scope | |----------------|---------------|-------| | TwinCAT 2 | bInit | Global (in Standard.lib ) | | TwinCAT 3 | FirstScan | Per-POU (automatic) |

// Check if the flag is still false IF NOT bFirstCycleDone THEN // Place your one-time initialization logic here // ...

| Variable | Data type | Description | | :--- | :--- | :--- | | active | BOOL | TRUE if the task is active. | | taskName | STRING(16) | The name of the task. | | | BOOL | TRUE only on the first scan cycle after system startup; FALSE thereafter. | | cycleTimeExceeded | BOOL | TRUE if the task's execution time exceeds the configured cycle time. | | cycleTime | UDINT | The configured cycle time of the task (in 100 ns increments). | | lastExecTime | UDINT | The execution time of the task during the previous cycle (in 100 ns increments). | | priority | BYTE | The priority of the task. | | cycleCount | UDINT | The number of completed cycles since the task started. |

: Sending a "Reset" or "Init" command to external devices (like drives or Vision systems) over EtherCAT. Beckhoff system architecture dependent

: At the end of your program or inside the conditional block, set the variable to FALSE . Manual Code Example:

PROGRAM MAIN VAR bFirstScan : BOOL := TRUE; // Initialize as TRUE bInitialized : BOOL; END_VAR // --- Logic --- IF bFirstScan THEN // Place all your initialization code here iSpeed := 100; bInitializeDone := TRUE; bFirstScan := FALSE; // Turn off after first run END_IF; // ... Rest of your program Use code with caution. Method B: Using PLC Attributes (TwinCAT 3 Best Practice)

The primary utility of the First Scan bit lies in initialization. It serves as the logical "clean slate" mechanism. For instance, in complex motion control applications involving Beckhoff’s NC (Numerical Control) or robotics, the First Scan routine is used to verify the actual position of axes against their commanded positions. It allows the programmer to suppress motion commands until the system has verified that communication with servo drives is healthy. Furthermore, it is instrumental in state machine logic. By forcing the state machine into a specific "Init" or "Home" state on the first scan, the engineer ensures the machine follows a strict, safe sequence of startup events, regardless of the state the machine was in when it was last powered off.

Why it matters