Beckhoff First Scan Bit Fix -

: Triggering a TP (Timer Pulse) or R_TRIG that needs to fire immediately upon startup.

// Main logic uses bIsFirstScan... // Final line of code: bIsFirstScan := FALSE; Use code with caution. Copied to clipboard 3. SFC Initialization Flag

found in Allen-Bradley controllers. Instead, developers typically use one of two methods to detect the first scan cycle of a PLC task: a built-in system variable or a manual "home-made" boolean flag. 1. Built-in Method: PlcTaskSystemInfo TwinCAT provides a structured data type called PlcTaskSystemInfo that contains a specific boolean member for this purpose: FirstCycle

FUNCTION_BLOCK FB_DriveController VAR_INPUT bEnable : BOOL; END_VAR VAR_OUTPUT bReady : BOOL; END_VAR VAR fSpeed : REAL; END_VAR

The first scan code should be fast and deterministic. Avoid long WHILE loops or complex, time-consuming operations that could cause a cycle time watchdog fault. beckhoff first scan bit

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

Beckhoff TwinCAT offers built-in program-local initialization flags that many users overlook: INIT and EXIT sections within a 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.

: This method is portable across different PLC brands and doesn't require specific TwinCAT system function calls. Example Implementation: : Triggering a TP (Timer Pulse) or R_TRIG

A common point of confusion during commissioning revolves around what actually triggers a "first scan" in TwinCAT. _TaskInfo[x].FirstCycle Behavior Custom Software Flag Behavior (Switching Config -> Run) Triggers TRUE Triggers TRUE Cold Reset / PLC Download Triggers TRUE Triggers TRUE Stop & Start PLC Code (Red Square / Green Triangle in VS) Remains FALSE Triggers TRUE (Variables re-initialize) Online Change (Modifying code on the fly) Remains FALSE Remains FALSE (Memory is preserved) The Stop/Start Runtime Trap

A common practice among developers is to manually declare a global variable that initializes as and is immediately set to after use. Structured Text Example:

// 1. Set global output safe state GVL.bEmergencyStop := FALSE; GVL.nMotorSpeed := 0;

At the very end of your MAIN routine, add: bFirstScan := FALSE; . Copied to clipboard 3

// EXIT section runs when program stops EXIT myOutput := FALSE;

// Directly accessing the system array (often index 1 is the default PLC task) bFirstScan := SystemTaskInfoArr[1].firstCycle; Use code with caution.

Highly accurate and managed entirely by the TwinCAT runtime.

PROGRAM MAIN VAR bFirstScan : BOOL := TRUE; // Initialize to TRUE bIsInitialized : BOOL := FALSE; END_VAR // ---------------------------------------------------- // FIRST SCAN LOGIC // ---------------------------------------------------- IF bFirstScan THEN // --- Place Initialization Code Here --- // Example: Set initial values // AxisRef.bExecute := FALSE; // CurrentState := E_State.Idle; // ------------------------------------- bFirstScan := FALSE; // Turn off for next scan bIsInitialized := TRUE; // Mark as initialized END_IF // ---------------------------------------------------- // NORMAL CYCLIC CODE // ---------------------------------------------------- // ... rest of your program Use code with caution. The "TwinCAT Runtime" Method (Recommended)