Beckhoff First Scan Bit |best| < Extended – Release >

This comprehensive article breaks down the technical reasons behind this architectural difference and explains the three best practices for implementing a "First Scan Bit" behavior inside Beckhoff TwinCAT 3 . The Core Concept: Why Initialization Matters in TwinCAT

IF bFirstScan THEN // Perform Initialization Tasks here iTargetVelocity := 1500; bMachineReady := FALSE; END_IF // All other machine logic goes here... // The very last line of the program: bFirstScan := FALSE; Use code with caution. 2. Using FB_GetCurTaskIndex (The Pro Method)

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 bIsFirstScan THEN // Your one-time initialization code bIsFirstScan := FALSE; // Flag is cleared for future scans END_IF beckhoff first scan bit

bFirstScan := FALSE; (* Set to false so this never runs again *)

TwinCAT provides an array called _TaskInfo that holds real-time diagnostic data for every task running on the processor. You must fetch the specific task index first to avoid hardcoding errors.

PROGRAM MAIN VAR bFirstScan : BOOL; END_VAR // Check the boot count or execution cycle of the current task // Task ID 1 is typically the standard PLC task bFirstScan := (_TaskInfo[1].CycleCount = 0); IF bFirstScan THEN // Execute initialization routines here END_IF; Use code with caution. This comprehensive article breaks down the technical reasons

⚠️ This method can fail if the PLC is stopped/started without power cycle. Always prefer the system library method.

If you use the manual variable method, ensure the line bFirstScan := FALSE; is at the very bottom of your MAIN task. If you put it in a sub-function, other parts of your program might miss the "True" state.

PROGRAM MAIN VAR bFirstScan : BOOL; // True only during the first execution cycle bInitialized : BOOL; // Global flag showing init status fbReadConfig : FB_FileRead; // Example init function block END_VAR // Determine First Scan using the TwinCAT Task Info structure bFirstScan := (_TaskInfo[1].CycleCount = 1); IF bFirstScan THEN // Execute Startup-Only Logic Here bInitialized := FALSE; // Example: Set default hardware overrides GVL_Hardware.TargetVelocity := 100.0; END_IF; // Rest of your cyclic PLC program runs below Use code with caution. Why this works: Evaluate it at the start of your main

METHOD FB_init : BOOL VAR_INPUT bInitRetains : BOOL; bInCopyCode : BOOL; END_VAR

If you store data in VAR_RETAIN or VAR_PERSISTENT , these variables survive a PLC restart. If your first scan bit blindly overwrites these variables with hardcoded default values on every boot, you defeat the entire purpose of persistent memory. Ensure your initialization logic leaves persistent recipes intact unless explicitly forced by a "Factory Reset" flag.