Beckhoff First Scan Bit !!better!! Here

If you perform a or a Reset Original , the memory is cleared, variables re-initialize to their default values, and bFirstScan will correctly fire again on the next startup. Method 2: Utilizing TwinCAT System Variables

Use the first scan to ensure all physical outputs are in a "Safe/Off" state before the logic takes over.

TwinCAT provides internal system information via the Tc2_System library. You can check if the current cycle is the very first one by looking at the system task info.

// Obtain the index of the current task fbGetCurTaskIdx();

// Set the flag to true so this block will not execute again bFirstCycleDone := TRUE; beckhoff first scan bit

For Object-Oriented Programming (OOP) in TwinCAT, encapsulate your startup logic within a dedicated FB_Init method or an explicit M_Initialize() method called during the first cycle. If you are working on a specific architecture, let me know:

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

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

The first scan is the perfect place to set default values, clear buffers, or initialize complex data structures that are not retentive. If you perform a or a Reset Original

Implementation Method 3: Using the TwinCAT 3 Event Functions

PROGRAM MAIN VAR bInitTriggered : BOOL := FALSE; // Instantiated as FALSE at boot bFirstScan : BOOL; // Your usable First Scan bit END_VAR // If the init flag hasn't been set yet, this is the first scan IF NOT bInitTriggered THEN bFirstScan := TRUE; bInitTriggered := FALSE; // Prepare to lock it out next line ELSE bFirstScan := FALSE; END_IF; // --- CRITICAL STEP --- // At the very end of your first scan logic, lock out the trigger permanently IF bFirstScan THEN // Execute initialization routines here... // Lock the gate for subsequent cycles bInitTriggered := TRUE; END_IF; Use code with caution. How it works:

a global or local variable: bInitialized : BOOL := FALSE; Logic :

Beckhoff TwinCAT offers built-in program-local initialization flags that many users overlook: INIT and EXIT sections within a program. You can check if the current cycle is

This is the most common way to implement the first scan bit.

PROGRAM MAIN VAR bFirstCycleDone : BOOL := FALSE; END_VAR

Typical uses and patterns

For engineers working with Beckhoff TwinCAT 3 (and TwinCAT 2), the concept of the (often implemented via the bInit or bFirst variable) is the cornerstone of safe and robust machine initialization.