Beckhoff First Scan Bit – Genuine & Top-Rated

A simpler, more manual approach uses a boolean variable to track if the first scan has already occurred. This method is particularly useful for quick prototyping or simple programs.

: This method is portable across different PLC brands and doesn't require specific TwinCAT system function calls. Example Implementation:

Executing specific logic only once during the very first PLC task cycle is essential for several reasons:

METHOD FB_init : BOOL VAR_INPUT bInitRetains : BOOL; // If TRUE, retain variables are initialized bInCopyCode : BOOL; // If TRUE, instance was copied (e.g., during online change) END_VAR // Initialization code here nTargetVelocity := 1500; bEnableAxis := TRUE; Use code with caution. Why Use It? beckhoff first scan bit

You can access this feature through the implicit array, which contains data for every task running in your PLC project. Syntax (Structured Text):

FB_init is a specialized method that executes implicitly the PLC task starts its cyclic execution. It runs during the initialization code phase. How to use FB_init :

Demystifying the Beckhoff "First Scan Bit": 3 Ways to Handle PLC Initialization in TwinCAT 3 A simpler, more manual approach uses a boolean

PROGRAM MAIN VAR bFirstScan : BOOL := TRUE; // Initialize as TRUE bFirstScanDone: BOOL := FALSE; END_VAR

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.

// INIT section runs once when program is loaded/started INIT myOutput := FALSE; // Set safe state // Home axes, clear arrays, etc. Syntax (Structured Text): FB_init is a specialized method

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

Have a specific first scan challenge? Visit the Beckhoff Community forums or consult your local Beckhoff support engineer.