Beckhoff First Scan Bit -

PROGRAM MAIN VAR fbGetCurTaskIdx : GETCURTASKINDEX; END_VAR

The most straightforward way is to declare a global variable that resets itself after the first cycle.

: The timer’s output starts FALSE . On the first cycle, IN is TRUE , but the timer hasn't elapsed, so Q remains FALSE . Thus bFirstScan = TRUE . On the second cycle, Q becomes TRUE , IN becomes FALSE , and bFirstScan becomes FALSE permanently. beckhoff first scan bit

VAR_GLOBAL FirstScan : BOOL := TRUE; END_VAR

Beckhoff's TwinCAT 3 environment does not have a dedicated pre-defined "first scan" system bit like Allen-Bradley's S:FS . Instead, developers typically implement this functionality manually using an initial value or by referencing specific PLC task variables. Thus bFirstScan = TRUE

PROGRAM MAIN VAR fbGetCurTaskIndex : GETCURTASKINDEX; // Function block to find active task index bFirstScan : BOOL; // Our dedicated local first scan variable END_VAR // 1. Invoke the system function block to determine the current task index fbGetCurTaskIndex(); // 2. Extract the FirstCycle boolean flag from the global system task array bFirstScan := _TaskInfo[fbGetCurTaskIndex.index].FirstCycle; // 3. Execute your targeted one-time initialization routine IF bFirstScan THEN // Place one-time initialization code here FormatStorageDrives(); LoadDefaultCalibrationValues(); SetStateMachineToDefault(); END_IF Use code with caution. Why Choose Method 1?

Understanding and Implementing the Beckhoff First Scan Bit in TwinCAT Beckhoff introduced the Tc3_Standard library

With TwinCAT 3, Beckhoff introduced the Tc3_Standard library, which includes a dedicated function block: F_TRIG combined with a system flag is no longer needed. Instead, use:

It becomes FALSE for all subsequent cycles.