Beckhoff First Scan Bit -

If you forget this line, your initialization logic will run every single cycle, overriding any changes made to those variables later in the program.

Does not waste execution time checking an IF statement during normal cyclic operation. Critical Use Cases for First Scan Logic

: If your project has multiple PLC tasks, _AppInfo.bFirstCycle is local to the context of the task it is called in.

In TwinCAT 3, you can inspect the implicit global structure _TaskInfo . This structure contains an array of system data relating to your specific tasks. beckhoff first scan bit

A common "best practice" for portability across different PLC brands is to create your own flag in a Global Variable List (GVL) . :

PROGRAM MAIN VAR bFirstScan : BOOL; rst : BOOL; END_VAR

// -- First scan detection -- fbFirstScan(CLK := bInit); IF fbFirstScan.Q THEN bFirstScanDone := FALSE; If you forget this line, your initialization logic

For advanced TwinCAT 3 users working with Function Blocks, the most elegant and robust "first scan" is not a bit at all—it's the object constructor: FB_Init .

On older hardware like the , some technical references mention a specific status bit (Bit 4) in certain control parameters that is set for the first scan after power-up. Key Comparison PlcTaskSystemInfo.FirstCycle Custom bInitialized Variable Best for System-level startup/initialization Logic-level resets/warm restarts Reset Trigger TwinCAT Runtime Restart PLC Program Start (Start/Stop) Setup Requires GETCURTASKINDEX Simple declaration

If you cannot use PlcTaskSystemInfo , you can create a local BOOL variable that is initialized to TRUE . In TwinCAT 3, you can inspect the implicit

When the TwinCAT PLC runtime transitions to , memory is allocated and bFirstScan is initialized to TRUE .

Purging old data arrays, resetting FIFO (First-In, First-Out) pointers, and sending initial handshakes to Modbus, EtherCAT, or EtherNet/IP slaves.