Beckhoff First Scan Bit !link!

If you are transitioning to the Beckhoff ecosystem from Rockwell Automation (Studio 5000) or Siemens (TIA Portal), you might quickly notice that . Because Beckhoff uses an object-oriented, multi-tasking real-time environment based on CODESYS, initialization is handled through explicit system data types, initialization code blocks, or custom software variables.

In , this is achieved using the First Scan Bit (also known as the First Cycle Bit).

Do not assume your fieldbus networks (EtherCAT, PROFINET) are fully operational and exchanging valid data during the first scan. EtherCAT slaves require a few cycles to transition from Safe-Op to Op mode. If your first scan bit relies on reading live data from an external sensor, it will likely read zeroes or trigger a bus fault. Use the first scan bit for . 3. Mismanaging Retain Variables

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

Always place your first-scan detection logic at the of your application entry point (usually MAIN ). If you calculate bFirstScan at the bottom of the program, blocks higher up in the execution order will miss the initialization event on the first cycle. Summary Matrix: Choosing the Right First Scan Method Target Use Case _TaskInfo.CycleCount Standard TwinCAT 3 Applications Native, highly reliable, zero manual flags to maintain. Beckhoff system architecture dependent. Inverted Flag ( BOOL ) Code portability across CoDeSys / IEC platforms Simple to understand, works on any PLC hardware.

Because it is part of the task info structure, it correctly identifies the first scan for the specific task it is called within.

Never make your First Scan bit a RETAIN or PERSISTENT variable. It needs to reset every time the PLC power cycles. If you are transitioning to the Beckhoff ecosystem

TwinCAT 3 allows you to use attributes to ensure initialization happens properly, especially when dealing with complex function blocks or specialized task handling.

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

In Beckhoff TwinCAT, there is no single global system bit like the Allen-Bradley S:FS . Instead, you use task-specific system information or a manual initialization variable. Do not assume your fieldbus networks (EtherCAT, PROFINET)

In TwinCAT, you can run multiple independent tasks at different cycle times (e.g., a 1ms motion control task and a 50ms visualization/HMI task). A single global first scan bit would fail because different tasks start at different times or may be restarted independently. Therefore, initialization logic in TwinCAT must be managed at the application, task, or function block level. Method 1: The Local Variable Approach (Recommended)

You can use the attribute 'call_after_init' pragma to mark a method to be executed immediately after the FB_init call but still before the first cyclic call of the FB. This gives you another layer of control over the object's lifecycle.

Another method, often considered more accurate for truly catching the first PLC tick after the system is fully booted and ready, involves using the TwinCAT system library functions if needed, but the method above is generally robust for most applications.