Beckhoff First Scan Bit Verified <Limited Time>

along the rung when bInitDone is FALSE . Set the Coil bInitDone at the end of that same rung.

In Beckhoff TwinCAT 3 , efficiently capturing the is essential for initializing variables, resetting state machines, and executing one-time startup routines . Unlike many legacy PLC platforms that offer a simple, hardcoded system flag (like S₀ or SM0.1) for the very first execution cycle, TwinCAT handles this dynamically through system structures.

Are you using standard or Object-Oriented Programming (methods/properties) ?

// EXIT section runs when program stops EXIT myOutput := FALSE;

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)

END_IF

: Establishing initial handshake bits with HMIs or other PLCs.

FB_init is a specialized method that executes the first cycle of the PLC task. It runs during the transition from Stop to Run or during an Online Change. How to use FB_init: Right-click your Function Block in the solution tree. Select Add > Method .

If you are currently setting up the initialization routines for your TwinCAT 3 project, I can help you tailor this implementation to your specific hardware. Let me know:

On the second scan, bInitialized is TRUE , meaning NOT bInitialized is FALSE . The initialization block is bypassed for as long as the PLC remains in Run Mode. Method 2: System Variables ( TwinCAT_SystemInfoVarList )

Requires code sequencing management; easy to accidentally overwrite. Modular, Object-Oriented Architecture Clean encapsulation, executes before cyclic tasks start. Can be harder to debug for developers new to OOP.

Do you need to initialize variables that must be ?

Why it matters