Beckhoff First Scan Bit Now

Name it something like GVL_System : VAR_GLOBAL bFirstScan : BOOL := TRUE; END_VAR Use code with caution. 2. Evaluate it at the start of your main code

Proper utilization of the first scan bit is a hallmark of clean, structured Beckhoff programming. Common use cases include:

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

PROGRAM MAIN VAR bFirstCycleExecuted : BOOL := FALSE; // Instantiated to FALSE on startup END_VAR IF NOT bFirstCycleExecuted THEN // Place one-time code here // Lock out the loop permanently bFirstCycleExecuted := TRUE; END_IF Use code with caution. How it works beckhoff first scan bit

Note: It is worth highlighting that restarting or stopping/starting the PLC code via the engineering UI may not always toggle this bit. It strictly relies on the full initialization of the TwinCAT runtime system.

If you need a first scan bit that works across multiple programs and tasks, use the Tc2_System or Tc3_System library.

: For complex setups, some developers prefer using a dedicated Initialization (INIT) block Name it something like GVL_System : VAR_GLOBAL bFirstScan

When working with the first scan bit, keep these points in mind to avoid common pitfalls:

Without a proper first scan routine, your machine might start with actuators in unpredictable positions or unfinished states from a previous run.

If you encounter difficulties with your specific Beckhoff hardware platform or TwinCAT version, consult the Beckhoff Information System (Infosys) or reach out to your local Beckhoff support representative for platform-specific guidance. Common use cases include: Have a specific first

IF bIsFirstScan THEN // Your one-time initialization code bIsFirstScan := FALSE; // Flag is cleared for future scans END_IF

Setting default temperatures, speeds, or timers.