In Beckhoff TwinCAT, the "first scan bit" is a fundamental tool used to execute initialization code only once when the PLC starts or transitions into Run mode. Unlike some other platforms that use a fixed system bit (like Siemens' S:FS ), TwinCAT provides a more flexible approach using built-in system structures or manual variable initialization. Direct Solution for First Scan Bit
If you use the manual variable method, ensure the line bFirstScan := FALSE; is at the very bottom of your MAIN task. If you put it in a sub-function, other parts of your program might miss the "True" state. beckhoff first scan bit
: At the very end of your MAIN program (or the last program executed in your task), set this bit to FALSE . In Beckhoff TwinCAT, the "first scan bit" is
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: If you put it in a sub-function, other
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The Beckhoff First Scan bit is your "clean slate" button. Whether you use a simple boolean flag or the system's cycle counter, implementing this ensures that your machine starts up with the correct parameters every time, preventing "ghost" data from causing erratic behavior during commissioning.
You can use this bit in to set default values at startup: