Beckhoff First Scan Bit Jun 2026

: Do not use the first scan bit to overwrite PERSISTENT or RETAIN variables unless you intentionally want to ignore their saved values upon every reboot.

In this example, the FirstScan system variable is used to execute an initialization code segment during the first scan cycle. Once the initialization is complete, the FirstScan bit is reset to FALSE. beckhoff first scan bit

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TwinCAT provides a dedicated global data structure called PlcTaskSystemInfo , which contains a FirstCycle boolean for this exact purpose. The official and recommended approach to access it is: : Do not use the first scan bit

Beckhoff's TwinCAT 3 environment offers multiple methods to implement a first scan, ranging from a simple, self-created flag to using the more official, built-in system structures. END_IF TwinCAT provides a dedicated global data structure

It is more robust than manual "first scan" flags (like using a boolean that you set to false at the end of the code), as the PLC runtime handles its state directly. Usage Example