Beckhoff First Scan Bit -

In Beckhoff TwinCAT (2 and 3), there is no single "magic" global bit like the S:FS in Allen-Bradley . Instead, you can access the "First Scan" status through built-in system variables or by creating a custom initialization flag. 1. Using Built-in System Info (FirstCycle)

// Also override certain retained values if needed
IF NOT bRetainValid THEN
    // Force defaults
END_IF

A common "best practice" for portability across different PLC brands is to create your own flag in a Global Variable List (GVL) . Declare it with an initial value: beckhoff first scan bit

A common practice among developers is to manually declare a global variable that initializes as and is immediately set to after use. Structured Text Example: In Beckhoff TwinCAT (2 and 3), there is

The First Scan Bit is a digital output that is automatically set by the PLC during its startup sequence. When the PLC is powered on or reset, it executes a series of internal checks and initializations before starting to execute the user program. During this first scan cycle, the First Scan Bit is set to TRUE (or 1). A common "best practice" for portability across different

. At the very end of your main program, set this variable to

What is the First Scan Bit?

It’s a system flag that is TRUE only during the first PLC cycle after:

Edge detection using task cycle counter: