Beckhoff First Scan Bit [new] (2026)

In Beckhoff TwinCAT systems, there is no single global "S:FS" bit like those found in Rockwell (Allen-Bradley) controllers . Instead, users typically leverage the PlcTaskSystemInfo

Part 8: Comparison with Other PLC Brands

| Brand | First Scan Mechanism | Beckhoff Equivalent | |--------|----------------------|----------------------| | Siemens | OB100 | INIT section or FB_FirstScan | | Rockwell | S:FS (First Scan) bit | bInit variable manually set | | Codesys | bInit in PLC_PRG | Same (TwinCAT is based on Codesys) | | Beckhoff Native | INIT, FB_Init, or F_TRIG | Choose based on scope | beckhoff first scan bit

• The first scan bit is a boolean flag that is true only during the PLC’s initial program cycle after the runtime transitions to RUN mode (or after a cold start). It indicates that the PLC is executing its very first program scan.
• In TwinCAT environments, this bit is typically provided by the runtime system (for example, as SysInit or a compiler/runtime variable) rather than being a user-created flag. Different Beckhoff/TwinCAT versions may expose named system variables or recommended idioms to detect first-scan behavior.

Edge-detected approach