In Beckhoff TwinCAT, there is no single global system bit like the Allen-Bradley S:FS . Instead, you use task-specific system information or a manual initialization variable.
The most robust and elegant way to detect the first scan in TwinCAT 3 is by utilizing the built-in PLC Task System Information. TwinCAT automatically tracks execution data for every task group configured in your project. 1. The TwinCAT_SystemInfoVarList beckhoff first scan bit
The array index _TaskInfo[1] points to your primary PLC task. The property .CycleCount increments automatically on every cycle. On the very first pass, it evaluates to 1 , rendering bFirstScan true. On cycle two and all subsequent cycles, bFirstScan becomes false automatically. Method 2: The Classic IEC 61131-3 "Inverted Flag" Approach In Beckhoff TwinCAT, there is no single global
If your TwinCAT project has multiple tasks (e.g., a fast 1ms task and a slow 10ms task), remember that each task has its own "first cycle." TwinCAT automatically tracks execution data for every task
The most robust way is to use the PlcTaskSystemInfo structure, which contains a FirstCycle boolean. This bit is only during the very first execution of that specific task after the TwinCAT runtime starts. Implementation Example (Structured Text):
To use this system variable, you must include the TcSystem library in your TwinCAT project. Once included, you can directly access the variable SystemTaskInfoArr[1].firstCycle in your PLC logic. Its definition states: "During the first PLC task cycle, this variable has the value: TRUE".
Mastering the Beckhoff First Scan Bit: A Complete Guide to PLC Initialization