For a more "built-in" feel, you can access internal PLC task information. Beckhoff provides a structure that tracks the scan count of a task. : _TaskInfo[Index].CycleCount
Because it is part of the task info structure, it correctly identifies the first scan for the specific task it is called within. beckhoff first scan bit
The First Scan Bit is a flag that is for exactly one PLC cycle when the controller moves from "Config" or "Stop" mode into "Run" mode. After that first execution of the logic, the bit turns FALSE and remains so until the PLC is restarted or the code is re-downloaded. Why Do You Need It? For a more "built-in" feel, you can access
The primary utility of the First Scan bit lies in initialization. It serves as the logical "clean slate" mechanism. For instance, in complex motion control applications involving Beckhoff’s NC (Numerical Control) or robotics, the First Scan routine is used to verify the actual position of axes against their commanded positions. It allows the programmer to suppress motion commands until the system has verified that communication with servo drives is healthy. Furthermore, it is instrumental in state machine logic. By forcing the state machine into a specific "Init" or "Home" state on the first scan, the engineer ensures the machine follows a strict, safe sequence of startup events, regardless of the state the machine was in when it was last powered off. The First Scan Bit is a flag that
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