The difference between a and a warm start in relation to this bit. Beckhoff CX1010 first scan | PLCtalk - Interactive Q & A
Note: This method is more robust because it relies on the system's own cycle counter rather than a variable you might accidentally overwrite elsewhere. Best Practices
| Platform | System Variable Availability | Behavior Notes | |---|---|---| | | SystemTaskInfoArr[1].firstCycle via TcBaseBCxx50.lbx | Requires library linking | | BX Series | SystemTaskInfoArr via TcBaseBX.lbx | Full structure support | | TwinCAT 2 (PC) | SystemTaskInfoArr via TcSystem.lib | Standard implementation | | TwinCAT 3 (PC) | SystemTaskInfoArr via TcSystem.lib | Same as TwinCAT 2, fully compatible | | CX Series | Same as PC platforms | Consistent behavior across all PC-based controllers | 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
The is a boolean variable that is TRUE only during the very first cycle of the PLC program after transitioning from Stop to Run mode [1, 2]. Immediately after that first cycle completes, the bit automatically turns FALSE and remains that way until the PLC is stopped and restarted [1]. Why Do You Need It? The difference between a and a warm start
Large projects with multiple tasks or complex initialization. Quick, single-task projects or basic logic. Summary of Benefits
This is the standard, robust method in TwinCAT 3. You check the FirstCycle property within the PlcTaskSystemInfo array. The property
VAR fbGetCurTaskIdx : GETCURTASKINDEX; // Fetches the current task's index bFirstScan : BOOL; // Your usable First Scan bit END_VAR // 1. Get the current task index fbGetCurTaskIdx(); // 2. Read the FirstCycle boolean from the task system info bFirstScan := _TaskInfo[fbGetCurTaskIdx.index].FirstCycle; Use code with caution. Copied to clipboard
boolean, which signals the initial task execution, or by creating a custom global variable initialized to TRUE. Alternative methods include utilizing
Beyond just the firstCycle , these additional variables are incredibly valuable for real-time diagnostics. For instance, the cycleTimeExceeded flag can alert you to performance bottlenecks, helping to debug why a particular cycle might be taking too long.
However, in the ecosystem, there is no single, hardcoded global system variable explicitly named "First Scan Bit." Instead, Beckhoff provides a highly flexible architecture utilizing system variables, initialization properties, and standard IEC 61131-3 logic to achieve the exact same functionality. Why Do You Need a First Scan Bit?