The standard method to detect the first cycle in TwinCAT is to use two key tools: the GETCURTASKINDEX function block and the global _TaskInfo array.
. It will then act as a first-scan bit for every subsequent cycle. : Simple to understand and portable across different IEC 61131-3 platforms. Beckhoff Information System User Experience and "Reviews" Behavioral Quirks : Some users have noted that the FirstCycle
The most straightforward way is to declare a global variable that resets itself after the first cycle. beckhoff first scan bit
// Implementation fbFirstScan(IN := NOT fbFirstScan.Q, PT := T#1MS); bFirstScan := NOT fbFirstScan.Q;
To make your machine's startup routine bulletproof, consider these best practices. The standard method to detect the first cycle
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
Understanding and Implementing the Beckhoff First Scan Bit in TwinCAT : Simple to understand and portable across different
: Use bInit in FB_Init – it respects online changes differently. Or explicitly handle a "reinit" via a variable that you toggle manually.
IF _TaskInfo[fbGetCurTaskIndex.index].FirstCycle THEN // Your initialization code here END_IF Use code with caution. Copied to clipboard
TwinCAT provides an array called _TaskInfo that holds real-time diagnostic data for every task running on the processor. You must fetch the specific task index first to avoid hardcoding errors.