In Beckhoff TwinCAT, there is no single pre-defined global "First Scan" variable like the
: Highly accurate; resets every time the PLC transitions from STOP to RUN or after a power cycle. Method 2: Manual Flag (The "Standard" Way)
In legacy TwinCAT 2 and early TwinCAT 3 projects using the Tc2_Standard library, the standard way to get a first scan bit is: beckhoff first scan bit
Version and platform considerations
On older hardware like the , some technical references mention a specific status bit (Bit 4) in certain control parameters that is set for the first scan after power-up. Key Comparison PlcTaskSystemInfo.FirstCycle Custom bInitialized Variable Best for System-level startup/initialization Logic-level resets/warm restarts Reset Trigger TwinCAT Runtime Restart PLC Program Start (Start/Stop) Setup Requires GETCURTASKINDEX Simple declaration In Beckhoff TwinCAT, there is no single pre-defined
There are two primary ways to access or create a "First Scan" signal in Beckhoff TwinCAT:
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 Get the current task index fbGetCurTaskIdx(); // 2
// -- Wait for EtherCAT sync -- nState := fbEcMaster.GetState(); IF nState <> 8 THEN RETURN; // Don't run logic until bus is operational END_IF