The Beckhoff First Scan bit is your "clean slate" button. Whether you use a simple boolean flag or the system's cycle counter, implementing this ensures that your machine starts up with the correct parameters every time, preventing "ghost" data from causing erratic behavior during commissioning.
IF bFirstScan THEN // Perform Initialization Tasks here iTargetVelocity := 1500; bMachineReady := FALSE; END_IF // All other machine logic goes here... // The very last line of the program: bFirstScan := FALSE; Use code with caution. 2. Using FB_GetCurTaskIndex (The Pro Method)
In the world of Beckhoff TwinCAT and industrial automation, the "First Scan Bit" is a fundamental tool for ensuring your PLC starts in a predictable, safe state. If you’ve ever worked with Siemens (where it’s a system bit like FirstScan ) or Allen-Bradley (using the S:FS bit), you know how vital this is. beckhoff first scan bit
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
Use the first scan to ensure all physical outputs are in a "Safe/Off" state before the logic takes over. The Beckhoff First Scan bit is your "clean slate" button
TwinCAT provides internal system information via the Tc2_System library. You can check if the current cycle is the very first one by looking at the system task info.
VAR fbGetTaskIndex : FB_GetCurTaskIndex; nCycleCount : UDINT; END_VAR fbGetTaskIndex(); nCycleCount := _TaskInfo[fbGetTaskIndex.index].CycleCount; IF nCycleCount = 1 THEN // This is the first scan END_IF Use code with caution. // The very last line of the program:
Setting default temperatures, speeds, or timers.