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.
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. 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?
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. The Beckhoff First Scan bit is your "clean slate" button
If you use the manual variable method, ensure the line bFirstScan := FALSE; is at the very bottom of your MAIN task. If you put it in a sub-function, other parts of your program might miss the "True" state.
Without a initialization bit, your PLC logic simply resumes from its last state or starts with default values that might not be appropriate for a running machine. Common use cases include: Best Practices Use the first scan to ensure
Wiping the slate clean on startup so old errors don't prevent a start.
Ensuring your sequences (SFC) start at "Step 0."
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.