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.
In Beckhoff’s TwinCAT 3 environment, there isn’t a single hard-coded bit in the global memory by default, but the system provides a specialized mechanism to create one that is far more powerful than a simple boolean. What is the First Scan Bit?
Most TwinCAT developers create a global boolean variable and set it to TRUE by default. At the very end of their main program, they set it to FALSE . VAR_GLOBAL bFirstScan : BOOL := TRUE; END_VAR Use code with caution. Main Logic (MAIN PRG): beckhoff first scan bit
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.
Never make your First Scan bit a RETAIN or PERSISTENT variable. It needs to reset every time the PLC power cycles. The Beckhoff First Scan bit is your "clean slate" button
Ensuring your sequences (SFC) start at "Step 0."
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. What is the 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
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.
Wiping the slate clean on startup so old errors don't prevent a start.
