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.
Never make your First Scan bit a RETAIN or PERSISTENT variable. It needs to reset every time the PLC power cycles. beckhoff 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): In the world of Beckhoff TwinCAT and industrial
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? It needs to reset every time the PLC power cycles