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.
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:
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) beckhoff first scan bit
Wiping the slate clean on startup so old errors don't prevent a start.
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. The Beckhoff First Scan bit is your "clean slate" button
Never make your First Scan bit a RETAIN or PERSISTENT variable. It needs to reset every time the PLC power cycles.
Establishing a "heartbeat" or initial connection status with HMIs or third-party devices. How to Implement "First Scan" in TwinCAT 3 There are two primary ways to handle this in Beckhoff. 1. The Manual Method (Most Common) Common use cases include: IF bFirstScan THEN //
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
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?
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.