Beckhoff First Scan Bit

If you have nested function blocks, they won't automatically know a "first scan" occurred in the main program.

: If your project has multiple tasks (e.g., a fast 1ms task and a slow 100ms task), each task has its own FirstCycle flag. Ensure you are checking the flag for the specific task where your initialization logic resides . RSLogix 5000 First Scan Bit (S:FS) Programming Guide

Remember that the first scan logic runs before the main logic, meaning inputs are read, but the outputs have not yet been written for the very first time.

Maintained directly by the TwinCAT real-time kernel. beckhoff first scan bit

VAR bFirstScan : BOOL := TRUE; // Starts as TRUE on boot END_VAR // Place your first-scan initialization logic here IF bFirstScan THEN // Execute your one-time startup code // Turn it off at the end of the first scan bFirstScan := FALSE; END_IF Use code with caution. Copied to clipboard Extremely fast and simple.

#Beckhoff #TwinCAT #PLCProgramming #Automation #ControlsEngineering #IEC61131

A "first scan bit" is a special internal flag found in most PLCs (Programmable Logic Controllers). It turns for only the very first execution cycle after the controller is powered on or after it transitions from a stopped state to the run state. Once that first cycle is complete, the flag is automatically set to FALSE for all subsequent cycles. If you have nested function blocks, they won't

IF fbFirstScan.bFirstScan THEN // First cycle only bResetDrives := TRUE; bStartupComplete := FALSE; tStartupTimer(IN:=FALSE); END_IF

Conclusion The Beckhoff first scan bit is a simple but essential tool for controlled startup in TwinCAT PLC programs. Properly used, it ensures variables and outputs are initialized predictably, prevents unsafe transient actions, and supports reliable commissioning and diagnostics. Adhering to concise, documented first-scan patterns and combining them with broader safety practices produces safer, more maintainable control software.

: If you call the program multiple times (e.g., as an action or method), the INIT behavior may change. Use it carefully within the main cyclic task. RSLogix 5000 First Scan Bit (S:FS) Programming Guide

PROGRAM MAIN VAR bFirstScan : BOOL := TRUE; // Initialize to TRUE bIsInitialized : BOOL := FALSE; END_VAR // ---------------------------------------------------- // FIRST SCAN LOGIC // ---------------------------------------------------- IF bFirstScan THEN // --- Place Initialization Code Here --- // Example: Set initial values // AxisRef.bExecute := FALSE; // CurrentState := E_State.Idle; // ------------------------------------- bFirstScan := FALSE; // Turn off for next scan bIsInitialized := TRUE; // Mark as initialized END_IF // ---------------------------------------------------- // NORMAL CYCLIC CODE // ---------------------------------------------------- // ... rest of your program Use code with caution. The "TwinCAT Runtime" Method (Recommended)

VAR fbGetCurTaskIdx : GETCURTASKINDEX; // Function block to get current task index bFirstScan : BOOL; END_VAR fbGetCurTaskIdx(); // Call the FB to refresh current task info bFirstScan := _TaskInfo[fbGetCurTaskIdx.index].FirstCycle; IF bFirstScan THEN // Logic here only runs on the very first PLC scan // e.g., Initializing setpoints or resetting state machines END_IF Use code with caution. Copied to clipboard 2. Manual Global Variable Flag

Without a first scan flag, variables might retain their last value before a power failure, or default to zero, which might not be the desired safe state.