Beckhoff First Scan Bit __top__ Jun 2026

For complex architectures where initialization requires checking hardware topology, EtherCAT master states, or waiting for external databases to link, a simple bit might not suffice. In these scenarios, you can leverage the system PLC task variables or PLC status functions.

// Rest of the user program... END_PROGRAM

Let me know in the comments — I’ve debugged many tricky startup issues and can help!

Beckhoff's TwinCAT 3 environment offers multiple methods to implement a first scan, ranging from a simple, self-created flag to using the more official, built-in system structures. beckhoff first scan bit

This distinction is crucial for:

// Last line of the program bFirstScan := FALSE; Use code with caution. Copied to clipboard Why use a First Scan Bit? Initialization

The first scan is the perfect place to set default values, clear buffers, or initialize complex data structures that are not retentive. END_PROGRAM Let me know in the comments —

📡 Triggers a "Hello" or synchronization pulse to external devices, such as HMIs or SQL databases, to signal that the PLC is back online. Best Practices and Pitfalls

This method is commonly used in MAIN (PRG) or in a dedicated initialization block.

You can access this feature through the implicit array, which contains data for every task running in your PLC project. Syntax (Structured Text): Copied to clipboard Why use a First Scan Bit

PROGRAM MAIN VAR fbGetCurTaskIndex : GETCURTASKINDEX; // Function block to find active task index bFirstScan : BOOL; // Our dedicated local first scan variable END_VAR // 1. Invoke the system function block to determine the current task index fbGetCurTaskIndex(); // 2. Extract the FirstCycle boolean flag from the global system task array bFirstScan := _TaskInfo[fbGetCurTaskIndex.index].FirstCycle; // 3. Execute your targeted one-time initialization routine IF bFirstScan THEN // Place one-time initialization code here FormatStorageDrives(); LoadDefaultCalibrationValues(); SetStateMachineToDefault(); END_IF Use code with caution. Why Choose Method 1?

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

PROGRAM MAIN VAR bInitDone : BOOL; myCounter : INT; END_VAR

Commonly used to load recipes, initialize communication drivers, or reset state machines to their starting position upon a PLC cold or warm start. If you'd like, I can show you:

Use a local BOOL := TRUE; variable and clear it at the end of the IF statement. Use TwinCAT_SystemInfoVarList._TaskInfo[x].CycleCount = 1 . HMI Safe-State