Beckhoff First Scan Bit Upd [ Safe — 2025 ]

TwinCAT, which is built on the IEC 61131-3 standard, handles startup execution uniquely through built-in system variables and structured initialization structures. The Anatomy of the TwinCAT First Scan Bit

IF _TaskInfo[fbGetCurTaskIndex.index].FirstCycle THEN // Logic here only executes on the very first scan END_IF Use code with caution. Copied to clipboard 2. Manual Method: Initialized Boolean

In legacy TwinCAT 2 and early TwinCAT 3 projects using the Tc2_Standard library, the standard way to get a first scan bit is:

📡 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 beckhoff first scan bit

bFirstScanSys := TwinCAT_SystemInfoVarList._FirstScan;

The core of this tracking relies on the system structure , which contains a dedicated Boolean flag named FirstCycle . This variable reads TRUE during the very first cycle of the task and switches permanently to FALSE for every subsequent cycle. Method 1: The Standard Global Task Info Hook

When you perform an in TwinCAT, existing variables retain their current values in memory. Because bFirstScan was already set to FALSE during the very first cyclic loop of the previous download, it remains FALSE during the online change. This prevents your initialization code from accidentally re-running and disrupting a live, moving machine. TwinCAT, which is built on the IEC 61131-3

In TwinCAT 2, developers frequently monitored the SystemInfoCtrl or utilized standard system flags. However, for TwinCAT 3, is highly preferred due to its independence from specific system task architectures and superior portability across different hardware targets (ARM vs. x86/x64).

Are you looking to initialize , hardware axes (NC) , or communication blocks ?

IF NOT bInitDone THEN // Do startup logic here bInitDone := TRUE; END_IF Use code with caution. Copied to clipboard Manual Method: Initialized Boolean In legacy TwinCAT 2

: Simplest to implement and easy to read for engineers coming from other platforms.

: Setting default values for non-persistent variables.