Beckhoff First Scan Bit -

bFirstScanSys := TwinCAT_SystemInfoVarList._FirstScan;

VAR bFirstScan : BOOL := TRUE; // Initializes to TRUE on PLC startup bInitializationDone : BOOL; // Your application variables nState : INT; fTargetVelocity : LREAL; END_VAR // --- PLC Execution Code --- IF bFirstScan THEN // 1. Initialize State Machines nState := 10; // 2. Set Default Configurations fTargetVelocity := 1500.0; // 3. Clear Historical Fault Arrays or Buffers // (Your code here) // 4. Reset the first scan bit so this block never runs again bFirstScan := FALSE; END_VAR // Normal, cyclic PLC logic continues here Use code with caution. How It Works during Online Changes

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. beckhoff first scan bit

VAR fbGetCurTaskIndex : GETCURTASKINDEX; END_VAR

Inside this method, you can place all code required to initialize that specific function block instance. You do not need to manage any cyclic bits; TwinCAT handles the execution sequence automatically before the main cycles begin. Common Pitfalls and Best Practices 1. Task Synchronicity Pitfall bFirstScanSys := TwinCAT_SystemInfoVarList

: If you prefer not to use the system global, you can create a local "Init" flag:

In the world of industrial automation, a clean start is everything. When a Beckhoff PLC (Programmable Logic Controller) boots up—whether from a power cycle, a download of a new TwinCAT configuration, or a manual restart—the system enters a critical, fleeting state. Variables are uninitialized, previous runtime values linger in memory, and physical outputs may hold their last state. How do you reliably distinguish this “first scan” from normal cyclic operation? Clear Historical Fault Arrays or Buffers // (Your

For Object-Oriented Programming (OOP) in TwinCAT, encapsulate your startup logic within a dedicated FB_Init method or an explicit M_Initialize() method called during the first cycle. If you are working on a specific architecture, let me know:

structure or create a custom initialization variable to manage first-scan logic. Beckhoff Information System Key Ways to Implement a First Scan Bit