Beckhoff First Scan Bit !new! 【FHD | 8K】

Without a proper first scan routine, your machine might start with actuators in unpredictable positions or unfinished states from a previous run.

Clear old error flags that might have occurred during the previous shutdown. Establish Communication:

State machines are a staple of PLC programming. Using the first scan bit to set the initial state ensures a predictable and safe start-up.

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.

// Implementation fbFirstScan(CLK := bInit); bFirstScan := fbFirstScan.Q; bInit := FALSE; beckhoff first scan bit

: In your Global Variable List (GVL) or program, declare a BOOL with an initial value of TRUE . VAR_GLOBAL bFirstScan : BOOL := TRUE; END_VAR Use code with caution. Copied to clipboard

Be careful when trying to clear or initialize variables marked as RETAIN or PERSISTENT using a classic first scan bit. Persistent variables are designed to survive power cycles. If your first scan bit overwrites them every time the PLC boots, you defeat the purpose of persistent storage (like saving machine calibration offsets). 3. Execution Order Matters

This relies on the fact that standard, non-retained PLC variables always initialize to FALSE (or 0 ) upon a cold reset. Code Implementation (Structured Text)

If you click the "Stop PLC" button inside Visual Studio and hit "Start PLC" again, . This is because the underlying TwinCAT real-time operating system kernel was never restarted—only the execution engine of that specific sub-program was halted. If you explicitly require an initialization pulse whenever the application logic restarts independently of the kernel, use Method 3 (The Custom Software Flag) . Practical Use Cases for First Scan Bits Without a proper first scan routine, your machine

The most elegant, efficient, and standard-compliant way to create a first scan bit in TwinCAT 3 is by utilizing implicit variable initialization. In the IEC 61131-3 standard, variables can be assigned an initial value directly in the declaration block.

Proper testing ensures your initialization code works correctly in all scenarios:

// -- Wait for EtherCAT sync -- nState := fbEcMaster.GetState(); IF nState <> 8 THEN RETURN; // Don't run logic until bus is operational END_IF

To use this natively, you can reference the global structure _TaskInfo . Below is the industry-standard implementation pattern in Structured Text (ST): Using the first scan bit to set the

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.

If you have multiple programs ( MAIN , Safety , HMI ), ensure you know which program runs first to set the bFirstScan bit. It is usually best to handle initialization in the main task of your main program.

Without a mechanism to detect the first scan, your program would execute initialization logic (like setting default values, clearing counters, or resetting state machines) every single time the code runs. Key scenarios requiring the First Scan Bit include: