Beckhoff First Scan Bit High Quality Page

fbFirstScan();

:Place this line at the very bottom of your MAIN program or the last task to execute :

If your initialization code runs after a simple "stop" and "start" from the TwinCAT engineering interface , it might be because you are not truly restarting the runtime. The first scan is typically for the .

Use a timer or a counter after the first scan to delay initialization by a few cycles. 3. Missing Initializations in Function Blocks beckhoff first scan bit

: Essential for setting default values for retentive memory variables.

METHOD FB_init : BOOL VAR_INPUT bInitRetains : BOOL; // IF TRUE, the retain variables are initialized bInCopyCode : BOOL; // IF TRUE, the instance afterwards is moved into copy code END_VAR // Any code placed here executes once before the main PLC scan starts. // Ideal for calculating static constants, initializing pointers, or clearing arrays. Use code with caution. Common Use Cases for the First Scan Bit

With TwinCAT 3, Beckhoff introduced the Tc3_Standard library, which includes a dedicated function block: F_TRIG combined with a system flag is no longer needed. Instead, use: fbFirstScan(); :Place this line at the very bottom

The refers to a signal used in TwinCAT PLC programming to execute initialization logic exactly once when the controller starts or enters run mode. Unlike some other PLC platforms that have a fixed system bit like Allen-Bradley's S:FS , Beckhoff TwinCAT provides this functionality through specific system variables or custom logic. Standard Implementation Methods

: This bit usually only triggers when the TwinCAT Runtime is started or restarted. Simply stopping and starting the PLC code with the "Start/Stop" commands in the IDE may not reset this bit. 2. Custom Initialization Variable

Remember: A well-implemented first scan routine separates unreliable prototypes from industrial-grade automation. Take the time to initialize deliberately—your machine's safe operation depends on it. Why use a First Scan Bit?

Declare instance:

If you are using Function Blocks, TwinCAT 3 supports the FB_init method. This is a specialized sub-method that runs when the block is instantiated (during PLC startup or after a download), making it the cleanest way to handle block-specific initializations. Why use a First Scan Bit?