Beckhoff First Scan Bit !link! Jun 2026

The first scan bit is a fundamental concept for writing robust and predictable PLC programs in Beckhoff TwinCAT. Whether you choose the system variable SystemTaskInfoArr[1].firstCycle for standard cold-start initialization or a custom RETAIN flag for more control, understanding this feature will help you build more reliable automation systems.

Large projects with multiple tasks or complex initialization. Quick, single-task projects or basic logic. Summary of Benefits

If you perform an "Online Change" (modify code without full download), the first scan bit does trigger. Your initialization code will not run. To force reinitialization, use Reset or Reset Cold from the TwinCAT runtime.

// Your logic here... IF bFirstScan THEN // Initialization code END_IF // Last line of the program bFirstScan := FALSE; Use code with caution. Copied to clipboard

For scenarios where you need the flag to be TRUE on every program start (e.g., after a STOP→RUN transition), you can create your own. This method offers more control but is less standardized. beckhoff first scan bit

In the realm of industrial automation, the difference between a smoothly running machine and a catastrophic collision often comes down to timing. While the cyclical nature of Programmable Logic Controllers (PLC) implies a repetitive, predictable existence, the transition from a powered-down state to an operational one is a critical window of uncertainty. It is in this precise moment that the "First Scan" bit proves its worth. Within the Beckhoff automation ecosystem—specifically utilizing TwinCAT software—the First Scan bit acts as the essential sentinel of initialization, ensuring that logic executes correctly before the physical world is engaged.

In traditional PLCs like Siemens Step 7 ( OB100 ) or Allen-Bradley ControlLogix ( S:FS ), dedicated "First Scan" flags or organization blocks are built directly into the system. In Beckhoff’s TwinCAT environment, which is based on the IEC 61131-3 standard, handling the first scan requires a slightly different approach.

Here’s a concise guide to the in Beckhoff TwinCAT (IEC 61131-3).

Then, in your main program or task, you reset this flag after the first cycle: The first scan bit is a fundamental concept

: Allows initialization to be tied to the specific timing of individual tasks rather than just global power-up. RSLogix 5000 First Scan Bit (S:FS) Programming Guide

The First Scan Bit is essential for several reasons:

for function block-specific initialization. These approaches enable reliable initialization of communication, variables, and logging upon system startup. AllTwinCAT First cycle - AllTwinCAT

Engineers transitioning from Rockwell Automation or Siemens to TwinCAT often search the system variables for a universal TwinCAT_System_Variables.FirstScan flag. It does not exist globally for one primary reason: Quick, single-task projects or basic logic

a global or local variable: bInitialized : BOOL := FALSE; Logic :

TwinCAT controllers have distinct boot phases. The first scan bit is set during the transition from to RUN mode. Understanding this can help diagnose why certain initialization tasks might not execute as expected. The SystemTaskInfoArr[1].firstCycle variable will be TRUE during this transition, but not during a simple program restart from the development environment.

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

IF TRUE THEN // This will run every cycle, not just first Initialize(); END_IF