Beckhoff First Scan Bit Repack ✓

Are you handling during this first scan?

In TwinCAT, you can run multiple independent tasks at different cycle times (e.g., a 1ms motion control task and a 50ms visualization/HMI task). A single global first scan bit would fail because different tasks start at different times or may be restarted independently. Therefore, initialization logic in TwinCAT must be managed at the application, task, or function block level. Method 1: The Local Variable Approach (Recommended)

On startup, the variable initializes to TRUE. The logic runs once, and then the assignment at the bottom kills the bit for all subsequent cycles. 3. Using the 'Init' Attribute beckhoff first scan bit

In TwinCAT 3, the First Scan Bit is represented by the system variable FirstScan . Here's an example of how to use it in a simple PLC program:

When working with the first scan bit, keep these points in mind to avoid common pitfalls: Are you handling during this first scan

This is particularly useful for initializing complex data structures, allocating memory with __NEW , or passing parameters to function blocks before they are used in the program.

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. Therefore, initialization logic in TwinCAT must be managed

: Sending a "Reset" or "Init" command to external devices (like drives or Vision systems) over EtherCAT.

Which of these two methods are you planning to use for your specific project? Beckhoff CX1010 first scan | PLCtalk - Interactive Q & A

BEGIN IF FirstScan THEN // Execute initialization code here // e.g., set default values, initialize variables FirstScan := FALSE; END_IF

When you transition to Beckhoff TwinCAT, you won’t find a system bit named exactly "First Scan" in the global variable list. This often leads to the question: How do I run logic exactly once when the PLC starts?