Beckhoff First Scan Bit -

// Your main cyclic program logic continues here

VAR fbGetCurTaskIndex : GETCURTASKINDEX; bFirstScan : BOOL; END_VAR fbGetCurTaskIndex(); // Get the index of the current PLC task bFirstScan := _TaskInfo[fbGetCurTaskIndex.index].FirstCycle; // Extract the bit IF bFirstScan THEN // Place initialization logic here (e.g., loading presets or clearing memory) END_IF Use code with caution. Copied to clipboard 1. Understanding System Information Structure

This simple pattern relies on the fact that static variables ( VAR declared in a program, POU, or function block) are initialized exactly once at program start. While functional, this method is generally discouraged in favor of the system-provided firstCycle because it adds an extra boolean variable and a constant check to every cycle. However, it is a useful fallback and helps illustrate the fundamental concept.

The Beckhoff “first scan” bit is a small but important concept in TwinCAT PLC programming that helps ensure deterministic, safe, and predictable behavior during controller startup. While its implementation is straightforward, understanding its purpose and correct use is essential for robust automation systems. beckhoff first scan bit

Version and platform considerations

BEGIN IF FirstScan THEN // Initialize variables MyVar1 := 10; MyVar2 := 20.5; InitDone := TRUE; END_IF END_PROGRAM

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. // Your main cyclic program logic continues here

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.

: In industrial settings, a first-scan bit is considered essential for resetting retentive memory and ensuring equipment starts in a safe, predictable state. Alternative for Advanced Users

VAR RETAIN powerCycleCounter : UDINT; END_VAR While functional, this method is generally discouraged in

Have a specific first scan challenge? Visit the Beckhoff Community forums or consult your local Beckhoff support engineer.

Because IEC 61131-3 guarantees that variables are initialized to their default values when the PLC boots, you can leverage this behavior to capture the first execution loop. Structured Text (ST) Implementation