Beckhoff First Scan Bit Page

In legacy TwinCAT 2 and early TwinCAT 3 projects using the Tc2_Standard library, the standard way to get a first scan bit is:

Pass the bFirstScan flag down to the function blocks or create a dedicated Initialize method/input within your FB that is called during the first scan.

// Initialization code here // This runs before the first PLC cycle begins

To help tailor this implementation, what are you currently using, and are you initializing local variables or code inside a reusable Function Block ? Share public link beckhoff first scan bit

IF bFirstScan THEN // Perform one-time tasks bFirstScan := FALSE; // Permanent off for the remainder of runtime END_IF Use code with caution. Copied to clipboard

The Beckhoff First Scan bit is a small, easily overlooked tool that separates professional, robust PLC code from fragile, “works-most-of-the-time” logic. By taking explicit control of the first cycle, you eliminate startup surprises, protect hardware, and ensure your TwinCAT application starts every time in a predictable, safe state.

The principle is to declare a global BOOL variable and ensure it retains its value across cycles: In legacy TwinCAT 2 and early TwinCAT 3

The standard method to detect the first cycle in TwinCAT is to use two key tools: the GETCURTASKINDEX function block and the global _TaskInfo array.

Software-driven flag (common)

FB_init is a specialized method you can add to any Function Block. It executes automatically before the PLC task starts its cyclic execution. Right-click your Function Block →right arrow →right arrow Method . Name the method exactly FB_init . It automatically generates the required signature: Copied to clipboard The Beckhoff First Scan bit

PROGRAM MAIN VAR fbGetCurTaskIndex : GETCURTASKINDEX; // Function block to find current task index END_VAR

While effective, this method is not automatically linked to the task's state. For instance, it may not re-trigger after a "Reset Cold" or "Reset Origin" command in TwinCAT, which could lead to initialization code not being run when it should be.

The most common, portable, and standard-compliant way to create a first scan bit in TwinCAT Structured Text (ST) is by declaring a local or global initialization variable. Implementation

: The logic trapped inside this IF statement evaluates only once when the controller boots up or transitions from CONFIG to RUN mode. Why is the First Scan Bit Important?