Bit: Beckhoff First Scan

In all subsequent cycles, the condition evaluates to false, skipping the initialization block.

In Beckhoff TwinCAT, there is no single global system bit like the Allen-Bradley S:FS . Instead, you use task-specific system information or a manual initialization variable.

: Simplest to implement and easy to read for engineers coming from other platforms.

PROGRAM PRG_MachineControl VAR bFirstScan : BOOL := TRUE; // Initializes to TRUE on startup Variables : ST_MachineVariables; END_VAR Use code with caution. beckhoff first scan bit

By pairing a boolean variable defaulted to TRUE with an immediate reset to FALSE inside the code, you create an automatic, single-cycle trigger. Structured Text (ST) Implementation

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

TwinCAT provides a global structure called TwinCAT_SystemInfoVarList which contains runtime metrics about the PLC. Within this structure, you can access the current cycle counter of the task executing the code. In all subsequent cycles, the condition evaluates to

To use this natively, you can reference the global structure _TaskInfo . Below is the industry-standard implementation pattern in Structured Text (ST):

The most common, efficient, and standard-compliant way to create a first scan bit in TwinCAT Structured Text (ST) is by using a local or global BOOL variable that defaults to FALSE (or TRUE , depending on your logic preference).

A common practice among developers is to manually declare a global variable that initializes as and is immediately set to after use. Structured Text Example: : Simplest to implement and easy to read

The First Scan Bit, also known as the "First Cycle Bit" or "Initialization Bit," is a special bit in Beckhoff PLCs that indicates when the PLC is executing its first scan cycle. In other words, it signals that the PLC is starting up and executing its program for the first time.

I can provide more specialized code to fit your exact setup. PlcTaskSystemInfo - Beckhoff Information System - English

While the GVL method is easy to read, the _TaskInfo.FirstCycle method is strictly preferred in modern TwinCAT 3 architectures, especially when writing Object-Oriented (OOI) code or using modular Function Blocks, as it guarantees a pure first-execution cycle across individual tasks. Diving Deeper into TwinCAT System Information