Beckhoff First Scan Bit 【Ad-Free】

// Last line of the program bFirstScan := FALSE; Use code with caution. Copied to clipboard Why use a First Scan Bit? Initialization

To use the First Scan Bit in a Beckhoff PLC, you need to access the PLC's system variables. The First Scan Bit is typically represented by a specific system variable, such as FirstScan or InitDone .

To understand the importance of the First Scan bit, one must first appreciate the architecture of a PLC. A PLC operates on a scan cycle: reading inputs, executing logic, and writing outputs. Under normal operation, this cycle repeats endlessly. However, the very first cycle after a power-up or a program reset presents a unique problem. At this specific moment, input data may not have settled, variables may be holding default values rather than retained ones, and physical actuators might be in unknown positions. If the control logic were to execute standard commands immediately, it could lead to unintended consequences, such as commanding a cylinder to extend before verifying it is retracted, or resetting a recipe to default values instead of loading the last saved state.

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The principle is to declare a global BOOL variable and ensure it retains its value across cycles:

In this snippet, the fbTerminalConfig function block (likely a custom or library FB) is triggered only on the first cycle, ensuring the hardware is configured once and not repeatedly during normal operation.

If your TwinCAT project has multiple tasks (e.g., a fast 1ms task and a slow 10ms task), remember that each task has its own "first cycle." // Last line of the program bFirstScan :=

In this example, the FirstScan system variable is used to execute an initialization code segment during the first scan cycle. Once the initialization is complete, the FirstScan bit is reset to FALSE.

Note: While functional, this method hardcodes the task index ( [1] ), making it less adaptable if your PLC project structure changes down the road. Method 3: The Custom Software-Based Flag

The built-in PlcTaskSystemInfo method is the most elegant and robust solution. However, in some legacy systems or when a developer prefers a more hardware-independent approach, a manual first-scan flag can be created. This DIY method demonstrates the underlying logic behind the system flag. The First Scan Bit is typically represented by

There are two primary methods used in TwinCAT to achieve "first scan" functionality: System Variable Method : The most robust way is using the FirstCycle member of the PlcTaskSystemInfo structure. How it works : Every PLC task has a system variable which contains a boolean FirstCycle . This bit is only during the very first cycle of that specific task.

VAR fbGetCurTaskIndex : GETCURTASKINDEX; END_VAR

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

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