Beckhoff First Scan Bit < Newest ✓ >

For advanced applications, TwinCAT provides internal system structures that expose the state of the PLC task execution. You can inspect the task information via the TwinCAT_SystemInfoVarList . Code Implementation

In this article, we've provided a comprehensive overview of the Beckhoff First Scan Bit, including its definition, importance, and usage. By following the guidelines outlined here, you'll be able to harness the power of the First Scan Bit in your own PLC projects.

With TwinCAT 3, Beckhoff introduced the Tc3_Standard library, which includes a dedicated function block: F_TRIG combined with a system flag is no longer needed. Instead, use:

TwinCAT controllers have distinct boot phases. The first scan bit is set during the transition from to RUN mode. Understanding this can help diagnose why certain initialization tasks might not execute as expected. The SystemTaskInfoArr[1].firstCycle variable will be TRUE during this transition, but not during a simple program restart from the development environment.

The First Scan Bit is a digital output that is automatically set by the PLC during its startup sequence. When the PLC is powered on or reset, it executes a series of internal checks and initializations before starting to execute the user program. During this first scan cycle, the First Scan Bit is set to TRUE (or 1). beckhoff first scan bit

Ensure all actuators are locked out until initialized.

IF bFirstScan THEN bFirstScan := FALSE; // One-time init code END_IF

Caveat : This method is cycle-dependent. If your cycle time is 10ms, set PT to at least 1ms — but ensure it's longer than one cycle but shorter than two.

The First Scan Bit is a that is TRUE only for a single PLC cycle immediately after the runtime system starts executing the application. On the second cycle, it becomes FALSE and remains FALSE until the next controller reboot or program download. By following the guidelines outlined here, you'll be

This system variable is automatically managed by the TwinCAT runtime.

Instead of an absolute global bit, TwinCAT provides an array of structures named _TaskInfo . This array tracks the real-time performance and diagnostics of every active PLC task running inside the TwinCAT execution environment. Within the PlcTaskSystemInfo structure , Beckhoff natively updates a boolean property called FirstCycle . During the first-ever execution cycle of that specific task, this variable is set to TRUE ; on every subsequent cycle, it automatically remains FALSE .

END_IF

For Object-Oriented Programming (OOP) in TwinCAT, encapsulate your startup logic within a dedicated FB_Init method or an explicit M_Initialize() method called during the first cycle. If you are working on a specific architecture, let me know: The first scan bit is set during the

A standard Programmable Logic Controller (PLC) operates on a continuous loop: it reads physical inputs, executes user program logic, and updates physical outputs.

In Beckhoff TwinCAT, there is no single pre-defined global "First Scan" variable like the

It becomes FALSE for all subsequent cycles.