Beckhoff First Scan Bit Guide

Name the method exactly FB_init . TwinCAT will automatically populate the required input parameters: bInitRetains : BOOL and bInCopyCode : BOOL .

In TwinCAT, you can run multiple independent tasks at different cycle times (e.g., a 1ms motion control task and a 50ms visualization/HMI task). A single global first scan bit would fail because different tasks start at different times or may be restarted independently. Therefore, initialization logic in TwinCAT must be managed at the application, task, or function block level. Method 1: The Local Variable Approach (Recommended)

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

Without a mechanism to detect the first scan, your program would execute initialization logic (like setting default values, clearing counters, or resetting state machines) every single time the code runs. Key scenarios requiring the First Scan Bit include:

PROGRAM MAIN VAR bFirstScan : BOOL; // True only during the first execution cycle bInitialized : BOOL; // Global flag showing init status fbReadConfig : FB_FileRead; // Example init function block END_VAR // Determine First Scan using the TwinCAT Task Info structure bFirstScan := (_TaskInfo[1].CycleCount = 1); IF bFirstScan THEN // Execute Startup-Only Logic Here bInitialized := FALSE; // Example: Set default hardware overrides GVL_Hardware.TargetVelocity := 100.0; END_IF; // Rest of your cyclic PLC program runs below Use code with caution. Why this works: Name the method exactly FB_init

Understanding the Beckhoff First Scan Bit: Implementation, Use Cases, and Best Practices

Unlike traditional PLCs (such as Rockwell Automation’s Allen-Bradley or Siemens STEP 7) which provide a built-in system variable for this purpose (e.g., S:FS or FirstScan ), Beckhoff’s TwinCAT environment handles execution differently due to its PC-based control architecture. TwinCAT developers must implement their own first scan logic or utilize specific system variables provided by the TwinCAT PLC Framework. Why TwinCAT Lacks a Default "FirstScan" Bit A single global first scan bit would fail

IF bFirstScan THEN // Perform initialization (e.g., setting default values) END_IF