Beckhoff First Scan Bit !link! May 2026

Beckhoff First Scan Bit — Comprehensive Treatise

Overview

The "First Scan" bit in Beckhoff TwinCAT PLC systems is a boolean status flag indicating the PLC program's initial execution cycle after startup, download, or reset. It enables safe, deterministic initialization of variables, hardware states, and communication interfaces before normal cyclic operation proceeds.

Here’s a concise guide to the First Scan Bit in Beckhoff TwinCAT (IEC 61131-3).

// -- Wait for EtherCAT sync -- nState := fbEcMaster.GetState(); IF nState <> 8 THEN RETURN; // Don't run logic until bus is operational END_IF beckhoff first scan bit

// First scan detection IF bFirstScan AND NOT bFirstScanDone THEN bFirstScanDone := TRUE; // First scan logic here END_IF

⚙️ How to Implement First Scan in TwinCAT 3

Method 1: Using Tc2_System Library (Recommended)

1. Step 3: At the end of your program or inside the conditional block, set the variable to FALSE.  Manual Code Example:  Beckhoff First Scan Bit — Comprehensive Treatise Overview

   Behavior: This bit usually only triggers when the TwinCAT Runtime is started or restarted. Simply stopping and starting the PLC code with the "Start/Stop" commands in the IDE may not reset this bit. 2. Custom Initialization Variable

   Part 3: The Native TwinCAT Method – Using PRG.INIT and PRG.EXIT

   Beckhoff TwinCAT offers built-in program-local initialization flags that many users overlook: INIT and EXIT sections within a program. Step 3 : At the end of your

   Clearing Alarms: Wiping the slate clean on startup so old errors don't prevent a start.