Beckhoff First Scan Bit -

, FALSE , or NULL ). However, your machine might need specific initial states, such as: Setting a default speed for a conveyor. Loading default recipes. Activating a specific user interface screen. Clearing temporary alarms. Initializing function blocks.

TwinCAT 3 introduces Object-Oriented Programming (OOP) extensions to IEC 61131-3. If you are developing reusable Function Blocks (FBs), you do not need to check a first scan bit inside the body of your code. Instead, you can use the implicit FB_init method.

Add library: Tc2_System

boolean, which signals the initial task execution, or by creating a custom global variable initialized to TRUE. Alternative methods include utilizing

You can implement this programmatically in Structured Text (ST) by declaring the system information variables and the GETCURTASKINDEX function block. beckhoff first scan bit

Pass the bFirstScan flag down to the function blocks or create a dedicated Initialize method/input within your FB that is called during the first scan.

// Rest of the user program... END_PROGRAM , FALSE , or NULL )

If you use PERSISTENT variables to save machine parameters across power outages, do not overwrite them in your first scan logic. Overwriting them defeats the purpose of making them persistent.

Telling other machines or HMI that the PLC has just powered on. 2. Implementing the First Scan Bit in TwinCAT 3 (ST) Activating a specific user interface screen

If you are coming from the Allen-Bradley world, you are likely very comfortable with the (First Scan) bit. It’s a staple for initializing logic, resetting counters, or clearing buffers on startup.