: Sending a "Reset" or "Init" command to external devices (like drives or Vision systems) over EtherCAT.
This is particularly useful for initializing complex data structures, allocating memory with __NEW , or passing parameters to function blocks before they are used in the program.
In Beckhoff’s TwinCAT environment, the First Scan Bit is a fundamental diagnostic tool used to initialize logic, reset variables, or trigger specific startup sequences the moment the PLC transitions from Config/Stop
For modern TwinCAT 3 development utilizing Object-Oriented Programming (OOP), using a simple boolean flag inside a Function Block (FB) is often inefficient. Instead, Beckhoff supports the implicit FB_init method. Implementation
Mastering the first scan bit is not just about writing code; it is about adopting a design philosophy that prioritizes safety, reliability, and predictability from the very first moment of operation. Whether you use the robust _TaskInfo[].FirstCycle or a simple manual boolean flag, correctly implementing startup logic is a fundamental requirement for any professional TwinCAT project. By understanding the methods and best practices outlined in this guide, you can ensure your applications always start in a safe, controlled, and predictable state. beckhoff first scan bit
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.
IF _TaskInfo[GETCURTASKINDEX()].FirstCycle THEN // Your initialization logic here END_IF; Use code with caution. Copied to clipboard Custom Variable Method
This is the standard, robust method in TwinCAT 3. You check the FirstCycle property within the PlcTaskSystemInfo array.
If you cannot use PlcTaskSystemInfo , you can create a local BOOL variable that is initialized to TRUE . : Sending a "Reset" or "Init" command to
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
This article provides a comprehensive guide to understanding, implementing, and best-practicing the use of the bFirstScan bit in TwinCAT 2 and TwinCAT 3. 1. What is the Beckhoff First Scan Bit?
Are you handling during this first scan?
To make your machine's startup routine bulletproof, consider these best practices. Instead, Beckhoff supports the implicit FB_init method
Beyond just the firstCycle , these additional variables are incredibly valuable for real-time diagnostics. For instance, the cycleTimeExceeded flag can alert you to performance bottlenecks, helping to debug why a particular cycle might be taking too long.
: Establishing initial handshake bits with HMIs or other PLCs.
The most elegant, efficient, and standard-compliant way to create a first scan bit in TwinCAT 3 is by utilizing implicit variable initialization. In the IEC 61131-3 standard, variables can be assigned an initial value directly in the declaration block.