If you have nested function blocks, they won't automatically know a "first scan" occurred in the main program.
property, the code can detect if it is running for the first time. For example, in Structured Text (ST):
Built-in FirstScan flag (recommended if provided) beckhoff first scan bit
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.
If you are working on older legacy controllers (such as the Beckhoff CX1010 or CX1020 series) or maintaining an legacy system, you may find the global system array variable explicit: If you have nested function blocks, they won't
: In large OOP-based TwinCAT projects with many reusable function blocks.
In conclusion, the Beckhoff First Scan bit is far more than a simple Boolean flag; it is a foundational element of reliable software engineering in automation. It bridges the gap between the static, powered-down world and the dynamic, moving machine. By providing a deterministic method to execute initialization logic exactly once, it safeguards machinery, protects processes, and ensures that every production cycle begins with a known, safe, and calculated start. In the symphony of industrial control, the First Scan bit is the conductor’s initial tap of the baton—the signal that establishes order before the performance truly begins. Overwriting them defeats the purpose of making them
If your project features an explicit slow background task and a fast 1 ms motion task, each task tracks its own independent FirstCycle trigger.
The most robust, precise, and professional way to find out if the runtime is on its very first loop execution is by tapping into TwinCAT's built-in task system structures.
// In your implementation section bFirstScan := NOT rst; rst := TRUE;
If you are developing your application using Ladder Logic or Function Block Diagram (FBD) within TwinCAT, you can replicate Method 1 visually using a sealed coil or an interlocking contact. Visual Logic Sequence