Beckhoff First Scan Bit · Authentic

When you transition to Beckhoff TwinCAT, you won’t find a system bit named exactly "First Scan" in the global variable list. This often leads to the question: How do I run logic exactly once when the PLC starts?

Use the bit to set initial setpoints ( iSpeed := 100 ) rather than to initialize complex state machines.

In Beckhoff TwinCAT, there is no single pre-defined global "First Scan" variable like the beckhoff first scan bit

When you instantiate a function block, TwinCAT automatically calls its FB_Init method once.

Telling other machines or HMI that the PLC has just powered on. 2. Implementing the First Scan Bit in TwinCAT 3 (ST) When you transition to Beckhoff TwinCAT, you won’t

In the world of industrial automation, the moment a PLC (Programmable Logic Controller) transitions from "Stop" to "Run" is fraught with both opportunity and danger. Uninitialized variables, rogue previous states, and half-configured hardware can lead to catastrophic machine behavior.

In PLC programming, ensuring that a system starts in a known state is critical for safety and process reliability. The (sometimes called First Cycle bit) is a special bit that is TRUE only during the very first scan of a PLC program after powering on or transitioning from STOP to RUN mode. In Beckhoff TwinCAT, there is no single pre-defined

Beckhoff's TwinCAT 3 environment offers multiple methods to implement a first scan, ranging from a simple, self-created flag to using the more official, built-in system structures.

While using _TaskInfo[index].FirstCycle is highly precise, the standard software pattern (Method 1) is generally preferred by TwinCAT programming teams. Method 1 is lightweight, easier to read for field technicians transitioning from other PLC platforms, and does not require managing task index function blocks. Method 3: Ladder Logic (LD) Implementation