Beckhoff First Scan Bit ❲ULTIMATE · 2024❳

In essence, a (also commonly referred to as the First Cycle Bit ) is a system-provided flag within a PLC. This boolean variable is set to TRUE for the very first execution cycle of a PLC program after a specific startup event and is FALSE for every subsequent cycle. This flag acts as a one-time trigger, signaling to the program that it is running for the very first time since the controller was powered up, reset, or transitioned into RUN mode.

In TwinCAT 3, the First Scan Bit is represented by the system variable FirstScan . Here's an example of how to use it in a simple PLC program:

// Obtain the index of the current task fbGetCurTaskIdx(); beckhoff first scan bit

For advanced TwinCAT 3 users working with Function Blocks, the most elegant and robust "first scan" is not a bit at all—it's the object constructor: FB_Init .

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 essence, a (also commonly referred to as

Assign setpoints to PID controllers or load recipe parameters from memory. Reset Faults:

: Provides the exact number of cycles the task has completed since boot—a fantastic diagnostic tool for tracking task execution frequency. In TwinCAT 3, the First Scan Bit is

| Brand | First Scan Mechanism | Beckhoff Equivalent | |--------|----------------------|----------------------| | Siemens | OB100 | INIT section or FB_FirstScan | | Rockwell | S:FS (First Scan) bit | bInit variable manually set | | Codesys | bInit in PLC_PRG | Same (TwinCAT is based on Codesys) | | Beckhoff Native | INIT , FB_Init , or F_TRIG | Choose based on scope |

Add the TwinCAT_SystemInfoVarList to your project, then: