Beckhoff First Scan Bit

With TwinCAT 3, Beckhoff introduced the Tc3_Standard library, which includes a dedicated function block: F_TRIG combined with a system flag is no longer needed. Instead, use:

PROGRAM MAIN
VAR
    bFirstScan AT %Q* : BOOL;  // Not directly. Better:
    fbFirstScan : F_TRIG;
    bInit : BOOL := TRUE;
END_VAR
// Implementation
fbFirstScan(CLK := bInit);
bFirstScan := fbFirstScan.Q;
bInit := FALSE;

Better yet: The official Tc3_Standard provides a clean helper:

PROGRAM MAIN
VAR
    bFirstScan : BOOL;
    rst : BOOL;
END_VAR
// In your implementation section
bFirstScan := NOT rst;
rst := TRUE;
 beckhoff first scan bit

But this is still a workaround. The most professional method in Tc3_Standard is to use the StandardLib (from Beckhoff Automation) which includes FB_FirstScan directly.

Never do this:

IF TRUE THEN   // This will run every cycle, not just first
    Initialize();
END_IF

VAR
    bFirstScan : BOOL := TRUE;
END_VAR
IF bFirstScan THEN
bFirstScan := FALSE;
// One-time init code
END_IF
 Better yet : The official Tc3_Standard provides a

⚠️ Caution: This resets on warm start. For cold start retention, use VAR RETAIN.

Note: exact symbol names can vary by TwinCAT version and project conventions.

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. But this is still a workaround

For engineers working with Beckhoff TwinCAT 3 (and TwinCAT 2), the concept of the First Scan Bit (often implemented via the bInit or bFirst variable) is the cornerstone of safe and robust machine initialization.

But here’s the catch: unlike traditional PLCs (e.g., Siemens with OB100 or Rockwell with FirstScan), Beckhoff’s approach is more flexible—but also more confusing for newcomers. This article will dissect every method to detect and utilize the first scan cycle in TwinCAT, from standard PLC libraries to advanced object-oriented techniques.

Without a first scan flag, you cannot reliably distinguish between:

This distinction is crucial for:

| Use Case | Purpose | |----------|---------| | Initialize outputs | Set outputs to safe defaults | | Clear retentive variables selectively | Override old retentive values on first start | | Home axes | Trigger homing sequence only once | | Reset alarms | Clear boot-time fault flags | | Load configuration | Load parameters from file or EEPROM once |