Beckhoff First Scan Bit -

⚠️ If you use Method 1 in multiple tasks (e.g., a fast cyclic task and a slower periodic task), it will evaluate properly for each task's respective first execution loop.

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

The underlying system automatically forces the bit to FALSE when the second scan begins.

⚠️ Always use the first scan bit to put your machine into a Safe State . Never assume the hardware is in the same position it was when the power went out.

If you use a manual first scan bit, ensure it is set to FALSE at the very end of your program. If you do it at the top, the rest of your logic won't see the TRUE state. beckhoff first scan bit

: Beckhoff's system does not have a mandatory, automatic FirstScan system bit. You must explicitly create it. This gives you more control but requires discipline.

Note: This method is more robust because it relies on the system's own cycle counter rather than a variable you might accidentally overwrite elsewhere. Best Practices

In legacy TwinCAT 2 and early TwinCAT 3 projects using the Tc2_Standard library, the standard way to get a first scan bit is:

Are you handling during this first scan? ⚠️ If you use Method 1 in multiple tasks (e

: This bit is TRUE during the very first execution cycle of the PLC task and automatically switches to FALSE for all subsequent cycles. Implementation Example (ST)

The FB_FirstScan function block monitors the system’s cycle counter and reliably returns TRUE for exactly one cycle after application start, even if multiple programs call the same FB instance.

// Startup timer after first scan IF NOT fbFirstScan.bFirstScan AND NOT bStartupComplete THEN tStartupTimer(IN:=TRUE, PT:=T#2S); IF tStartupTimer.Q THEN bStartupComplete := TRUE; // Enable normal operation END_IF END_IF

: The FirstCycle bit typically triggers when the Runtime starts . Simple PLC "Stop" and "Start" commands in the IDE might not always reset this bit unless the entire TwinCAT system is restarted or a new configuration is activated . ⚠️ Always use the first scan bit to

TwinCAT provides a built-in system variable for this purpose within the PlcAppSystemInfo structure. You do not need to create a global variable manually; you can access it via the . Variable Name : _AppInfo.bFirstCycle Data Type : BOOL

Version and platform considerations

It acts as a gatekeeper for initialization code. Without a first scan bit, logic intended to run only once would execute continuously, overwriting live operational data or wasting processing power. Why TwinCAT Doesn't Have a Global "S:FS"

The most common, efficient, and standard-compliant way to create a first scan bit in TwinCAT Structured Text (ST) is by using a local or global BOOL variable that defaults to FALSE (or TRUE , depending on your logic preference).

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: Triggering handshake or connection logic that only needs to happen once upon startup. Safety Resets