Beckhoff First Scan Bit

The "First Scan" bit in Beckhoff TwinCAT PLC systems is a boolean status flag indicating the PLC program's initial execution cycle after startup, download, or reset. It enables safe, deterministic initialization of variables, hardware states, and communication interfaces before normal cyclic operation proceeds.

Force a state machine to the "Idle" or "Home" position on start.

Resetting historical error flags or step indexes in Sequential Function Charts (SFC) to ensure the machine starts from a known, safe state.

Do not call asynchronous function blocks (like file I/O operations, ADS read/write blocks, or socket communications) inside the first scan IF statement. These blocks require multiple PLC cycles to return a bBusy = FALSE status. They will stall or fail if executed only once. beckhoff first scan bit

METHOD FB_init : BOOL VAR_INPUT bInitRetains : BOOL; // if TRUE, the retain variables are initialized bInCopyCode : BOOL; // if TRUE, the instance afterwards gets moved into the copy code END_VAR Use code with caution.

Here’s a complete, ready-to-use post explaining the in TwinCAT PLC.

FirstScan gives you a deterministic moment to reset, preset, or home everything before normal operation begins. The "First Scan" bit in Beckhoff TwinCAT PLC

Next time you write a new PLC program, ask yourself: “If this were a cold start right now, would my system behave safely?” If the answer isn’t an immediate “yes,” you need FirstScan .

The IF bFirstScan block should only contain variables that need to be set once. Do not put complex, long-running calculations inside it, as this can increase the scan time of the very first cycle.

In TwinCAT 2, developers frequently monitored the SystemInfoCtrl or utilized standard system flags. However, for TwinCAT 3, is highly preferred due to its independence from specific system task architectures and superior portability across different hardware targets (ARM vs. x86/x64). Resetting historical error flags or step indexes in

When implementing first-scan logic, adhering to established best practices is crucial for creating robust and maintainable code.

: Allows initialization to be tied to the specific timing of individual tasks rather than just global power-up. RSLogix 5000 First Scan Bit (S:FS) Programming Guide