Beckhoff First Scan Bit [updated] -

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.

The most common, portable, and standard-compliant way to create a first scan bit in TwinCAT Structured Text (ST) is by declaring a local or global initialization variable. Implementation

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 the realm of industrial automation, the difference between a smoothly running machine and a catastrophic collision often comes down to timing. While the cyclical nature of Programmable Logic Controllers (PLC) implies a repetitive, predictable existence, the transition from a powered-down state to an operational one is a critical window of uncertainty. It is in this precise moment that the "First Scan" bit proves its worth. Within the Beckhoff automation ecosystem—specifically utilizing TwinCAT software—the First Scan bit acts as the essential sentinel of initialization, ensuring that logic executes correctly before the physical world is engaged. beckhoff first scan bit

In TwinCAT, the PROGRAM or FUNCTION_BLOCK structure has a specific order of execution. The VAR section declares variables, but it doesn't execute logic. To run logic on the first scan, you declare a boolean flag and check its state.

If your project features an explicit slow background task and a fast 1 ms motion task, each task tracks its own independent FirstCycle trigger.

Best practices

Purging old data arrays, resetting FIFO (First-In, First-Out) pointers, and sending initial handshakes to Modbus, EtherCAT, or EtherNet/IP slaves.

PROGRAM MAIN VAR bInitDone : BOOL; myCounter : INT; END_VAR

Use these in combination:

PROGRAM MAIN VAR // Variable initializes to FALSE automatically upon startup bInitialized : BOOL := FALSE; // Your operational variables nTargetVelocity : INT; sSystemStatus : STRING; END_VAR // The First Scan Logic IF NOT bInitialized THEN // This code runs EXACTLY ONCE on the very first PLC cycle nTargetVelocity := 100; sSystemStatus := 'System Initialized'; // Set the flag to TRUE so this block is skipped in the next cycle bInitialized := TRUE; END_IF; // --- Your regular PLC program starts here --- Use code with caution. Why this works:

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 .

Sending initialization pulses to Fieldbus nodes (EtherCAT, Profinet) or external software layers (HMI, SCADA, MES) to declare the PLC is online. METHOD FB_init : BOOL VAR_INPUT bInitRetains : BOOL;

Populating operational variables with safe baseline data before the operator inputs custom values.