Beckhoff First Scan Bit [2021] | Easy – 2026 |

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.

PROGRAM MAIN VAR bFirstScan : BOOL; END_VAR // Check the boot count or execution cycle of the current task // Task ID 1 is typically the standard PLC task bFirstScan := (_TaskInfo[1].CycleCount = 0); IF bFirstScan THEN // Execute initialization routines here END_IF; Use code with caution.

VAR bFirstScan : BOOL := TRUE; // Starts as TRUE on boot END_VAR // Place your first-scan initialization logic here IF bFirstScan THEN // Execute your one-time startup code // Turn it off at the end of the first scan bFirstScan := FALSE; END_IF Use code with caution. Copied to clipboard Extremely fast and simple. beckhoff first scan bit

The First Scan Bit is the "ignition switch" of a TwinCAT program. By isolating startup logic into this single-cycle window, engineers ensure that the system begins its operation from a known, predictable state

: At the end of your program or inside the conditional block, set the variable to FALSE . Manual Code Example: Copied to clipboard Extremely fast and simple

Pass the bFirstScan flag down to the function blocks or create a dedicated Initialize method/input within your FB that is called during the first scan.

IF FirstScan THEN // Clear receive/transmit buffers receiveBuffer := ''; sendBuffer := ''; // Reset bus coupler ETC_ClearDeviceState(); Manual Code Example: Pass the bFirstScan flag down

PROGRAM MAIN VAR bInit : BOOL := TRUE; // Our first scan flag bFirstScanDone : BOOL; fbFirstScan : F_TRIG; fbEcMaster : FB_EcCoEADsRead; // EtherCAT utility nState : INT; END_VAR

For systems with motion control, ensuring all axes are in a known position is crucial. The first scan is the standard place to trigger initial homing routines for drives and servos.

Implementation Method 1: The Local Initialization Attribute (Recommended)