Beckhoff First Scan Bit Now

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.

In PLC (Programmable Logic Controller) programming, initialization is a critical phase of machine control. When a control system transitions from to Run mode, or when the controller power-cycles, variables must be set to safe default states, communication links must be verified, and system handshakes must be established.

Suppose we have a PLC program that requires the initialization of several variables during the startup of the PLC. We can use the First Scan Bit to execute this initialization task only once, during the first scan of the PLC.

Ensure your first scan flag is initialized to TRUE in its declaration. beckhoff first scan bit

a global or local variable: bInitialized : BOOL := FALSE; Logic :

This relies on the fact that standard, non-retained PLC variables always initialize to FALSE (or 0 ) upon a cold reset. Code Implementation (Structured Text)

The most elegant, efficient, and standard-compliant way to create a first scan bit in TwinCAT 3 is by utilizing implicit variable initialization. In the IEC 61131-3 standard, variables can be assigned an initial value directly in the declaration block. Do not call asynchronous function blocks (like file

Many EtherCAT terminals require a one-time configuration command after start-up to function correctly. The first scan bit is the perfect place to send these commands.

Ensuring your sequences (SFC) start at "Step 0."

VAR RETAIN bInitialized : BOOL; END_VAR VAR bFirstScanSys : BOOL; END_VAR We can use the First Scan Bit to

END_IF

Unlike some traditional PLCs (such as Siemens S7 with OB100 or Allen-Bradley Logix with the S:FS bit) that feature a dedicated, globally defined system variable for this purpose, Beckhoff’s TwinCAT environment handles the "first scan" concept through its standardized IEC 61131-3 implementation. Mechanics of the Initialization Phase

In all subsequent cycles, the condition evaluates to false, skipping the initialization block.