Bit - Beckhoff First Scan

You can access this feature through the implicit array, which contains data for every task running in your PLC project. Syntax (Structured Text):

When a TwinCAT configuration activates and the PLC task starts, the system runtime allocates memory and sets variables to their default values. The very first execution cycle—Cycle 0—is where initialization logic must execute.

But here’s the catch: unlike traditional PLCs (e.g., Siemens with OB100 or Rockwell with FirstScan ), Beckhoff’s approach is more flexible—but also more confusing for newcomers. This article will dissect every method to detect and utilize the first scan cycle in TwinCAT, from standard PLC libraries to advanced object-oriented techniques.

In the world of industrial automation and TwinCAT programming, ensuring a PLC program initializes correctly is crucial. Whether you are setting initial positions, resetting counters, or activating safety interlocks, you need a way to execute code only once—the very first time the PLC runs. beckhoff first scan bit

Method 1: The Standard Structured Text Pattern (Recommended)

The most reliable way to access a "first scan" state in TwinCAT 3 is through the global array _TaskInfo . This structure contains real-time information for each task running on the controller.

// -- First scan detection -- fbFirstScan(CLK := bInit); IF fbFirstScan.Q THEN bFirstScanDone := FALSE; You can access this feature through the implicit

Function block instances can include an FB_init method, which is automatically called by the TwinCAT runtime system before the PLC program starts. This method is similar to a constructor in object-oriented programming:

VAR currentState : E_MachineState := E_MachineState.STATE_IDLE; END_VAR

Edge detection using task cycle counter: But here’s the catch: unlike traditional PLCs (e

When TwinCAT loads the program into memory, bFirstScan is explicitly set to TRUE . During the first cycle, the IF condition evaluates to true, and the initialization code runs. Before the execution pointer leaves the IF block, bFirstScan is forced to FALSE . For every subsequent cycle, the condition evaluates to false, introducing zero processing overhead for the rest of the uptime duration.

The first scan can be used to run a quick diagnostic of critical I/O or system functions. For example, you can verify the integrity of a backup battery or the status of a safety relay before the main control logic begins its cycle.

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

The array index _TaskInfo[1] points to your primary PLC task. The property .CycleCount increments automatically on every cycle. On the very first pass, it evaluates to 1 , rendering bFirstScan true. On cycle two and all subsequent cycles, bFirstScan becomes false automatically. Method 2: The Classic IEC 61131-3 "Inverted Flag" Approach

Executing specific logic only once during the very first PLC task cycle is essential for several reasons: