: If your project has multiple tasks (e.g., a fast 1ms task and a slow 100ms task), each task has its own FirstCycle flag. Ensure you are checking the flag for the specific task where your initialization logic resides . RSLogix 5000 First Scan Bit (S:FS) Programming Guide
Without a first scan flag, you cannot reliably distinguish between:
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
You can implement this programmatically in Structured Text (ST) by declaring the system information variables and the GETCURTASKINDEX function block. beckhoff first scan bit
In older TwinCAT 2 systems, sometimes developers used specialized _Init programs. However, in modern TwinCAT 3, using a bFirstScan boolean in the main task is the preferred approach for readability and consistency. 5. Common Pitfalls and Debugging
: Simplest to implement and easy to read for engineers coming from other platforms.
Ensuring old data is cleared before new operations begin. : If your project has multiple tasks (e
IF SystemTaskInfoArr[1].firstCycle THEN // Perform one-time initialization code here bInitializationDone := FALSE; // ... other one-time tasks ... bInitializationDone := TRUE; END_IF
While using _TaskInfo[index].FirstCycle is highly precise, the standard software pattern (Method 1) is generally preferred by TwinCAT programming teams. Method 1 is lightweight, easier to read for field technicians transitioning from other PLC platforms, and does not require managing task index function blocks. Method 3: Ladder Logic (LD) Implementation
Using the first scan bit improperly can lead to hard-to-debug startup issues. Follow these best practices: Mechanics of the Initialization Phase You can implement
// Check if the flag is still false IF NOT bFirstCycleDone THEN // Place your one-time initialization logic here // ...
PROGRAM MAIN VAR bFirstScan : BOOL := TRUE; // Initialize to TRUE bIsInitialized : BOOL := FALSE; END_VAR // ---------------------------------------------------- // FIRST SCAN LOGIC // ---------------------------------------------------- IF bFirstScan THEN // --- Place Initialization Code Here --- // Example: Set initial values // AxisRef.bExecute := FALSE; // CurrentState := E_State.Idle; // ------------------------------------- bFirstScan := FALSE; // Turn off for next scan bIsInitialized := TRUE; // Mark as initialized END_IF // ---------------------------------------------------- // NORMAL CYCLIC CODE // ---------------------------------------------------- // ... rest of your program Use code with caution. The "TwinCAT Runtime" Method (Recommended)