Beckhoff First Scan Bit Direct

Ensure that the POU (Program Organization Unit) containing your first scan initialization logic is placed at the very top of your task execution list. Initializing data after other code has already processed it for one cycle can lead to erratic machine behavior. To help tailor this to your current project, let me know:

Understanding the First Scan Bit in Beckhoff TwinCAT In industrial automation, executing specific logic only once when the PLC switches from Stop to Run mode is a critical requirement. This initialization phase establishes safe initial states, resets counters, pre-loads configuration parameters, and clears temporary buffers.

VAR fbGetCurTaskIdx : GETCURTASKINDEX; // Function block to find the current task ID bFirstScan : BOOL; END_VAR fbGetCurTaskIdx(); // Call the FB // Access the system's internal task info array bFirstScan := _TaskInfo[fbGetCurTaskIdx.index].FirstCycle; IF bFirstScan THEN // Insert initialization logic here END_IF Use code with caution. Copied to clipboard

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. beckhoff first scan bit

Because bInitDone is now TRUE , the inverted contact opens on the second scan, disabling the rung until the next PLC Stop-to-Run transition. Best Practices for First Scan Initialization

When the TwinCAT PLC runtime transitions to , memory is allocated and bFirstScan is initialized to TRUE .

Use the bit to set initial setpoints ( iSpeed := 100 ) rather than to initialize complex state machines. Ensure that the POU (Program Organization Unit) containing

Method 3: Using the FB_init Method (Object-Oriented Approach)

VAR fbGetCurTaskInfo : FB_GetCurTaskInfo; bFirstScanSystem : BOOL; END_VAR // --- PLC Execution Code --- // Fetch the current task configuration information fbGetCurTaskInfo(); // Check if this is the very first cycle of the task IF _TaskInfo[fbGetCurTaskInfo.index].FirstCycle THEN bFirstScanSystem := TRUE; ELSE bFirstScanSystem := FALSE; END_IF // Use the bit for initialization logic IF bFirstScanSystem THEN // Execute initialization routines END_IF Use code with caution. Considerations for System Variables

IF bFirstScan THEN // Perform initialization (e.g., setting default values) END_IF Because bInitDone is now TRUE , the inverted

: Setting initial values for variables that aren't retentive. Communication Setup

Caveat : This method is cycle-dependent. If your cycle time is 10ms, set PT to at least 1ms — but ensure it's longer than one cycle but shorter than two.

If you perform an "Online Change" (modify code without full download), the first scan bit does trigger. Your initialization code will not run. To force reinitialization, use Reset or Reset Cold from the TwinCAT runtime.

If you have nested function blocks, they won't automatically know a "first scan" occurred in the main program.

Do not put long, blocking loops (like heavy WHILE loops) inside your first scan logic. If the first cycle takes too long, it will exceed the configured Task Cycle Time, throwing a and stopping the PLC immediately. 3. Execution Order Matters