Beckhoff First Scan Bit -

Never set the bFirstScan variable back to TRUE during normal operation. It should only be set to FALSE .

VAR RETAIN powerCycleCounter : UDINT; END_VAR

VAR bFirstScan : BOOL := TRUE; END_VAR

If you perform a or Download , the variables reset to their initial values, and the first scan logic will execute again. 2. Keep Logic Lightweight

The Beckhoff First Scan bit is a small, easily overlooked tool that separates professional, robust PLC code from fragile, “works-most-of-the-time” logic. By taking explicit control of the first cycle, you eliminate startup surprises, protect hardware, and ensure your TwinCAT application starts every time in a predictable, safe state. beckhoff first scan bit

Engineers transitioning from Rockwell Automation or Siemens to TwinCAT often search the system variables for a universal TwinCAT_System_Variables.FirstScan flag. It does not exist globally for one primary reason:

: Triggering handshake or connection logic that only needs to happen once upon startup. Safety Resets

If your initialization code runs after a simple "stop" and "start" from the TwinCAT engineering interface , it might be because you are not truly restarting the runtime. The first scan is typically for the .

: Setting initial values for PID loops or communication buffers. Resetting Sequences : Ensuring SFC (Sequential Function Chart) sequences start at the initial step. Communication Setup Never set the bFirstScan variable back to TRUE

Name the method exactly FB_init . TwinCAT will automatically populate the required input parameters: bInitRetains : BOOL and bInCopyCode : BOOL .

: The code inside your IF firstCycle THEN block executes repeatedly. Common Cause : You forgot to reset the flag or the condition remains TRUE. Solution : Ensure you reset the flag within the same cycle. Use R_TRIG (rising edge detection) if you're using a custom flag approach, or simply call SystemTaskInfoArr[1].firstCycle directly, which automatically resets after the first cycle.

In Beckhoff TwinCAT 3 , efficiently capturing the is essential for initializing variables, resetting state machines, and executing one-time startup routines . Unlike many legacy PLC platforms that offer a simple, hardcoded system flag (like S₀ or SM0.1) for the very first execution cycle, TwinCAT handles this dynamically through system structures.

Add the TwinCAT_SystemInfoVarList to your project, then: bInitializationDone := TRUE

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

The Beckhoff First Scan Bit is a powerful feature that allows PLC programmers to execute specific code segments during the first scan cycle of a PLC. By understanding the concept of the First Scan Bit, developers can create more efficient, safe, and reliable PLC programs. Whether you're a seasoned PLC programmer or just starting out, the First Scan Bit is an essential concept to grasp when working with Beckhoff PLCs.

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Set PID controllers to manual, initialize communication buffers, or define setpoints 1.2.5 .

IF SystemTaskInfoArr[1].firstCycle THEN // Perform one-time initialization code here bInitializationDone := FALSE; // ... other one-time tasks ... bInitializationDone := TRUE; END_IF