Beckhoff First Scan Bit 〈SIMPLE | 2024〉

IF TwinCAT_SystemInfoVarList._FirstScan THEN // One-time actions END_IF

In Beckhoff TwinCAT, first scan detection is achieved via the PlcTaskSystemInfo.FirstCycle

State machines are a staple of PLC programming. Using the first scan bit to set the initial state ensures a predictable and safe start-up.

Highly accurate and managed entirely by the TwinCAT runtime.

Implementing a 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 fundamental requirement. This initial execution cycle is crucial for initializing variables, resetting state machines, clearing faults, and establishing baseline communications. beckhoff first scan bit

Always ensure you are querying the task information index that corresponds explicitly to the task executing your specific program. Alternatively, use the local FB_init methods for localized logic. 2. Retain Variables Conflict

VAR currentState : E_MachineState := E_MachineState.STATE_IDLE; END_VAR

If you forget this line, your initialization logic will run every single cycle, overriding any changes made to those variables later in the program.

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 IF TwinCAT_SystemInfoVarList

IF fbFirstScan.bFirstScan THEN (* Force reset retain values on fresh download *) rMotorPosition := 0.0; // Even if retain, reset on first scan bRecipeLoaded := FALSE; END_IF

VAR CycleCount : UDINT := 0; END_VAR

In Beckhoff TwinCAT, the First Scan Bit is accessed via the _TaskInfo system array using the FirstCycle property. This bit is uniquely TRUE during the first execution cycle of a task, allowing for precise system initialization. First Scan Bit - OpenPLC Forum

: When set to TRUE , the sequence is reset to the initial step . Implementing a First Scan Bit in Beckhoff TwinCAT

FUNCTION_BLOCK FB_DriveController VAR_INPUT bEnable : BOOL; END_VAR VAR_OUTPUT bReady : BOOL; END_VAR VAR fSpeed : REAL; END_VAR

: Use this variable as a condition for your initialization code.

Here’s a complete, ready-to-use post explaining the in TwinCAT PLC.

PROGRAM MAIN VAR myOutput : BOOL; END_VAR