Note: Depending on your exact TwinCAT revision and system architecture, relying purely on CycleCount = 0 can sometimes be risky if the task executes an implicit cycle during hardware abstraction layers. Method 1 remains the safest industry standard.
This relies on the fact that standard, non-retained PLC variables always initialize to FALSE (or 0 ) upon a cold reset. Code Implementation (Structured Text)
Then, in your main program or task, you reset this flag after the first cycle:
The refers to a signal used in TwinCAT PLC programming to execute initialization logic exactly once when the controller starts or enters run mode. Unlike some other PLC platforms that have a fixed system bit like Allen-Bradley's S:FS , Beckhoff TwinCAT provides this functionality through specific system variables or custom logic. Standard Implementation Methods beckhoff first scan bit
Implementation examples (conceptual)
The Beckhoff First Scan Bit, also known as the "FirstScan" bit, is a special bit in the TwinCAT 3 system that indicates when the PLC (Programmable Logic Controller) is executing its first scan cycle. In other words, it is a flag that is set during the initial scan of the PLC program. This bit is typically used to execute specific code or actions only once, during the first scan of the PLC.
It’s a system flag that is after:
In PLC programming, the first scan bit (also known as the first cycle bit) is a special flag that is after the controller is started. After this initial cycle, the flag is reset to FALSE for the remainder of the program's execution. Its primary purpose is to distinguish the initial start-up sequence from all subsequent normal operation cycles.
But this is still a workaround. The most professional method in Tc3_Standard is to use the StandardLib (from Beckhoff Automation) which includes FB_FirstScan directly.
The most robust and recommended method is to use the firstCycle variable from the TcSystem library. This variable is part of the SYSTEMTASKINFOTYPE structure, which is accessible via the SystemTaskInfoArr array. The index in this array (typically 1) corresponds to the PLC task ID. Note: Depending on your exact TwinCAT revision and
PROGRAM MAIN VAR // Variable initializes to FALSE automatically upon startup bInitialized : BOOL := FALSE; // Your operational variables nTargetVelocity : INT; sSystemStatus : STRING; END_VAR // The First Scan Logic IF NOT bInitialized THEN // This code runs EXACTLY ONCE on the very first PLC cycle nTargetVelocity := 100; sSystemStatus := 'System Initialized'; // Set the flag to TRUE so this block is skipped in the next cycle bInitialized := TRUE; END_IF; // --- Your regular PLC program starts here --- Use code with caution. Why this works:
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