In PLC (Programmable Logic Controller) programming, initialization is a critical phase of the execution lifecycle. When a controller switches from Config Mode or Stop Mode into Run Mode, variables must be set to safe initial states, communication links must be established, and system histories must often be cleared.
PROGRAM MAIN VAR bFirstScan : BOOL; rst : BOOL; END_VAR
Method 2: System Task Array Linkage (TwinCAT 2 & 3 Compatibility)
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This is the standard, robust method in TwinCAT 3. You check the FirstCycle property within the PlcTaskSystemInfo array.
// Check if this is the very first cycle of the PLC task IF nTaskIdx > 0 AND _TaskInfo[nTaskIdx].FirstCycle THEN // Place your one-time initialization code here // (e.g., resetting latched alarms, setting initial states) bInitializationDone := TRUE; END_IF
If you are using Object-Oriented Programming (OOP) with Function Blocks, you should generally use the method for hardware checks or setup, rather than a "First Scan" bit inside the body logic. This runs before the first cyclic call and is cleaner for object initialization. beckhoff first scan bit
| Platform | Naming / Implementation | | :--- | :--- | | | _TaskInfo[].FirstCycle via GETCURTASKINDEXEX() | | Siemens TIA Portal | OB100 (Organization Block for startup) | | Rockwell Studio 5000 | S:FS (System Status Bit for First Scan) | | CODESYS | Accessible via _TaskInfo[].FirstCycle or PlcTaskSystemInfo |
Ensuring old data is cleared before new operations begin.
Unlike some legacy PLCs that have a dedicated global address (like S7’s | Platform | Naming / Implementation | |
Here’s a technical feature article exploring the in Beckhoff TwinCAT systems.
Have a tricky Beckhoff initialization issue? Share your experience in the comments or contact your local Beckhoff automation partner.
(* Logic Section ) IF bFirstScan THEN ( --- This runs ONLY on the first scan --- ) nCounter := 0; ( Reset variables ) ( Perform other startup routines *) resetting latched alarms