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If you are developing your application using Ladder Logic or Function Block Diagram (FBD) within TwinCAT, you can replicate Method 1 visually using a sealed coil or an interlocking contact. Visual Logic Sequence
You can utilize the implicit variables provided by the TwinCAT System library. The TwinCAT System library contains global constants and variables that reflect the state of the PLC task.
END_IF
Method 1: The Standard Structured Text Pattern (Recommended)
IF FirstScan THEN IF bInit_Cold THEN // Full factory reset ELSIF bInit_Warm THEN // Restore retain variables, but reinit comms END_IF END_IF
In TwinCAT, you can run multiple independent tasks at different cycle times (e.g., a 1ms motion control task and a 50ms visualization/HMI task). A single global first scan bit would fail because different tasks start at different times or may be restarted independently. Therefore, initialization logic in TwinCAT must be managed at the application, task, or function block level. Method 1: The Local Variable Approach (Recommended)
VAR_GLOBAL FirstScan : BOOL := TRUE; END_VAR
fbGetCurTaskIndex(); // Call the function block
In TwinCAT, the PROGRAM or FUNCTION_BLOCK structure has a specific order of execution. The VAR section declares variables, but it doesn't execute logic. To run logic on the first scan, you declare a boolean flag and check its state.
PROGRAM MAIN VAR // Initialize as TRUE. It will only be TRUE on the first cycle bFirstScan : BOOL := TRUE; END_VAR IF bFirstScan THEN // Perform one-time tasks // ... // Set to FALSE so it never runs again until power cycle bFirstScan := FALSE; END_IF Use code with caution. 4. Best Practices for First Scan Logic
If initialization logic runs continuously instead of only on the first scan, it can cause severe runtime bugs: Overwriting live HMI setpoints with default values. Perpetually resetting runtime timers and counters.
