If you use the manual variable method, ensure the line bFirstScan := FALSE; is at the very bottom of your MAIN task. If you put it in a sub-function, other parts of your program might miss the "True" state.
Because it is part of the task info structure, it correctly identifies the first scan for the specific task it is called within.
In Beckhoff TwinCAT, first scan detection is achieved via the PlcTaskSystemInfo.FirstCycle beckhoff first scan bit
(* Clear alarms *) bAlarmReset := TRUE;
The First Scan Bit is a read-only bit that is automatically set by the TwinCAT 3 system during the first scan of the PLC. Once the bit is set, it remains set until the PLC is restarted or reset. If you use the manual variable method, ensure
Next time you write a new PLC program, ask yourself: “If this were a cold start right now, would my system behave safely?” If the answer isn’t an immediate “yes,” you need FirstScan .
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. In Beckhoff TwinCAT, first scan detection is achieved
FB_init is a specialized method you can add to any Function Block. It executes automatically before the PLC task starts its cyclic execution. Right-click your Function Block →right arrow →right arrow Method . Name the method exactly FB_init . It automatically generates the required signature:
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
: Clearing OTL (Latches) or variables that must start in a known state.
The First Scan Bit is a flag that is for exactly one PLC cycle when the controller moves from "Config" or "Stop" mode into "Run" mode. After that first execution of the logic, the bit turns FALSE and remains so until the PLC is restarted or the code is re-downloaded. Why Do You Need It?