In Beckhoff TwinCAT, the equivalent of a "first scan bit" is the firstCycle variable found within the system task information. This bit is automatically set to TRUE only during the very first execution cycle of a PLC task, making it ideal for one-time initialization logic. How to Access the First Scan Bit
You can access this feature through the implicit _TaskInfo array, which contains data for every task running in your PLC project. Syntax (Structured Text):
IF _TaskInfo[GETCURTASKINDEXEX()].firstCycle THEN // Your initialization code here (e.g., setting default parameters) END_IF Use code with caution. Copied to clipboard Key Characteristics
Automatic Reset: The system automatically resets this bit to FALSE after the first cycle completes.
Task-Specific: Because it is part of the task info structure, it correctly identifies the first scan for the specific task it is called within. beckhoff first scan bit
Reliability: It is more robust than manual "first scan" flags (like using a boolean that you set to false at the end of the code), as the PLC runtime handles its state directly. Usage Example
Commonly used to load recipes, initialize communication drivers, or reset state machines to their starting position upon a PLC cold or warm start. If you'd like, I can show you:
How to manually create a first-scan bit if you're using an older version of TwinCAT.
How to use it to initialize persistent variables from a file. In Beckhoff TwinCAT, the equivalent of a "first
The difference between a cold start and a warm start in relation to this bit. Beckhoff CX1010 first scan | PLCtalk - Interactive Q & A
Here’s a technical feature article exploring the First Scan Bit in Beckhoff TwinCAT systems.
✅ Always use FirstScan in your main program – Never assume default values.
✅ Set outputs to safe states first – Protect machinery and personnel.
✅ Avoid heavy computation inside FirstScan – Keep it fast; one cycle only.
✅ Test all restart scenarios – Power cycle, online change, cold start, warm start.
✅ Log first scan events – Useful for debugging unexpected initializations.
Without a first scan flag, you cannot reliably distinguish between: Best Practices Summary ✅ Always use FirstScan in
This distinction is crucial for:
| Use Case | Purpose | |----------|---------| | Initialize outputs | Set outputs to safe defaults | | Clear retentive variables selectively | Override old retentive values on first start | | Home axes | Trigger homing sequence only once | | Reset alarms | Clear boot-time fault flags | | Load configuration | Load parameters from file or EEPROM once |
If you set outputs on the first scan before the EtherCAT bus is fully operational (state OP), your writes may be ignored or cause errors. Always wait for EtherCAT Master State = OP before critical I/O initialization.
IF fbFirstScan() AND (nEtherCATState = 8) THEN // 8 = OP
EnableOutputs();
END_IF