Beckhoff First Scan Bit

Clear old error flags that might have occurred during the previous shutdown. Establish Communication:

A common misconception is that the first scan bit is only relevant upon a full system power-up. In TwinCAT, however, using the SystemTaskInfoArr[1].firstCycle variable will be TRUE when the controller physically starts up (a "cold start"). It will not be set to TRUE when you simply restart the PLC program from STOP to RUN.

Conclusion The Beckhoff first scan bit is a simple but essential tool for controlled startup in TwinCAT PLC programs. Properly used, it ensures variables and outputs are initialized predictably, prevents unsafe transient actions, and supports reliable commissioning and diagnostics. Adhering to concise, documented first-scan patterns and combining them with broader safety practices produces safer, more maintainable control software.

: Use this variable as a condition for your initialization code.

The principle is to declare a global BOOL variable and ensure it retains its value across cycles: beckhoff first scan bit

It becomes FALSE for all subsequent cycles.

Initiating communication handshakes with external fieldbus devices or vision systems. Best Practices & Pitfalls to Avoid 1. Beware of Online Changes

Typical uses and patterns

: Sending a "Reset" or "Init" command to external devices (like drives or Vision systems) over EtherCAT. Clear old error flags that might have occurred

// -- Wait for EtherCAT sync -- nState := fbEcMaster.GetState(); IF nState <> 8 THEN RETURN; // Don't run logic until bus is operational END_IF

Unlike some legacy PLCs that have a dedicated global address (like S7’s

: Reading a default parameter set from a file or database during the first task execution. Important Considerations

:

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.

Do not place complex, time-consuming code within the IF FirstScan block. The first cycle should be fast to avoid triggering watchdog errors.

IF FirstScan THEN IF bInit_Cold THEN // Full factory reset ELSIF bInit_Warm THEN // Restore retain variables, but reinit comms END_IF END_IF