Beckhoff First Scan Bit 【Recent】

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.

Method 3: Using the FB_init Method (TwinCAT 3 Object-Oriented Approach)

IF fbFirstScan.bFirstScan THEN // First cycle only bResetDrives := TRUE; bStartupComplete := FALSE; tStartupTimer(IN:=FALSE); END_IF

One of the most critical uses is to reset latched alarms or safety circuits at startup. This prevents a machine from automatically restarting a potentially dangerous process after a power cycle. beckhoff first scan bit

: Retrieves the Distributed Clock (DC) timestamp, which is absolutely vital for high-precision motion control and EtherCAT synchronization.

When a TwinCAT configuration activates and the PLC task starts, the system runtime allocates memory and sets variables to their default values. The very first execution cycle—Cycle 0—is where initialization logic must execute.

If you are coming from the Allen-Bradley world, you are likely very comfortable with the (First Scan) bit. It’s a staple for initializing logic, resetting counters, or clearing buffers on startup. If you use the manual variable method, ensure

This public link is valid for 7 days and shares a thread, including any personal information you added. This link or copies made by others cannot be deleted. If you share with third parties, their policies apply. Can’t copy the link right now. Try again later.

fbFirstScan();

Remember that the first scan bit is a tool, not a complete solution. For complex systems, combine it with other initialization techniques like FB_init methods, state machines, and proper variable typing to create truly robust automation code. Test your initialization logic thoroughly on your specific target hardware, as subtle differences between platforms can lead to unexpected behavior in production environments. This prevents a machine from automatically restarting a

If the PLC goes to STOP mode and then back to RUN, the bit becomes TRUE again for one cycle. 2. Why Use a First Scan Bit in TwinCAT?

// First scan detection IF bFirstScan AND NOT bFirstScanDone THEN bFirstScanDone := TRUE; // First scan logic here END_IF