Beckhoff First Scan Bit -

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.

TwinCAT provides internal system information via the Tc2_System library. You can check if the current cycle is the very first one by looking at the system task info.

Ensuring your sequences (SFC) start at "Step 0." beckhoff first scan bit

In Beckhoff’s TwinCAT 3 environment, there isn’t a single hard-coded bit in the global memory by default, but the system provides a specialized mechanism to create one that is far more powerful than a simple boolean. What is the First Scan Bit?

Without a initialization bit, your PLC logic simply resumes from its last state or starts with default values that might not be appropriate for a running machine. Common use cases include: If you use the manual variable method, ensure

IF bFirstScan THEN // Perform Initialization Tasks here iTargetVelocity := 1500; bMachineReady := FALSE; END_IF // All other machine logic goes here... // The very last line of the program: bFirstScan := FALSE; Use code with caution. 2. Using FB_GetCurTaskIndex (The Pro Method)

Never make your First Scan bit a RETAIN or PERSISTENT variable. It needs to reset every time the PLC power cycles. You can check if the current cycle is

Use the first scan to ensure all physical outputs are in a "Safe/Off" state before the logic takes over.

Wiping the slate clean on startup so old errors don't prevent a start.

In the world of Beckhoff TwinCAT and industrial automation, the "First Scan Bit" is a fundamental tool for ensuring your PLC starts in a predictable, safe state. If you’ve ever worked with Siemens (where it’s a system bit like FirstScan ) or Allen-Bradley (using the S:FS bit), you know how vital this is.