Beckhoff First Scan Bit -

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.

Most TwinCAT developers create a global boolean variable and set it to TRUE by default. At the very end of their main program, they set it to FALSE . VAR_GLOBAL bFirstScan : BOOL := TRUE; END_VAR Use code with caution. Main Logic (MAIN PRG): beckhoff first scan bit

Establishing a "heartbeat" or initial connection status with HMIs or third-party devices. How to Implement "First Scan" in TwinCAT 3 There are two primary ways to handle this in Beckhoff. 1. The Manual Method (Most Common) In the world of Beckhoff TwinCAT and industrial

Never make your First Scan bit a RETAIN or PERSISTENT variable. It needs to reset every time the PLC power cycles. At the very end of their main program, they set it to FALSE

Ensuring your sequences (SFC) start at "Step 0."

Note: This method is more robust because it relies on the system's own cycle counter rather than a variable you might accidentally overwrite elsewhere. Best Practices

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: