Beckhoff First Scan Bit

Unlike some traditional PLCs (like Allen-Bradley’s S:FS bit) that have a predefined system variable, Beckhoff’s TwinCAT allows for several ways to achieve this functionality depending on your version and preference. Methods to Implement First Scan in TwinCAT 1. Using the TwinCAT System Info (The Pro Way)

: Triggering handshake or connection logic that only needs to happen once upon startup. Safety Resets beckhoff first scan bit

✅ – 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. Safety Resets ✅ – Never assume default values

If you prefer a portable method that works across almost any IEC 61131-3 platform, you can create your own logic using a global variable. ✅ Test all restart scenarios – Power cycle,

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.

In Beckhoff TwinCAT, first scan detection is achieved via the PlcTaskSystemInfo.FirstCycle

PROGRAM MAIN VAR fbFirstScan : FB_FirstScan; bInitDone : BOOL; END_VAR