[best]: Beckhoff First Scan Bit

First Scan Bit

In Beckhoff TwinCAT, the is a system flag used to execute logic exactly once when the PLC transitions from Config/Stop to Run mode . It is essential for initializing variables, resetting timers, or triggering one-time communication handshakes. How to Access the First Scan Bit

• if FIRST_SCAN and NOT Initialized then
  • The _FirstScan from TwinCAT_SystemInfoVarList is reset on each login – useful for debugging.
  • For persistent one-time init across power cycles, store a flag in VAR RETAIN.
  • Normal runtime operation
  • The very first cycle after boot

  Multiple Tasks

  : If your project has multiple PLC tasks, _AppInfo.bFirstCycle is local to the context of the task it is called in. beckhoff first scan bit

  Key difference

  : Beckhoff's system does not have a mandatory, automatic FirstScan system bit. You must explicitly create it. This gives you more control but requires discipline. First Scan Bit In Beckhoff TwinCAT, the is

  TwinCAT does not have a global hardware bit like Allen-Bradley's S:FS . Instead, it handles this through software or task-level data types. 🚀 Method 1: The Built-in System Variable (Best Practice) if FIRST_SCAN and NOT Initialized then