: Ensuring certain outputs or states are cleared before the main logic begins. hardware initialization How to Configure the FirstScan Bit in Siemens TIA Portal
: 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.
Without a proper first scan routine, your machine might start with actuators in unpredictable positions or unfinished states from a previous run. beckhoff first scan bit
Have a specific first scan challenge? Visit the Beckhoff Community forums or consult your local Beckhoff support engineer.
bFirstScanSys := TwinCAT_SystemInfoVarList._FirstScan; : Ensuring certain outputs or states are cleared
The primary utility of the First Scan bit lies in initialization. It serves as the logical "clean slate" mechanism. For instance, in complex motion control applications involving Beckhoff’s NC (Numerical Control) or robotics, the First Scan routine is used to verify the actual position of axes against their commanded positions. It allows the programmer to suppress motion commands until the system has verified that communication with servo drives is healthy. Furthermore, it is instrumental in state machine logic. By forcing the state machine into a specific "Init" or "Home" state on the first scan, the engineer ensures the machine follows a strict, safe sequence of startup events, regardless of the state the machine was in when it was last powered off.
✅ – 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. This gives you more control but requires discipline
To understand the importance of the First Scan bit, one must first appreciate the architecture of a PLC. A PLC operates on a scan cycle: reading inputs, executing logic, and writing outputs. Under normal operation, this cycle repeats endlessly. However, the very first cycle after a power-up or a program reset presents a unique problem. At this specific moment, input data may not have settled, variables may be holding default values rather than retained ones, and physical actuators might be in unknown positions. If the control logic were to execute standard commands immediately, it could lead to unintended consequences, such as commanding a cylinder to extend before verifying it is retracted, or resetting a recipe to default values instead of loading the last saved state.