: Highly accurate; resets every time the PLC transitions from STOP to RUN or after a power cycle. Method 2: Manual Flag (The "Standard" Way)
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. 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. : Highly accurate; resets every time the PLC
IF FirstScan THEN IF bInit_Cold THEN // Full factory reset ELSIF bInit_Warm THEN // Restore retain variables, but reinit comms END_IF END_IF Under normal operation, this cycle repeats endlessly
⚠️ If you use Method 1 in multiple tasks (e.g., a fast cyclic task and a slower periodic task), it will evaluate properly for each task's respective first execution loop.