Example — Codesys Sfc

The SFC jumped to Step 99. The crane rose. The drain opened.

15:47:32.100 - Enter Step 20 (DIP) 15:47:32.105 - Timer started: 45s 15:48:17.200 - Temp fault detected 15:48:17.205 - Exit Step 20 15:48:17.210 - Enter Step 99 (EMERGENCY_RETRACT) 15:48:21.400 - Acid level <5% 15:48:21.405 - Enter Step 0 (IDLE) The coil was perfect. The acid was safe. And Lena finally understood the power of SFC in CODESYS:

The SFC was in with a coil halfway submerged. codesys sfc example

She went to the Action Definition for Step 20. Instead of putting Drain_Valve := FALSE in the step's exit action, she created a Global Action called Acid_Safety and set its qualifier to SD (Set Dominant—stays TRUE until explicitly reset).

Transition from Step 20: Condition: (T#45s) AND NOT EStop_Pressed Supervisory Logic (Parallel Branch): IF EStop_Pressed THEN Jump to Step 99: EMERGENCY_RETRACT END_IF The SFC jumped to Step 99

The transition to Step 0 required Acid_Level < 5% . But the drain valve closed after 2 seconds because the "DIP" step's action had been deactivated. She forgot: Actions in SFC only run while their step is active.

Lena pointed at the HMI. "No. The SFC saved it. Look—step history." 15:47:32

The Pickle Paradox System: Industrial Pickling Line (Acid Bath for Steel Coils) Controller: CODESYS SoftPLC v3.5 SP20 Part 1: The Problem Engineer Lena Vasquez stared at the production log. Line 7, the steel coil pickling line, had just scrapped its third $40,000 coil of the week. The sequence: Load coil → Dip in HCl acid → Rinse → Dry → Unload .

But then... nothing.

Lena shook her head. "No. We need an SFC." She opened CODESYS and created a new POU (Program Organization Unit). She chose Sequential Function Chart (SFC) . No ladder. No structured text loops. Just pure, visual, time-tested sequence logic.