PLC Scan Cycle Execution in 2026: How IEC 61131-3 Changed the Playbook

Detail

Deterministic execution is no longer optional on safety-related machines: IEC 61131-3 Ed.4 and PL e / SIL 3 require a watched scan watchdog.

Modular PLCs (Allen-Bradley ControlLogix, Siemens S7-1500, Mitsubishi iQ-R) now ship multi-tasking runtimes that split programs into FAST / NORMAL / SLOW tasks.

Event-driven interrupts have replaced many legacy timed scans on packaging, motion, and vision lines.

For KOEED customers running 1990s SLC 500 or S7-300 systems, the practical path is parallel stocking: keep legacy spares and migrate at the next outage.

The 2026 PLC scan cycle keeps input → execute → output, but IEC 61131-3 Ed.4 multi-tasking and watched safety tasks have reshaped determinism. What is new and what is unchanged.

If you commissioned a PLC in 2004, the scan cycle was a single loop: read inputs, run the program top to bottom, write outputs, repeat. In 2026 that mental model still works, but the runtime has grown layered tasks, event-driven I/O updates, and a safety layer that refuses to let the watchdog stretch. This Engineering Note walks through what changed, what stayed, and where engineers most often stumble.

, execute the user program against the process image in the order defined by the runtime.