PLC Working Modes & Scan Process: 2026 Engineering Notes
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PLC Working Modes & Scan Process: 2026 Engineering Notes
SIMATIC S7-1500 , runs the same two-mode, three-stage loop. This Engineering Note revisits the 2024 original with the IEC 61131-3 determinism rules, modern task-priority execution, and field notes for engineers sourcing spare CPUs.
Detail
PLC has two working modes: Programming (STOP / offline edit) and Run (STOP / RUN execution with scan cycle).
IEC 61131-3 4th edition (2026) formalises deterministic task scheduling on top of the classical scan.
Scan time directly bounds safety response; on modern CPUs it is typically 1, 10 ms vs. 20, 50 ms in 1990s hardware.
Field tip: a stalled scan usually points to a runaway MSG, math overflow, or a watchdog on a remote I/O drop.
PLCs alternate between Programming and Run. Run loops Input, Program and Output Scan. IEC 61131-3:2026 layers deterministic tasks on top, trimming motion and safety latency 60, 80% vs 2024.
, every programmable controller exposes only two top-level states to the field engineer.
In Programming mode the CPU halts the scan loop. All physical outputs are driven to a vendor-defined safe state (off, hold-last, or pre-load). The technician uses Studio 5000, TIA Portal, GX Works or CX-One to create, edit and download the project file. In Programming mode no I/O is processed, which is why a hot plant should never be left in PROG while machinery is energised.
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