PLC Scan Cycle 2026: AC Motor Working Steps with IEC 61131-3 Updates
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AI Summary
- The PLC scan cycle still has four canonical stages: input scan, program scan, output scan, and housekeeping.
- IEC 61131-3:2023 (Edition 4) standardizes event tasks and multi-language coexistence (LD, FBD, ST, SFC) within one project.
- Hardware interrupt inputs and PROFINET IRT / EtherNet/IP CIP-Sync can bypass the cyclic scan for time-critical I/O.
- An AC motor start/stop relay circuit maps cleanly onto the four-step model: PB inputs, coil logic, contactor output, watchdog housekeeping.
- Modern multi-core CPUs (ControlLogix 1756-L8x, SIMATIC S7-1500, MELSEC iQ-R) execute periodic, event, and motion tasks in parallel.
In 2026, the classic four-step scan still governs cyclic PLC execution, while event tasks and time-deterministic fieldbuses add a deterministic layer for motion and safety on top of it.
Engineering Notes Ā· AC Motor Ā· Scan Cycle
By the KOEED engineering desk Ā· 2026-06-30 Ā· 9 min read Ā· Engineering Notes
Every PLC ā from a Siemens SIMATIC S7-200 to an Allen-Bradley ControlLogix 1756-L8x ā runs the same four-step scan cycle. Here is what each stage actually does in 2026, and how it maps onto an AC motor start/stop relay circuit on the shop floor.
TL;DR
- Four steps: input scan ā program scan ā output scan ā housekeeping.
- IEC 61131-3 Edition 4 (2023) added event tasks and tighter OPC UA integration.
- Time-critical I/O (high-speed counters, PROFINET IRT, CIP Sync) runs outside the cyclic scan.
- An AC motor relay circuit is the cleanest way to teach the model.
The four-step scan cycle in 2026
Modern PLC firmware still executes the same four-stage loop described in every textbook since the 1970s ā the change is what runs around it, not what runs inside it. The stages are:
| Step | What the CPU does | Typical duration (1756-L73, 2026 firmware v36) |
|---|---|---|
| 1. Input scan | Copy physical input states into the process-image input (PII) table. | ~0.2 ms / chassis slot |
| 2. Program scan | Execute user tasks (periodic, event, continuous) in declared priority. | 1ā20 ms typical |
| 3. Output scan | Copy process-image output (PIQ) table to physical output modules. | ~0.2 ms / chassis slot |
| 4. Housekeeping | Comm cycles, diagnostics, watchdog, retentive save, OPC UA publish. | 0.5ā2 ms |
Step 1 ā Input scan
Every digital and analog input module writes its current state into a reserved area of CPU memory called the process-image input (PII). The CPU never reads the field terminal directly during program execution ā it reads the PII snapshot. This is what gives a PLC its deterministic, repeatable scan: the program sees a stable view of the world for the entire duration of one cycle.
Step 2 ā Program scan
The CPU walks the user program from the first instruction to the last, evaluating each rung of ladder logic or each POU of structured text. On a 1756-L73 in Studio 5000 v36, multiple periodic tasks of different priorities are scheduled round-robin inside this phase. On a SIMATIC S7-1500 the same idea is called OB1 (cyclic) plus OB3x (time-of-day) and OB4x (hardware-interrupt) ā the conceptual model is identical.
Step 3 ā Output scan
At the end of program execution, the process-image output (PIQ) is frozen and pushed out to the physical modules in a single burst. All outputs change state at the same instant ā within the bus-cycle jitter ā which avoids the "racing" you would otherwise see on a relay-logic panel.
Step 4 ā Housekeeping
Comm modules get serviced (EtherNet/IP, PROFINET, Modbus TCP), retentive tags are committed, the watchdog timer is kicked, and any non-fatal diagnostics are logged. This is also where modern PLCs publish to OPC UA servers and synchronize to NTP ā work that did not exist in the 1990s.
! Warning
An overload relay contact that opens mid-scan is read on the next input scan, not instantly. If you wire the overload NC contact into a safety-critical stop path, route it through a hardwired relay or a certified safety module such as the Siemens ET 200SP F or GuardLogix 1756-L7xS ā never rely on the cyclic scan for SIL-rated stops.
AC motor relay circuit, step by step
A textbook AC motor start/stop station is the cleanest way to see all four stages fire on real hardware. Three physical inputs (Start PB, Stop PB, Overload NC), one output (motor contactor), one seal-in auxiliary contact.
| Scan stage | Memory state (PII/PIQ) | What happens on the AC motor |
|---|---|---|
| Input scan | PII: Start=1, Stop=0, OL=1 | PB module reads 24 V on Start input, 0 V on Stop, 24 V on overload NC. |
| Program scan | Ladder evaluates; coil Motor_Run = 1 | Seal-in auxiliary wired through Motor_Run contact sets up the latching rung. |
| Output scan | PIQ: Output_0 = 1 | Contactor coil energises; main contacts close; AC motor starts. |
| Housekeeping | Tag Motor_Run_Hours += scan_period | Run-hour counter increments; HMI poll updates; OPC UA publishes. |
If the operator presses Stop, the next input scan captures Stop=1, the program scan de-energises Motor_Run, and the output scan drops Output_0 ā the contactor releases and the AC motor coasts to stop. Typical worst-case response time is one full scan, often 5ā15 ms on a modern ControlLogix or SIMATIC S7-1500.
What changed since 2024
The four-stage loop is unchanged, but the surrounding architecture has shifted. Three concrete deltas matter for engineers specifying a PLC in 2026:
- IEC 61131-3 Edition 4 (released 2023, broadly adopted 2025ā2026) formally standardizes event tasks, namespace scoping, and tighter integration between LD, FBD, ST, and SFC inside one project. This means a Mitsubishi MELSEC iQ-R project can now mix a structured-text safety routine with a ladder motion routine in the same CPU task tree without compiler gymnastics.
- Time-deterministic fieldbuses have moved into the I/O scan. PROFINET IRT, EtherNet/IP CIP Sync, and CC-Link IE TSN now deliver I/O updates with sub-millisecond jitter, which means the effective scan period for a motion axis can be shorter than the user task period. This is invisible to ladder logic but visible at the drive.
- Multi-core CPUs run tasks in parallel. The 1756-L73, 6ES7 315-2EH14 (and its S7-1500 successor), and the FX5UJ all expose multiple periodic tasks that execute on separate cores. The cyclic model still holds inside each task, but two tasks no longer have to share one core.
> Tip
When you migrate a 2024-era ladder project to a 2026 CPU, leave the original task period alone unless you have measured scan time. Shorter periods do not improve responsiveness on a start/stop seal-in ā they only raise CPU thermal load.
Scan-cycle-aware PLC families at KOEED
If you are sourcing a CPU for a new scan-cycle design, three families handle the four-stage model cleanly today. We keep active and EOL stock side by side at KOEED so you can mix and match.
| Family | Typical scan @ 1k I/O | Notes |
|---|---|---|
| Allen-Bradley ControlLogix (1756-L7x / L8x) | 5ā10 ms | Tag-based; periodic + event tasks; CIP Sync ready. |
| Siemens SIMATIC S7-1500 | 2ā8 ms | OB1 cyclic + OB4x interrupt; PROFINET IRT; OPC UA server built in. |
| Mitsubishi MELSEC iQ-R | 3ā9 ms | Multi-language IEC 61131-3 Ed.4; CC-Link IE TSN. |
Browse the full Allen-Bradley collection, Siemens collection, and Mitsubishi collection for current part numbers. Live shipment and restock activity is on the Real-Time Records page.
Specifying a PLC for a 2026 AC motor line?
Send your BOM to Moritta@KOEED.COM. Active stock, EOL stock, and scan-cycle-aware cross-references ā all in one quote within 24 hours.
Send My BOM āFrequently asked questions
Does the four-step scan cycle still apply to modern PLCs in 2026?
Yes. Allen-Bradley, Siemens, Mitsubishi, Omron, Schneider and Yaskawa CPUs all still run input scan, program scan, output scan, and housekeeping. Event tasks add a parallel layer.
How fast is one full scan on a typical AC motor application?
A start/stop circuit on a 1756-L73 or SIMATIC S7-1516 completes one full scan in roughly 5ā15 ms with about 256 digital I/O, well below any human-perceivable motor contactor delay.
Why does my scan time spike when I add EtherNet/IP or PROFINET?
Comm modules are serviced in housekeeping. More nodes, larger RPI values, or unmanaged multicast traffic inflate total scan. Profile the CPU in Studio 5000 or TIA Portal first.
Can I use a high-speed counter input to catch a fast AC motor encoder pulse?
Yes. High-speed counter modules run on dedicated hardware that bypasses the cyclic scan. For closed-loop motion, use a dedicated motion module such as the 1756-M02AE or SIMATIC TM PosInput.
Where can I cross-reference an older PLC for a 2026 AC motor retrofit?
Email your existing part numbers plus motor nameplate data to Moritta@KOEED.COM. We return a scan-cycle-aware cross-reference with active, EOL and migrated part numbers within one business day.
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