PLC Classification in 2026: Nano, Micro, Edge, Soft & Safety

The classification chart

Scroll the table horizontally to view all columns.

Source: manufacturer product documentation for the named families. I/O ranges are typical bands for orientation; exact capacities are per CPU model.

Modular, compact or rack?

The old binary between fixed-compact and rack-modular has become a spectrum: current mid-range CPUs ship with on-board I/O, stackable local expansion, and an optional backplane for high-density projects, so one product family spans several of the older buckets. Pure rack still belongs to the high end, where redundant process CPUs, hot-swappable power and integrated safety are the requirements that justify it.

Edge PLCs and soft PLCs

An edge PLC is a standard controller with an additional container runtime running analytics, brokers or custom code alongside the deterministic control program; the major mid-range and flagship families now offer this. A soft PLC is an IEC 61131-3 runtime on an industrial PC with software-defined fieldbus; it shines where high-density motion and machine-vision pipelines converge. The caution for unattended duty: a soft PLC on a single industrial PC needs redundancy and a tested recovery procedure before it should carry a 24/7 process alone.

Safety PLCs

Safety-rated CPUs (the safety variants of the major families, certified to the functional-safety standards) are now specified by default in cells with e-stops, light curtains, muting, two-hand control or collaborative-robot zones. The selection rule is simple: if the application needs safety functions, pick the safety variant of whichever class fits the control requirement; a safety relay bolted onto a non-safety controller only covers the simplest circuits.

The legacy tier

The discontinued families (SLC 500 and PLC-5 on the Rockwell side, S7-300/400, MELSEC-A/Q predecessors, Omron C-series and peers) still run a large share of the world's plants. They are a real classification tier for maintenance purposes: repair windows are narrowing, so the tier's management is about sourcing spares and planning migration. KOEED keeps legacy and current families side by side in the catalog for exactly this reason.

How to pick the right class

Three questions in order: how many I/O (with honest expansion margin)? Is there a safety requirement (then take the safety variant of the fitting class)? Does the line need edge analytics or high-density motion (then add the edge runtime or consider a soft PLC)? Everything else in the chart is detail.

Common questions

Is the SLC 500 still in production?

No; the family has been in a mature phase for years. Rockwell supports installed bases through remanufactured modules and migration kits, and EOL stock sources such as KOEED keep the legacy catalog active for in-place repair.

What replaced the S7-400?

Siemens introduced a successor series for the S7-400's high-end redundant niche, while most new projects start on the S7-1500 family and escalate only when full hot-standby redundancy is required. Confirm current family names in Siemens selection data.

Do I need a safety PLC or is a safety relay enough?

For a simple e-stop circuit, a safety relay with a standard controller remains acceptable. Multi-zone safety, light curtains with muting, two-hand control and collaborative-robot zones are the territory of safety-rated controllers, which most OEMs now specify by default.

What replaced the PAC label?

The distinction blurred: current mid-range and flagship controllers all handle PC-style data handling, so the former PAC category has been absorbed into the compact and medium tiers. Choose by I/O scale, safety and edge needs rather than by the label.