PLC — Programmable Logic Controllers
The brain of discrete and manufacturing automation. Deterministic scan cycle, IEC 61131-3 languages, redundancy, fieldbus and OPC UA integration. Major vendors, architecture choices and 2026 trends — TSN, edge, security-by-design.
Core principles
Invented by Dick Morley (Modicon, 1968) to replace relay panels on automotive assembly lines, the PLC remains 55 years later the cornerstone of discrete automation. Four principles make it irreplaceable.
Scan cycle
Read inputs → execute program → write outputs → self-diagnostics. Typical cycle 1–20 ms, deterministic to the µs. Hardware watchdog catches infinite loops.
Distinct memories
Input image memory (I), outputs (Q), internal memory (M), data blocks (DB). Separated from program memory. Allows reasoning on "frozen" values within a scan.
Modular I/O
Rack or DIN rail: CPU + power supply + I/O modules (digital, analog, fast counter, comms). Hot-swap. Remote I/O over PROFINET/EtherCAT for extension.
Industrial robustness
Temperature range −25 °C to +60 °C, EMC immunity per IEC 61000-6-2, vibrations, industrial atmospheres (dust, condensation). Typical lifetime 15–25 yr.
IEC 61131-3 languages
The 1993 unifying standard defines 5 languages (4 active today). A modern PLC mixes languages: LD for visible interlocks, ST for calculations, FBD for control loops, SFC for sequences.
| Code | Full name | Typical use |
|---|---|---|
| LD | Ladder Diagram | Combinatorial/sequential logic, relay heritage |
| FBD | Function Block Diagram | Regulation, signal processing, continuous calculations |
| ST | Structured Text | Complex algorithms, loops, data manipulation |
| SFC | Sequential Function Chart | Step/transition sequences (state machines) |
| IL | Instruction List | Low-level, deprecated since IEC 61131-3:2013 |
Major vendors
| Brand | Flagship platform | Software | Region | Market share |
|---|---|---|---|---|
| Siemens | S7-1500 / S7-1200 | TIA Portal | EU, Asia | ~30% world |
| Rockwell Automation | ControlLogix / CompactLogix | Studio 5000 | Amériques | ~20% world |
| Schneider Electric | M340 / M580 / Momentum | EcoStruxure Control Expert | Global | ~10 % |
| Mitsubishi Electric | iQ-R / MELSEC | GX Works3 | Asia, EU | ~10% (Asia leader) |
| Beckhoff | CX / TwinCAT 3 | TwinCAT 3 | EU | Niche, EtherCAT |
| Omron | CJ / NX / NJ | Sysmac Studio | Asia, EU | ~5 % |
| B&R (ABB) | X20 / X90 | Automation Studio | EU, automotive | Niche |
Communication protocols
| Protocol | Org. | Layer | Use |
|---|---|---|---|
| PROFINET | PI | Industrial Ethernet | Siemens de-facto standard, deterministic |
| EtherNet/IP | ODVA | Industrial Ethernet | Rockwell standard, CIP |
| EtherCAT | ETG | Industrial Ethernet | Very low latency (<100 µs), motion control |
| Modbus TCP | Modbus Org | TCP/IP | Simple, open, ubiquitous — interoperability |
| OPC UA | OPC Foundation | Application | IT/OT interop, native security, Pub/Sub |
| OPC UA over TSN | OPC + IEEE | L2 + app layers | Convergence of real-time and openness — future |
2026 trends
Embedded OPC UA server
All new PLCs (S7-1500, ControlLogix 5380+, M580) embed a native OPC UA server — end of the dedicated gateway era.
Time-Sensitive Networking (TSN)
Convergence of standard Ethernet and guaranteed real-time. Allows PROFINET, EtherNet/IP, video, IT data on the same cable.
Edge computing in the PLC
Docker containers, Python, ML inference directly on the PLC CPU. Pre-processing before IIoT upload.
Security-by-design
Cyber Resilience Act (CRA) applicable end 2027 — manufacturer obligation. Secure boot, signed firmware, signed updates, strong auth, IEC 62443-4-2.
Standards
- IEC 61131-3 — PLC programming languages (LD, FBD, ST, SFC, IL)
- IEC 61131-9 — IO-Link — single-drop interface for sensors/actuators
- IEC 61499 — Distributed function blocks for distributed automation
- IEC 62443-4-2 — Cybersecurity requirements for IACS components — applies to PLCs
- IEC 61784-3 — Functional safety fieldbuses (PROFIsafe, CIP Safety, FSoE)