PLC Basic Technology Evolution: From Relays to Edge Computing (2026)
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How has PLC technology evolved?
Five eras tell the story: relay replacement (1968-1980), the microprocessor generation (1980-1995), network integration (1995-2010), intelligence integration (2010-2020), and the current edge-computing era. The core scan architecture (read inputs, execute logic, write outputs) has not fundamentally changed across any of them. What has changed is everything around that core: communication speed, memory capacity, security, integration capabilities, and analytics on the controller itself.
The evolution timeline
| Era | Key developments |
|---|---|
| 1968-1980: The beginning | Modicon 084, the first commercial PLC, designed to replace relay panels in automotive manufacturing. Ladder logic chosen because it looked like the wiring diagrams electricians already read. Limited memory (1-4 KB), serial communication only, no networking. |
| 1980-1995: Microprocessor era | Microprocessor-based PLCs; memory expanded past 64 KB; specialty modules (high-speed counters, PID) introduced; early proprietary networking. Allen-Bradley PLC-2 and Siemens S5 series dominated. |
| 1995-2010: Network integration | Industrial Ethernet adoption (PROFINET, EtherNet/IP); fieldbus technologies (PROFIBUS, DeviceNet); modular PLCs with hot-swappable modules; HMI integration standard; IEC 61131-3 programming languages standardised. |
| 2010-2020: Intelligence integration | Built-in web servers and advanced diagnostics; security focus; cloud connectivity; multi-core processors; edge computing in high-end PLCs; unified engineering environments (TIA Portal, Studio 5000, Sysmac). |
| 2020-2026: Edge computing era | PLCs as edge computing platforms: built-in analytics, local inference, predictive maintenance; integrated safety standard; native security; OPC UA as universal integration layer; TSN for converged IT/OT. |
Scroll the table horizontally to view all columns.
The core scan cycle architecture has not fundamentally changed across any of these eras. Read inputs, execute logic, write outputs, repeat. What has changed is everything around that core.
Technology comparison across eras
| Era | Scan time | Memory | Communication | Key feature |
|---|---|---|---|---|
| 1980s | 50-100 ms | 2-16 KB | Serial (RS-232) | Relay replacement |
| 1990s | 5-20 ms | 64 KB-1 MB | Fieldbus | Networked control |
| 2000s | 1-5 ms | 1-10 MB | Industrial Ethernet | Unified engineering |
| 2020s | 0.1-1 ms | 10-100 MB+ | TSN Ethernet | Edge computing |
Scroll the table horizontally to view all columns.
Key technology shifts
- Programming languages: ladder still dominates by installed volume, but structured text usage grows for complex algorithms, and all major platforms support the full IEC 61131-3 language set.
- Communication: OPC UA is the standard for enterprise integration. TSN enables converged IT/OT networks. Wireless I/O grows for remote monitoring.
- Hardware: multi-core CPUs separate real-time control from application logic. Built-in security is standard. Edge computing reaches mid-range platforms.
- Integration: PLCs connect natively to cloud platforms, run local analytics, and support digital twin integration.
When to modernise
Modernisation makes sense when the current PLC is over ten years old, when analytics or predictive maintenance is needed, when safety requirements have changed, when spares or support are drying up, or when IT/OT integration is a goal. Keeping the current system makes sense when it is reliable, meets current needs, has no spares issues, and the staff are trained on it.
When modernising legacy systems, focus on the communication infrastructure first. Upgrading to industrial Ethernet delivers most of the practical benefit at a fraction of the cost of a full CPU replacement. Replace the CPU only when the new capabilities (edge analytics, integrated safety) are actually needed.
Common questions
Is ladder logic still relevant?
Yes. It remains the most-used language for new applications, matches how electricians think about control, and maintenance teams read it fluently. Structured text complements it for math and data work; the two coexist in the same projects.
What is the most impactful first step in modernising a legacy system?
Upgrade the communication infrastructure to industrial Ethernet. This delivers connectivity, remote diagnostics and data collection without replacing the CPU, and it prepares the network for whatever controller comes next.
Is the PLC being replaced by PC-based control?
Not for the deterministic control layer. PC-based and soft-PLC platforms are growing for analytics and coordination, but the scan-cycle controller remains the standard for real-time machine and process control. The two coexist rather than compete.
Sourcing help
KOEED supplies PLC hardware from every era of this timeline, in active and EOL stock. Send the platform and part list for a quote.
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