The Development Process of Programmable Controllers: 1968 to 2026
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Engineering Notes · PLC history · Updated 2026-06
As PLC professionals who have spent two decades sourcing, programming, and supporting programmable controllers across Allen-Bradley, Siemens, Mitsubishi, and Schneider systems, we have watched this technology evolve from cabinet-filling relays to today’s software-defined controllers. This Engineering Note revisits the development process of programmable controllers and brings the story current to mid-2026, with the soft-PLC, OPC UA over TSN, and on-device AI milestones that have reshaped the field since 2024.
AI summary
- The first programmable controller, the Modicon 084, shipped in 1968 as a solid-state relay replacement for General Motors.
- Through the 1970s and 1980s, microprocessors turned PLCs into compact, affordable units with PID and analog I/O.
- The 1990s brought graphical programming (ladder logic, function blocks) and the first fieldbus networks.
- By the 2010s, PACs and PLCs converged, with Industrial Ethernet (Profinet, EtherNet/IP) becoming the default.
- Since 2024, IEC 61499, soft-PLC runtimes, OPC UA over TSN, and on-device AI inference have defined the 2026 control architecture.
In 2026, the PLC is no longer a dedicated black box on a DIN rail; it is a software workload that runs wherever it makes sense. Sourcing spans CPU, runtime, and edge cloud.
Why this 2026 update matters
The 2024 version of this article closed with the line that artificial intelligence, machine learning, and cloud computing would enhance PLCs. Two years on, those three capabilities are no longer speculative; they are line items in the same BOM as the CPU module. Engineers now routinely choose between a hardened PLC, a soft-PLC runtime on an industrial PC, and an AI-capable edge controller — often within the same machine. Maintenance teams discover that the spare they keep in the cabinet no longer matches the control architecture on the shop floor.
The full development process of programmable controllers
1968 — the Modicon 084 and the relay replacement
The concept of a programmable controller was first introduced in the late 1960s by Dick Morley. He wanted a solid-state replacement for the relay panels that dominated automotive lines. In 1968, he delivered the Modicon 084 to General Motors; the unit used magnetic-core memory, was the size of a refrigerator, and cost the equivalent of several months of an electrician’s salary. It still sold, because it changed the rules: control logic could now be edited without re-wiring the cabinet.
1970s — microprocessors change the economics
Through the 1970s, the development of microprocessors brought about a significant change. Allen-Bradley introduced the PLC-2 in the mid-1970s; Texas Instruments’ PM550 added analog I/O. By the end of the decade the PLC-2/15 and PLC-2/30 were the workhorses of automotive plants. Programming shifted from hand-punched tape to handheld programmers, and ladder logic became the de facto language on plant floors worldwide.
1980s — compact, networked, PID-capable
The 1980s saw rapid growth. The Allen-Bradley PLC-5 (1986) brought redundant CPUs and remote I/O. Siemens launched the SIMATIC S5 family with step-block language. PID loops, data logging, and communication modules turned the PLC from a relay substitute into a true process controller. End-of-line OEMs began standardizing on PLCs for everything from bottling to steel rolling.
1990s — graphical programming and fieldbus
The 1990s brought graphical programming software — RSLogix 5, STEP 5, and CX-Programmer — which made PLC programming accessible to maintenance electricians who were not engineers. Fieldbus networks (Profibus, DeviceNet, AS-Interface) replaced parallel wiring harnesses. In 1996, the IEC 61131-3 standard codified five programming languages: ladder logic, function block diagram, structured text, instruction list, and sequential function chart.
2000s — PACs, Industrial Ethernet, and safety
By the early 2000s, the term Programmable Automation Controller (PAC) appeared. Schneider introduced the Modicon Premium and later the M340; Allen-Bradley launched ControlLogix with the 1756 chassis. PROFINET and EtherNet/IP moved industrial networking onto standard Ethernet. Safety PLCs (SIL 3) emerged from Siemens F-CPU and GuardLogix, separating safety from process logic.
2010s — Ethernet everywhere and the cloud arrives
In the 2010s, Industrial Ethernet replaced most legacy fieldbus, and error-code libraries became part of every engineering toolbox. Edge gateways from ABB, Beckhoff, and Phoenix Contact pushed tag data to cloud historians. The PLC vendor roster stabilized: Allen-Bradley, Siemens, Mitsubishi, Omron held the top four slots globally, with Schneider, Rockwell, Yaskawa, Panasonic, and KEYENCE rounding out the B2B catalog. Runtime languages converged on IEC 61131-3 plus, increasingly, structured text and C/C++ integration.
What changed since 2024
Between 2024 and 2026, four shifts quietly changed what we recommend on a sourcing call. Each one matters for the part numbers on your BOM.
> Note from the field
Since 2024, customers are now asking whether to procure the CPU at all — they want the runtime license. Soft-PLC runtimes on industrial PCs have crossed the 1 ms cycle-time threshold that used to be the exclusive territory of a dedicated PLC. The BOM is shifting from hardware to software subscriptions, which has procurement, audit, and cybersecurity implications across the same project.
| Capability | 2024 state | 2026 state |
|---|---|---|
| Soft-PLC cycle time | 5–20 ms on x86 industrial PCs | Under 1 ms; Siemens, Schneider, and Allen-Bradley offer IEC 61131 virtual controllers |
| Industrial Ethernet | PROFINET, EtherNet/IP, CC-Link IE coexisting | OPC UA over TSN becomes the convergence layer |
| AI inference | Mostly cloud-based | 5–20 ms transformer models on edge controllers (ABB, Beckhoff, Siemens) |
| Distributed control | IEC 61131-3 polling programs | IEC 61499 function-block events for modular machines |
| Cybersecurity | IEC 62443 recommended, not mandated | ISA/IEC 62443-4-2 mandated for many EU and US critical-infrastructure tenders |
2026 architecture decisions an engineer faces today
Five choices show up on most of our quote lines in 2026. They are not mutually exclusive — a typical line mixes a hardened Siemens S7-1500 for safety with a CODESYS-based IPC for high-speed motion, an ABB edge controller for vision AI, and a remote I/O ring to keep the cabinet small.
1. Hardened PLC vs. soft-PLC runtime
If your line runs 24/7 in a Class 1 Div 2 area, a hardened PLC is still the right answer. If your line sits in an IP54 cabinet with predictable thermal envelope, a soft-PLC on an x86 IPC cuts hardware cost while keeping deterministic behavior — provided you license the runtime from the right vendor and respect the support contract.
2. OPC UA over TSN as the backbone
The promise of OPC UA over TSN — that one Ethernet layer carries PROFINET, EtherNet/IP, and CC-Link IE TSN with determinism — is now shipping in production lines we have seen quoted in 2025 and 2026. If your brownfield plant runs three fieldbus networks, this is the path to one.
3. Edge AI for vision and vibration
Edge AI controllers from Siemens SIMATIC ET200, Beckhoff TwinCAT 3 Vision, and KEYENCE CV-X run small transformer models at frame rates that used to require a server. If you spec vision or vibration analytics, the question is no longer “GPU or CPU?” but “edge or cloud?” Our recommendation in 2026 is edge for latency, cloud for trend analysis.
4. Migration from a 2010s PAC
Many plants now run 12-to-15-year-old CompactLogix, S7-300, or MELSEC-Q controllers past their support window. KOEED keeps active stock on legacy part numbers — see our multi-brand catalog for current inventory. For OEM lines still in production, planning a migration to a modern platform is cheaper than running out of spare CPU modules.
5. Cybersecurity posture
ISA/IEC 62443-4-2 is now a hard requirement for many EU tenders under NIS2 and for US critical-infrastructure sites that bid through CISA-aligned procurement. Update the security review of any new control cabinet before the BOM is signed.
Working on a legacy PLC migration or a 2026 greenfield?
Send your BOM and a one-line scope to Moritta@KOEED.COM. Active stock for nine PLC brands, EOL stock for the rest, and cross-reference on the legacy CPUs you cannot replace today.
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Frequently asked questions
Who invented the programmable controller?
Dick Morley is widely credited with inventing the PLC in 1968 with the Modicon 084, delivered to General Motors as a solid-state replacement for relay-based control panels. The name Modicon comes from Modular Digital Controller.
What is a soft-PLC in 2026, and is it as reliable as a hardened PLC?
A soft-PLC is an IEC 61131-3 runtime running on a general-purpose industrial PC instead of a dedicated CPU. By 2026, vendors like Siemens, Schneider, and Allen-Bradley certify cycle times under 1 ms on x86 hardware. For non-safety applications in IP54 cabinets, a soft-PLC matches a hardened PLC on determinism with more flexibility.
What is OPC UA over TSN, and do I need it?
OPC UA over TSN carries PROFINET, EtherNet/IP, and CC-Link IE TSN over a single deterministic Ethernet physical layer. You need it if your plant runs more than one fieldbus today and you are planning a multi-year modernization. For a small single-vendor line, classic PROFINET or EtherNet/IP is still fine.
Are 2010-era PACs still supported?
CompactLogix (1769), S7-300, and MELSEC-Q controllers are past their end-of-production date in many configurations. KOEED holds active and refurbished stock so that maintenance teams can keep legacy lines running while planning migrations.
Where can I source obsolete or end-of-life PLC modules?
Send your instruments list, the harder to find the better, to Moritta@KOEED.COM. KOEED supplies active, refurbished, and end-of-life stock across Allen-Bradley, Siemens, Mitsubishi, and seven more brands, with a quote within 24 hours.
Author: KOEED Industrial Automation editorial team. Reach the team at Moritta@KOEED.COM.