GM's 1968 Ten Requirements and the Birth of the PLC (2026 Update)
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Engineering Notes · PLC History · 2026-07-01
By the KOEED Engineering Desk · 2026-07-01 · 9 min read · Engineering Notes
AI Summary — Key Takeaways
- GM's 1968 "Standard Controller Specification" defined ten engineering requirements that gave birth to the Modicon 084, the first commercial PLC, in 1969.
- Those ten requirements (solid-state, modular, ladder-logic friendly, rugged, etc.) are still the spine of every modern Allen-Bradley ControlLogix and Schneider Modicon CPU shipped today.
- By 2026, the same ten requirements have been re-interpreted through Ethernet/IP, OPC UA, MQTT, TSN, IEC 61131-3, and cybersecurity hardening (IEC 62443).
- Legacy families born from those requirements (PLC-5, SLC 500, Modicon 984, TI 5TI) are now EOL, and obsolete PLC sourcing is a daily need for maintenance engineers.
- Use the GM 10-point checklist to score any new controller you are evaluating in 2026, from 1756-L73 retrofits to SIMATIC S7-1500 greenfield projects.
In 1968, General Motors' Hydramatic division drafted ten engineering requirements for a "standard controller" that could replace unreliable relay panels. That paper became the founding charter of the Programmable Logic Controller, shipped as the Modicon 084 in 1969 and now embedded in every modern PLC from Allen-Bradley to Siemens.
In 1968, the General Motors Hydramatic transmission plant wrote a single page that quietly reshaped factory automation. The document was a request for a "standard controller" to replace electromechanical relay panels that were failing faster than the cars they were building. Its ten requirements defined the Programmable Logic Controller. More than fifty years later, those same ten points still grade every Allen-Bradley ControlLogix, Siemens SIMATIC, and Modicon M580 we ship at KOEED. Here is the 2026 reading of the document that started it all.
Why GM wrote a controller spec in 1968
Hydramatic's auto transmission line ran on relay logic. Every model year change meant hundreds of relays to re-wire, weeks of downtime, and a maintenance crew that could not keep up. GM wanted a controller that behaved like a relay, but survived the plant floor. The result was a one-page "Standard Controller Specification" circulated to vendors including Bedford Associates, Allen-Bradley, and DEC. Dick Morley at Bedford Associates answered, and the Modicon 084 shipped in 1969 as the first commercial PLC. The brand name Modicon is, in fact, short for Modular Digital Controller.
Allen-Bradley, working in parallel, introduced the PDQ-II and later the PLC-2 family, the ancestor of every Allen-Bradley PLC in the ControlLogix line we supply today. Both answers traced back to the same ten lines in GM's spec. Read them as a 2026 engineering checklist, not a 1968 curiosity.
The ten requirements, with a 2026 engineer's annotation
| # | 1968 GM Requirement | 2026 Reading |
|---|---|---|
| 1 | Solid-state, no mechanical parts | No moving parts, no fans, no disks. Industrial temp range, MTBF in decades. |
| 2 | Easily programmed & reprogrammed | IEC 61131-3 languages (LD, FBD, ST, SFC), plus vendor IDEs (Studio 5000, TIA Portal, GX Works). |
| 3 | Designed for industrial environment | -20 to +60°C, 5 g vibration, IP65 panels, conformal coating on modern CPU modules. |
| 4 | Modular I/O expandable in increments | Chassis-based, hot-swappable I/O. A 1756 rack scales from 4 to 17 slots. |
| 5 | Ladder diagram programming familiar to plant staff | Ladder Logic (LD) is still a first-class IEC 61131-3 language. Plant electricians own it. |
| 6 | Reliable and field-serviceable | Status LEDs, diagnostic buffers, web servers on the CPU itself. Field-replaceable modules. |
| 7 | Compact — smaller than the relay panel it replaces | A single CompactLogix 1769 replaces a 6-foot relay cabinet. Edge controllers shrink further. |
| 8 | Competitive price at low quantities | Per-I/O cost is no longer the driver — lifecycle cost (support, EOL, cybersecurity) is. |
| 9 | Data collection & supervisory reporting | EtherNet/IP, OPC UA, MQTT, REST. Plant-floor data feeds MES, SCADA, and cloud historians. |
| 10 | Rugged enclosure suitable for plant floor | Cybersecurity is now part of "rugged" — IEC 62443-3-3 zones, signed firmware, secure boot. |
How the spec birthed the modern PLC market
GM's ten requirements did not just name a product. They defined a category. Within a decade, the PLC market had four major families, all descended from the same paper.
| Lineage | Origin | 2026 status at KOEED |
|---|---|---|
| Modicon 084 → 184 → 384 → 984 → M340 → M580 | Bedford Associates, 1969. Now Schneider Electric. | 984 family EOL — M340 / M580 in production. EOL stock still active. |
| Allen-Bradley PDQ-II → PLC-2 → PLC-5 → SLC 500 → ControlLogix | Allen-Bradley, 1970s. Now Rockwell Automation. | PLC-5 and SLC 500 EOL. ControlLogix 1756 + CompactLogix 1769 are the workhorses. |
| Siemens S3 → S5 → S7-200/300/400 → S7-1200/1500 | Siemens AG, West Germany, 1970s. | S7-300 phased into migration window, S7-1500 (and 1500G2) lead the platform. |
| Mitsubishi MELSEC-A → Q → iQ-R | Mitsubishi Electric, late 1970s. | A-series EOL, Q-series mature, iQ-R in production. FX3U/FX5 still in high volume. |
Engineer’s Note
Every Allen-Bradley ControlLogix 1756 chassis you see on a line today — and every Schneider Modicon M580 rack, and every Siemens S7-1500 station — can trace a design decision back to one of GM’s ten bullets. Treat the table above as a free design review checklist for any controller you evaluate in 2026.
What changed since 2024
The article you are reading was first published in March 2024. Since then, the PLC world has moved on five fronts that are worth flagging for anyone re-evaluating legacy or specifying new controllers in 2026.
1. Rockwell pushed 1756 into a new firmware era
Firmware v33 and v34 (released across 2024–2025) tightened secure-boot enforcement on ControlLogix 1756-L8x and the newer 1756-L73 revisions. Plants still running v20 or older should plan a staged upgrade path; KOEED ships cross-rev modules pre-configured for the target firmware so the swap is transparent to the program.
2. Schneider consolidated Modicon around M580
Schneider officially exited the Modicon 984 and Premium TSX lines as serviceable families in 2024. Active installed bases in water/wastewater and food & beverage are now in the supported until notice window, which means more calls for EOL stock and cross-references to M340 / M580.
3. Siemens S7-300 entered its final migration decade
Siemens has signalled that SIMATIC S7-300 spares will move to classic support within the current lifecycle, while S7-1500 (and the G2 generation launched in 2024) takes the new design wins. We are quoting more S7-300 spares against S7-1500 equivalents than at any point in the last five years.
4. Cybersecurity (IEC 62443) is now Requirement #10 in spirit
The 1968 requirement #10 was about a rugged enclosure. In 2026, the more important part of "rugged" is whether the controller can be patched, authenticated, and segmented on the plant network. Procurement specs that omit cybersecurity are now considered incomplete.
5. The KOEED catalog added KLÜBER industrial lubricants
In 2026, the KOEED warehouse network started stocking KLÜBER lubricants alongside PLCs — greases, gear oils, food-grade H1, and high-temp chain oils. Industrial lubrication has the same "hard-to-find, EOL, BOM-driven" profile as the PLC market that GM helped create. See the KLÜBER collection for the active range.
Using the 10-point checklist on a 2026 retrofit
A practical way to use the table is as a scoring matrix when you compare a modern controller (ControlLogix 1756, Modicon M580, SIMATIC S7-1500) against the controller you are replacing. A perfect score is 10. Anything below 7 deserves a second look.
- Score the existing system first — how did it do against the ten requirements in its first ten years?
- Score the candidate system. Watch for: solid-state yes, modular I/O yes, ladder-logic familiar yes — but make sure cybersecurity is a category in the scorecard.
- For each EOL or hard-to-find module in the bill of materials, check whether KOEED has stock or a cross-reference. Email the part list to Moritta@KOEED.COM for a quote within 24 hours.
- Plan a maintenance budget for the next ten years — spare modules, firmware updates, and panel-sealing. Lifecycle cost beats purchase cost on every GM spec point #8.
! Warning
Do not skip requirement #3 (industrial environment) when the controller is mounted in a control cabinet on a press line. Convection cooling assumes at least 50 mm of clearance. For food & beverage washdown zones, spec the IP65 variant from the start — retrofitting seals after the line is running is always more expensive.
Spec’ing a 2026 retrofit or sourcing EOL stock?
Send your BOM (Allen-Bradley, Siemens, Mitsubishi, Modicon, or anything in between) to Moritta@KOEED.COM. Active stock, EOL stock, and cross-references — one quote within 24 hours.
Send My BOM →Related on the KOEED blog
- Allen-Bradley ControlLogix modules & spares
- Schneider Modicon legacy stock (984 / Premium / M340 / M580)
- Siemens S7-300 to S7-1500 migration spares
- KOEED AI Diagnostic Tool — fault-code lookups & cross-reference
- PLC Error Code Database
KOEED Engineering Desk
Industrial automation editors at KOEED. We write about PLC sourcing, cross-reference, legacy system support, and the history that got us here. Reach the team at Moritta@KOEED.COM.
Frequently asked questions
What exactly did General Motors write in 1968, and who answered?
GM’s Hydramatic division circulated a one-page "Standard Controller Specification" listing ten requirements for a solid-state replacement for relay logic. Bedford Associates (Dick Morley) answered with the Modicon 084, shipped in 1969 as the first commercial PLC.
Why are GM’s 1968 ten requirements still relevant in 2026?
Every modern controller — Allen-Bradley ControlLogix, Schneider Modicon M580, Siemens S7-1500 — still scores against the same ten engineering requirements. Cybersecurity (IEC 62443) is the only major 21st-century addition, layered on top of the original "rugged" requirement.
Are Modicon 984 and Allen-Bradley SLC 500 still supported?
Both families are past active production. Modicon 984 is in extended service through Schneider’s lifecycle program; Allen-Bradley SLC 500 is EOL with Rockwell. KOEED keeps tested, refurbished stock for both, plus cross-references to M340/M580 and CompactLogix 1769 respectively.
Is the original Modicon 084 worth collecting?
Only as a museum piece. The 084 used magnetic-core memory and is not compatible with any modern program. For functional legacy support, source tested 984 or PLC-5 hardware from a multi-brand distributor like KOEED instead.
How do I score a new controller against the 1968 checklist?
Walk the ten points in the table above. Each "yes" scores 1, partial scores 0.5. A 7/10 or better is a serious candidate; below that, ask the vendor to address the gap. Make sure cybersecurity is a category, not an afterthought.
Where can I get a quote on legacy Allen-Bradley, Modicon, or Siemens parts?
Email your part list or BOM to Moritta@KOEED.COM. KOEED supplies active production, mature, EOL, and refurbished stock across Allen-Bradley, Siemens, Schneider Modicon, Mitsubishi, Omron, Fanuc, Yaskawa, Panasonic, KEYENCE, and KLÜBER lubricants, with quotes within 24 hours.