If your line still runs on an obsolete PLC, the risk grows every year: spares get harder to find, the old programming software won't run on modern laptops, and the people who understood the code move on. We replace legacy control systems with current, supported platforms in planned, phased stages, so you upgrade on your terms rather than after a breakdown.
Legacy control systems rarely fail all at once. The warning signs build up quietly, and most of them are visible long before a breakdown.
Your PLC is no longer made, and replacement cards come from refurbishers or eBay. When a critical card fails, you could be down for days while someone hunts for a compatible part.
The programming software needs an old operating system, and the one machine that still runs it is getting older too. If that laptop fails, the program may be out of reach entirely.
Years of quick fixes, no documentation, and the engineers who wrote it have retired or moved on. Fault finding turns into hours of guesswork every time the line stops.
Older systems often pre-date current machinery safety standards such as ISO 13849 and IEC 62061, and have no network security at all, which matters more as sites become more connected.
We look at how this shows up in your downtime figures in Unplanned Downtime Is an Obsolescence Problem.
We work across Siemens, Rockwell Allen-Bradley, Mitsubishi, Beckhoff, Omron and Schneider, so we can read the old system and recommend the right replacement for your site rather than for a particular brand.
| Legacy system | Typical replacement | Why sites are moving |
|---|---|---|
| Siemens S5 | Siemens S7-1500 / TIA Portal | Long discontinued, spares increasingly scarce, and the original programming tools are hard to run today. |
| Siemens S7-300 / S7-400 | Siemens S7-1500 / TIA Portal | Being phased out by Siemens. Planning the move now avoids a rush when spares dry up. |
| Allen-Bradley PLC-5 / SLC 500 | ControlLogix or CompactLogix / Studio 5000 | Discontinued by Rockwell, with support and spares increasingly limited. |
| Legacy Modicon | Rockwell ControlLogix, Siemens or current Schneider | Ageing hardware, shrinking knowledge base, and often no usable documentation. |
| Older Mitsubishi, Omron and others | Current platform from the same or a different vendor | Chosen around your installed base, engineer familiarity and long-term support. |
Not sure what is in your panels? That is exactly what a free line review establishes. For more on choosing a platform, see Modernise Legacy Control Systems.
The risk in a PLC migration is managed through preparation. By the time we touch your line, the new panels and software have already been tested.
We capture the existing control logic, I/O mapping, HMI screens and the undocumented behaviour that has built up over years of running. That record is valuable on its own, whatever you decide next.
The new PLC software is written against that baseline, with clearer structure and better diagnostics, and bench tested module by module. Where it helps, we virtually commission the sequence against a digital model first.
Replacement control and power panels are designed by our electrical engineers, then built and factory acceptance tested before they go anywhere near your line.
Sections of the line move across one at a time during weekends, night shifts or planned shutdowns, with a Duke engineer on site throughout, while the rest of the line keeps producing.
More detail on phased module swaps, shadow testing and code clean-up is in Reviving Legacy PLCs.
A major UK automotive manufacturer in the West Midlands was running production-critical conveyors on Modicon systems installed in the 1990s, with no vendor support, scarce spares and no usable documentation. We were appointed to replace every legacy control and power panel with a Rockwell ControlLogix 1756-L83E platform, covering electrical design, panel manufacture, PLC programming and phased commissioning in-house.
The logic was interpreted and rewritten from scratch, every panel was factory tested, and each cutover was rehearsed before going live. The programme is due to complete in December 2026, and there has been no unplanned downtime at any point so far.
Read the Case StudyEvery intervention on a live production site carries risk. The way you manage that risk is through preparation. By the time our engineers went live on each phase, the panel had been factory tested, the code had been bench tested, and the cutover sequence had been rehearsed.Duke Control Systems project team
We recommend the platform that suits your site, your engineers and your long-term support needs, not the one that is easiest for us.
Electrical design, panel build, software and site work under one team, with one point of responsibility.
Upgrades on running production lines in automotive, FMCG, logistics and life sciences, where keeping the line moving was the priority.
Structured, documented code with proper diagnostics, and training for your maintenance engineers at handover.
In most cases, yes. We develop and test the new software offline, factory test the panels, and then cut sections of the line across one at a time during weekends, night shifts or planned shutdowns, so the rest of the line keeps running. On our current Modicon to Rockwell migration for an automotive manufacturer, there has been no unplanned downtime at any point.
That is common, and it does not stop the project. We start by documenting the existing system: control logic, I/O mapping, HMI screens and any undocumented behaviour that has built up over the years. On our automotive migration the original system had no usable documentation, so the logic was interpreted and rewritten on the new platform with all of the original functions preserved.
It depends on your installed base, what your maintenance engineers already know, integration with other equipment, long-term support and safety requirements. We are vendor neutral and work across Siemens, Rockwell Allen-Bradley, Mitsubishi, Beckhoff, Omron and Schneider, so we recommend what suits your site rather than a particular brand.
No. Most sites do not need a mass upgrade programme. We help you rank systems by risk and impact, looking at age, support status, spares availability and how critical the machine is, so you tackle the systems that matter most first and plan the rest around your production schedule.
Book a free production line review. A senior engineer visits your site, reviews your control systems and gives you a written report with specific, prioritised recommendations. There is no cost and no obligation to proceed.
How to assess your legacy risk and choose the right replacement platform.
GuidePhased module swaps, SCADA refreshes and cleaning up undocumented code.
GuideWhere ageing controls erode OEE, and a simple way to prioritise upgrades.
A senior engineer will walk your site, review your control systems and give you a written, prioritised view of what to upgrade first. No cost, no obligation.