Walk around almost any UK manufacturing facility that has been running for more than ten years and you'll find at least one control cabinet housing a PLC that is no longer supported by its manufacturer. In many cases, there are several. The machines keep running, the operators know the quirks, and nobody wants to be the person who triggers a costly shutdown by changing something that isn't broken.
But legacy PLC systems are broken — just not in a way that shows up on a production report. The risk is invisible until the day it isn't.
Why Legacy PLCs Are a Bigger Problem Than Most Manufacturers Realise
When a PLC platform reaches end-of-life, the manufacturer stops producing spare parts and stops providing software or firmware updates. At first, this feels manageable — you might have a stock of spare cards, and the system keeps running. But the risk compounds over time in several ways.
- Spare parts dry up. Third-party stocks of obsolete PLCs are finite and unpredictable. When a critical card fails, you may face weeks of downtime searching for a replacement that no longer exists.
- Software becomes inaccessible. Legacy programming software often won't run on modern operating systems. When the one laptop that can still talk to the PLC fails, the programme may be inaccessible entirely.
- Safety compliance exposure. Control systems that pre-date modern safety standards may not meet current IEC 62061 or ISO 13849 requirements — a significant liability in the event of an incident.
- Cybersecurity vulnerability. Older PLCs often have no network security capability, creating risk in increasingly connected production environments.
- Institutional knowledge risk. The engineers who understand the legacy system retire or move on, taking undocumented knowledge with them.
The question is not whether a legacy PLC system will cause a problem — it's when. The only variable manufacturers control is whether they plan for it or react to it.
Assessing Your Legacy Risk
The starting point for any legacy upgrade project is an honest assessment of where you are. This means identifying every PLC in your facility, checking its support status with the manufacturer, reviewing spare parts availability and understanding what would happen to production if each system failed tomorrow.
A free production line review from Duke Control Systems will give you exactly this — an independent, written assessment of your legacy risk with specific recommendations on prioritisation and approach.
Not every legacy system needs to be upgraded immediately. A risk-prioritised approach considers the criticality of each machine to production, the availability of spares, the age and condition of the hardware, and the commercial impact of an unplanned failure.
The Case for Planned Upgrades Over Reactive Replacement
Manufacturers who upgrade legacy systems on their own terms — during planned shutdowns, with time to prepare — consistently achieve better outcomes than those who are forced to act by an emergency. The reasons are straightforward.
A planned upgrade allows time for proper electrical design, software development and factory acceptance testing before anything changes on site. A reactive replacement — driven by an unplanned failure — means rushing all of this under production pressure, with the risk of errors that create further problems.
Planned upgrades also allow the new system to be virtually commissioned before site work begins. Using tools like Siemens Process Simulate, we can test the complete control sequence in a digital twin of your machine — catching issues that would otherwise surface during live commissioning.
Choosing the Right Replacement Platform
One of the most common questions we hear from manufacturers approaching a legacy upgrade is: which PLC should we move to? The honest answer is that it depends on several factors specific to your operation — and any engineer who gives you a quick answer without understanding your situation is probably not giving you the right one.
At Duke Control Systems, we are vendor neutral. We work across Siemens, Rockwell Allen-Bradley, Mitsubishi, Beckhoff and other leading platforms, and we recommend what is right for your application — not what is most convenient for us. The considerations include:
- Your existing installed base and engineer familiarity
- Integration requirements with other machines and systems
- Long-term support and spare parts availability
- Safety requirements and functional safety standards
- Budget and total cost of ownership over ten or more years
How Duke Control Systems Approaches Legacy Upgrades
We have extensive experience upgrading legacy control systems across automotive, FMCG, logistics and life sciences manufacturing environments — including many brownfield projects on live production lines where minimising disruption was critical.
Our approach follows a structured process. We begin by fully documenting the existing system — capturing the control logic, I/O mapping, HMI screens and any undocumented behaviour that has built up over years of operation. This documentation alone is valuable, regardless of what happens next.
We then develop the new PLC software offline, using the existing documentation as the baseline and incorporating any improvements to control logic, diagnostics and operator interface. The new system is factory acceptance tested thoroughly before any site work begins.
On site, we use a phased approach wherever possible — bringing individual sections of the line across to the new system while maintaining production on the remainder. This approach significantly reduces the commercial risk of the upgrade.
A well-executed legacy upgrade doesn't just reduce risk — it typically improves machine performance, enhances fault diagnostics, and gives your maintenance team a system they can actually work with.
The Right Time to Act
If you have legacy PLC systems in your facility, the best time to plan their replacement is before you have to. That means starting the conversation now — understanding your risk, building a business case, and planning upgrades around your production schedule rather than around an emergency.
Duke Control Systems offers a free, no-obligation production line review that includes an assessment of your legacy control system risk. Our senior engineers visit your site, review your current systems and give you a clear, written report with specific recommendations.
To arrange a visit, call us on 0121 798 9063 or get in touch online. There is no cost and no obligation.
About the Author
Duke Control Systems Engineering Team
Our engineering team has 50+ years of combined experience delivering industrial automation projects — including legacy PLC upgrades — across automotive, FMCG, logistics and life sciences manufacturing.