Is Your PLC a Hidden Business Risk? Understanding the Risks of Legacy Siemens Automation Systems
For many industrial facilities, the most reliable piece of equipment is often the one everyone forgets about.
A PLC installed 15, 20, or even over 35 years ago may still be running perfectly, controlling pumps, conveyors, treatment processes or production lines without missing a beat.
Because it keeps running, it can be easy to assume there is no urgency to replace it. But an ageing automation system can quietly become a significant business risk.
At Goto Electrical, we regularly encounter facilities still operating Siemens S5, S7-200, S7-300 and S7-400 PLCs. Many continue to perform their intended function reliably.
The risk isn't necessarily that the PLC will fail tomorrow. The risk is everything surrounding that failure.
You need to ask the business:
Can you obtain a replacement processor or I/O module?
Do you have a current copy of the PLC program?
Do you still have the engineering software required to open it?
Can that software run on a modern computer?
Are the programming cables and interfaces still available?
Does anyone within the organisation — or its support network — still understand the platform?
And, most importantly:
How long can the business afford for that system to be unavailable?
The question isn't whether these systems still work. The question is what happens when they don't.
Siemens S5: Running on Borrowed Time
The Siemens S5 platform was a cornerstone of industrial automation throughout the 1980s and 1990s. Many of these systems remain operational today, a testament to their engineering quality.
The challenge is that S5 has been obsolete for many years. Common concerns include:
Extremely limited availability of spare parts
Difficulty sourcing specialised programming hardware
Limited engineering expertise still available in the market
Increased risk of extended downtime following hardware failure
When an S5 CPU or communication module fails, replacement parts often must be sourced from third-party refurbished suppliers, with no guarantee of long-term availability. Even then, you need to find someone who can restore the code and configuration.
For facilities that depend on continuous operation, this creates an unacceptable risk profile.
Siemens S7-200: Small PLC, Growing Risk
The S7-200 established itself as one of the most successful compact PLCs in the industry. It can still be found controlling:
Pump stations
Compressor systems
Small treatment plants
Packaging equipment
HVAC systems
Despite its popularity, Siemens discontinued the S7-200 several years ago and replaced it with the SIMATIC S7-1200 platform.
Many users face challenges such as:
Limited spare CPU availability
Aging communication modules
Obsolete programming software
Compatibility issues with modern SCADA and networking technologies
A single failed CPU can result in significant downtime if a replacement cannot be sourced quickly. Similarly, you need to find someone who can restore the code and configuration.
Siemens S7-300 and S7-400: The Industry Workhorses
The S7-300 and S7-400 families became the backbone of countless industrial facilities worldwide.
These systems are renowned for their reliability, which is precisely why many remain in service well beyond their intended lifecycle.
However, Siemens has progressively transitioned the market toward the S7-1500 platform.
While support remains available for some components, facilities should be asking several important questions:
Are critical spare parts still readily available?
How many years of support remain for installed hardware?
Do we have access to engineers skilled in STEP 7 Classic?
Can our current architecture meet modern cybersecurity requirements?
Is there a migration strategy in place before an emergency forces one?
Waiting until a major CPU, power supply or communications processor fails often leads to rushed decisions, emergency spending and extended outages.
The Hidden Costs of Delaying an Upgrade
Many organisations focus on the capital cost of an upgrade project. What is often overlooked are the hidden costs associated with maintaining obsolete systems. These may include:
Time Risk – A spare part can sometimes take days, weeks or even months to locate.
Rising Maintenance Costs – Engineering effort increases as systems age and documentation becomes outdated.
Loss of In-House Knowledge – The engineers who commissioned the system may have retired or moved on.
Cybersecurity Exposure – Legacy hardware and software were not designed for today's interconnected industrial environments.
Emergency Upgrade Costs – A planned migration can be staged and budgeted. An emergency upgrade is usually much more expensive, more disruptive and completed under significant operational pressure.
The Best Time to Upgrade Is Before You Have To
A successful PLC migration is not simply about replacing hardware. It is about reducing operational risk while improving maintainability and visibility. Modern Siemens platforms provide benefits including:
Improved diagnostics
Enhanced networking capability
Better cybersecurity features
Greater processing power
Increased data availability
Long-term support and spare part availability
Most importantly, a planned migration lets organisations choose the timing rather than letting a hardware failure choose it for them.
Developing a Practical Migration Strategy
Not every site needs an immediate replacement program. A sensible first step is to assess:
What legacy equipment is currently installed?
Do we have a disaster recovery plan (hardware, software, and backups?)
Which assets present the highest operational risk?
What spare parts are available on-site?
What support lifecycle remains?
What migration pathways exist?
Once these questions are answered, a phased upgrade strategy can typically be developed that aligns with operational and budgetary requirements.
Final Thoughts
Legacy PLC systems such as the Siemens S5, S7-200, S7-300 and S7-400 have delivered exceptional service to industry for decades.
But past reliability does not guarantee future reliability. The longer electronic equipment is in service, the more likely it is to fail.
Organisations that proactively assess and plan for automation obsolescence are significantly better positioned to avoid downtime, reduce risk and maintain operational continuity.
The real question isn't whether your PLC is obsolete. It's whether your business can afford the consequences of waiting until it fails.
About the Author
David Holman is the Managing Director of Goto Electrical with more than 30 years of experience in industrial automation, SCADA, telemetry and control systems.
Goto Electrical helps asset owners assess automation lifecycle risks, identify critical vulnerabilities and develop practical upgrade roadmaps for legacy PLC, SCADA and telemetry systems.
A proactive assessment today on your legacy systems can help avoid costly downtime tomorrow.
Contact Goto Electrical to discuss your automation lifecycle strategy.
