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Legacy PLCs Explained: How Long Can You Keep Running Yours?

by Bryan Hellman
Legacy Parts


Walk into almost any factory in the U.S., and you’ll likely see a surprising sight: production lines running on programmable logic controllers (PLCs) that are 15, 20, or even 30 years old. These systems—rock-solid Allen-Bradley SLC 500s, Siemens S5s, GE 90-30s, Mitsubishi A-series—were built to last, and many still do.

But time doesn’t stop, and neither does technology. Spare parts are disappearing, OEM support is ending, and downtime is more expensive than ever. For engineers and plant managers, the big question isn’t whether these systems work today—it’s how long can you keep running your legacy PLC without putting your operation at risk?

In this guide

What Defines a Legacy PLC?

The term “legacy” isn’t just another word for “old.” In industrial automation, a PLC enters legacy status as it moves through a typical lifecycle:

  1. Active production – Manufacturer is producing the PLC, providing updates, offering full support.
  2. Mature support – No new development, limited parts and updates remain.
  3. End-of-life (EOL) – Production is stopped, official support winds down or ends.
  4. Obsolete/legacy – Only secondary markets and third-party repair services remain.

Common signals you’re on legacy hardware include: discontinued series, no firmware/tool updates, shrinking technician familiarity, and scarce/expensive spares.

Why Do Plants Keep Running Them?

  • Reliability – Many legacy PLCs were engineered to survive harsh conditions for decades.
  • Cost of migration – Full modernization can run from hundreds of thousands to over a million dollars depending on I/O count, validation, and downtime.
  • Compatibility – New PLCs may not talk directly to older HMIs, drives, networks, or MES/SCADA without gateways.
  • Operator familiarity – Teams know the ladder logic, quirks, and maintenance steps.
  • Downtime avoidance – Taking a line down for migration can be costlier than squeezing extra life out of current gear.

The Risks of Holding On Too Long

  • Parts scarcity – Discontinued CPUs, power supplies, and I/O can take days or weeks to find.
  • Downtime exposure – If downtime costs $10,000/hr, a five-day wait for a rare CPU could mean ~$1.2M lost.
  • Rising costs – Scarcity pushes legacy module pricing 2–3× original list; certain repairs may exceed modern replacements.
  • Cybersecurity – Unpatched legacy controllers increase attack surface and recovery complexity.
  • Workforce gap – Fewer technicians are trained on S5/SLC programming environments and toolchains.


Scarce parts and fewer trained technicians increase repair time and risk.

How Long Can You Really Keep Running Yours?

There’s no single number; service life depends on environment, criticality, and spare availability.

  • Environment – Heat, dust, vibration, and humidity accelerate failure (e.g., stamping vs. clean assembly).
  • Criticality – A backup conveyor can tolerate risk; a primary bottleneck cannot.
  • Spares – If affordable hot spares are on the shelf, you can extend life with less risk.

Rule of thumb: you can safely run legacy PLCs until spare inventory becomes unpredictable or mean time between failures (MTBF) starts shrinking. Many plants push 10–20 years beyond EOL, but yearly risk—and cost of surprises—climbs.

Repair vs. Replace – A Budget-Friendly Framework

When Repair Makes Sense

  • Controls non-critical equipment or has redundancy.
  • Warranty-backed repair services are available.
  • Spares are reasonably priced and obtainable.
  • You need a bridge while capital for migration is approved.

Example: A packaging cell keeps an SLC 5/03 running with repaired power supplies and hot-spare I/O to extend life 3–5 years while planning a phased CompactLogix migration.

When Replacement Is Smarter

  • Repeated failures create unpredictable downtime.
  • Controls a mission-critical, high-OEE production line.
  • Repair costs approach/exceed replacement costs.
  • Broader modernization is already underway (safety, networking, data goals).

Example: A food & beverage plant replaces a GE 90-30 CPU after multiple batch losses; the upfront cost prevents recurring quality and downtime hits.

Practical Steps to Decide

1) Audit Your Installed Base

Document controller model, age, firmware, support status, role, enclosure conditions, and network interfaces. Note available spares and their condition (tested vs. unknown).

2) Rate Criticality

Score the operational impact of downtime (safety, quality, throughput, regulatory). Identify single points of failure.

3) Track Failures and MTBF

Log fault codes, repair actions, and time to restore. A shrinking MTBF is a red flag to escalate planning.

4) Build a Repair/Replace Matrix

Asset Criticality Spares On Hand Failure Trend Recommended Action
SLC 5/04 – Line 2 High CPU + PSU tested Stable (no trend) Repair + maintain hot spares (plan phased migration)
GE 90-30 – Batch Mix High No CPU spare MTBF declining Replace controller; prioritize migration budget
Siemens S5 – Utility Skid Medium Limited I/O spares Stable Repair until spare scarcity worsens; prepare adapter/gateway plan


5) Stage Spares and Gateways

For legacy networks (DH+, Remote I/O, PROFIBUS), stage gateways and cables for fast swaps. Validate backups for programs, comments, and I/O maps.

6) Set Triggers and Timelines

  • Define hard triggers (e.g., two critical failures in 12 months = initiate replacement).
  • Lock timelines to scheduled outages to minimize production impact.

How Industrial Automation Co. Helps

  • Extensive stock of legacy PLC parts across Allen-Bradley, Siemens, GE, Mitsubishi, and more.
  • Same-day emergency shipping to minimize downtime.
  • 2-year repair warranty for confidence in extending asset life.
  • Free technical support—even after the sale—for pragmatic troubleshooting.

Whether you’re repairing to stretch a few more years or replacing to modernize, we’ll help you balance risk, cost, and uptime.

Ready to talk through a specific line or controller?

Get Help With Legacy PLCs

Final Word

Legacy PLCs prove how durable industrial automation can be—but no controller lasts forever. The smartest strategy isn’t to gamble. It’s to plan: know your spares, track failures, and decide in advance whether you’ll repair or replace. That way, you control the timeline—not the equipment.



A simple installed-base audit turns reactive firefighting into proactive planning.
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