Many plants still run on programmable logic controllers that were installed decades ago. They often work perfectly, yet the day a CPU or an I/O card fails, the question comes up fast: can we get the same part, or do we have to migrate the whole system? This article gives maintenance teams and engineers a practical way to decide, and a plan to keep a legacy PLC running safely while a migration is prepared.
Understanding product lifecycle statuses
Most automation manufacturers publish a lifecycle status for each product. The exact names differ from one brand to another, but they usually follow the same general stages:
- Active: the product is fully sold and supported, and it is the recommended choice for new designs.
- Mature (or similar wording): the product is still sold and supported, but a successor exists and the manufacturer encourages new installations to use it.
- End of sale / phase-out: the product is no longer sold as a complete item, or only for a limited period; repair services and some spare parts may continue for a time.
- Discontinued: no more sales and, after a certain date, little or no factory support or repair.
Families such as Allen-Bradley SLC 500 and PLC-5, Siemens S5 and S7-300, Omron C-series or Moeller PS4 are frequently found in older machines. We do not state the official status of any of them here: statuses and dates change, so always check the manufacturer's own lifecycle page for the exact catalog number you use.
Typical lifecycle phases, without dates.
Assess the risk before you decide
A legacy PLC is not a problem in itself. The real question is how long the line can be stopped if a component fails. Score each controller on a few points:
- Criticality: what does one hour of downtime cost on this machine or process?
- Spare availability: do you have a spare CPU, power supply and each type of I/O card on the shelf, and can you still find more on the used market?
- Knowledge: is there someone on site, or a contractor, who can still troubleshoot and modify the program?
- Software and hardware tools: do you still have a licensed copy of the programming software, a computer that can run it, and the right programming cable or adapter?
- Documentation: are the program, the wiring diagrams and the I/O list current and stored in more than one place?
A controller that is critical, has no spare and no working programming setup is a high risk, even if it has never failed. A controller on a non-critical machine, with spares and a documented program, can often keep running for years.
Keep spares for the parts that stop the machine
When a legacy system stays in service, the simplest insurance is a small kit of spares matching what is installed. Prioritize:
- CPU / processor module, with the same catalog number and, where it matters, the same series or revision.
- Power supply for the rack or chassis; power supplies are a common failure point on older equipment.
- One of each I/O card type (digital input, digital output, analog, special modules such as counters or communication cards).
- Memory modules, batteries and communication adapters required by the CPU.
- Rack or chassis and terminal blocks, if they are specific to the family.
Write the series, revision and firmware level of every installed module on a list, and compare it with any spare you buy. Two modules with the same base catalog number can differ in series or firmware, and some CPUs accept only certain firmware versions or memory options. Check the manufacturer's documentation before you put a spare into service.
Back up programs and firmware information
A spare CPU is of little use if you cannot load the right program into it. Make backups a routine task:
- Upload the program from the running controller, including documentation (tag names, comments) if your software keeps it in a separate file.
- Record the firmware version of the CPU and of any intelligent module.
- Save the configuration of communication modules, drives and operator panels connected to the PLC.
- Store copies on at least two media and in two locations, with the date and the reason for each change.
- Compare the backup with the running program after every modification, so the archived copy is always the latest one.
Keep the old programming laptop working as well. Some legacy software needs an older operating system or a serial port, and some cables use adapters that are harder to find than the PLC itself. Note which software version, which cable and which driver settings connect to each controller.
Keep it running or migrate?
There is no single answer. Use the table below as a starting point.
| Situation | Usual direction |
|---|---|
| Machine is stable, spares are available, program is documented | Keep running; maintain the spare kit and backups |
| Machine will be replaced or retired within a few years | Keep running with spares rather than invest in a migration |
| Frequent changes to the process are needed | Plan a migration; old tools slow down every modification |
| No working programming setup or no one who knows the platform | Fix the tooling first, then plan a migration |
| Spares are getting hard to find and the machine is critical | Buy spares now to cover the transition, and migrate in phases |
For guidance only; every installation is different.
Plan a phased migration
Replacing an entire control system in one shutdown is possible, but it puts a lot of risk on a single weekend. A phased approach is often safer:
- Document first. Build a complete I/O list, network diagram and functional description of what the program actually does.
- Replace the network or HMI layer where it makes sense, using gateways that let old and current devices talk to each other during the transition.
- Convert the program with the manufacturer's conversion tools if they exist, then review it line by line; automatic conversion rarely gives a finished result.
- Reuse field wiring where possible. Some manufacturers offer wiring conversion kits that let the current I/O terminal wiring connect to a current-generation I/O system.
- Cut over one area at a time, with a rollback plan: keep the old CPU, cards and program ready to reinstall if the start-up goes wrong.
During the whole migration, the legacy system still needs spares. Keeping a few used modules on hand is what lets you take your time and do the migration properly instead of in an emergency.
Work on electrical equipment must be done by qualified personnel following the applicable electrical code and the manufacturer's instructions.
A phased approach to PLC migration.
Key takeaways
- Lifecycle names vary by brand; check the manufacturer's lifecycle page for your exact catalog number.
- Rank controllers by criticality, spares, knowledge, tools and documentation.
- Keep a spare CPU, power supply and one of each I/O card, and record series and firmware levels.
- Back up programs and configurations regularly, and keep a working programming setup.
- Migrate in phases with a rollback plan, and keep the old system supplied with spares until the end.
Related parts at 3JIndustry
We carry used legacy controllers, I/O cards and power supplies in our PLC collection, including Allen-Bradley / Rockwell and Siemens parts; all parts are used and come with a 30-day warranty. See also our article on Allen-Bradley, Moeller, Siemens and Omron automation parts.
Looking for a discontinued part? Send the full part number to sales@3jindustry.com or use our contact page.
