In short
A PLC is obsolete when the manufacturer stops making and supporting it. It keeps working, but a failure can stop the machine for weeks. Plan the replacement when new spares are no longer available, there is no copy of the program or the unit cannot communicate with current systems. The first step is always to save a copy of the program.
How do you know whether a PLC is obsolete?
Manufacturers publish the status of each product family. It usually goes through these phases:
| Phase | What it means | What to do |
|---|---|---|
| Active | Manufactured and sold as normal | Nothing special: keep copies of the program |
| Mature or phase-out | An end-of-sale date is announced | Buy critical spares and plan the migration |
| Discontinued | No longer made; only repair or second-hand | Migrate at the next planned stop |
| Unsupported | No spares and no official repair | High risk: any failure means a long stop |
What signs show it is unwise to wait?
- There is no copy of the program, or nobody knows whether the copy is the latest version.
- The program can only be opened with an old computer or a cable that is no longer sold.
- Spares are only available second-hand.
- The memory backup battery has never been changed.
- The person who knew the machine has left the company.
- Production data is needed and the unit has no communications.
What migration options are there?
The brand of the new PLC is best chosen according to what the plant already has: a single family of equipment simplifies spares and maintenance training.
| Option | What it involves | Advantage | Drawback |
|---|---|---|---|
| Same family, current model | Replace with the successor and convert the program | Short stop and wiring almost untouched | Automatic conversion needs review and testing |
| Reprogram from scratch | New PLC with a newly written program | Clean, documented program | More engineering and commissioning hours |
| In phases | Replace the panel or communications first, then the CPU | Spreads cost and risk | Old and new equipment coexist for a time |
What can be done today without changing anything?
- Read the program from the PLC and keep two copies, one off site.
- Take an inventory: model, cards, version and manufacturing status of each unit.
- Change the backup battery following the manufacturer’s procedure.
- Buy a spare of the critical parts while they exist.
- Keep the programming software and cables with the rest of the documentation.