HomeTechnical cornerTechnical cornerReplacing an end-of-life PLC: the 5 steps of a successful migration

Replacing an end-of-life PLC: the 5 steps of a successful migration

  • 14 July 2026
  • Posted by: Yann Chapron
  • Category: Technical corner
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Technical corner · Automation · 6 min read

Spare parts no longer available, software that only runs on an old PC, nobody who really knows the program: many plants live with end-of-life PLCs. As long as the machine runs, the decision is postponed. Until the day a card fails and the stop lasts days instead of hours.

Replacing a PLC is not just a hardware swap. Well prepared, it is also an opportunity to make the machine more reliable and easier to troubleshoot. Here are the 5 steps of a successful migration.

PLC migration: the 5 steps1Initial surveyexisting program, diagrams, I/O, links with other machines2Functional analysisdescribe what the machine must do, with production and maintenance3Hardware choice and preparationPLC, HMI, I/O, updated electrical diagram, office testing4Planned changeovershort stop, pre-wired panel, communication with neighbouring machines5Commissioning and documentationtests, tuning, training, status manual, as-built diagrams

1. Survey the existing installation

Gather everything that exists: program backup, electrical diagrams (often out of date), list of inputs and outputs, links with neighbouring machines. This is also the time to note known problems and the teams’ requests.

2. Write the functional analysis

This is the most often neglected step, and the most profitable. It describes what the machine must do: sequences, operating modes, safety functions, faults. Validated with production and maintenance, it becomes the reference for programming and testing.

3. Choose the hardware and prepare

The choice depends on the size of the machine and the existing installed base: for example a Siemens S7-1200 G2 for a compact machine, an S7-1500 for a more complex line. The new electrical diagram and the operator panel are prepared, and the program is tested in the office, in simulation, before going on site.

Convert the old program or rewrite it?CriterionConversionRewriteEngineering timeshorterlongerOriginal flawskeptfixedReadabilityold structureclear, documentedDiagnosisunchangedstatuses and faults on screenSuited if…same brand, clean codeother brand, poorly known code

Automatically converting the old program looks faster. But it keeps the original flaws and a structure that is hard to read. Starting again from the functional analysis gives a clear, documented program with a real diagnostic screen.

4. Plan the changeover

Everything that can be prepared in advance should be: cabinet or panel pre-wired, terminals labelled, communication with neighbouring machines tested. The on-site changeover then becomes a short, controlled stop rather than an adventure.

5. Commission and document

Function-by-function tests, tuning, training of operators and technicians, then handover of up-to-date documentation: diagrams, status manual, program backup. This is what will allow fast troubleshooting in the years that follow.

A real example

On the Biko unwinder at Elis Belgium, ITS replaced the B&R PLC with a Siemens S7-1200 G2, rewrote the program from a functional analysis, installed a new diagnostic panel and delivered complete EPLAN electrical diagrams. Commissioning: 3 days.

To go further on diagnosis, read also: Machine breakdowns: cut diagnostic time.

An end-of-life PLC on one of your machines? Let’s talk before it breaks down.