Who this is for: production and maintenance managers planning changes to existing industrial equipment.
A retrofit changes an existing machine to recover functionality, address obsolescence or introduce new capabilities. It does not necessarily mean replacing the PLC. The limitation may concern a drive, an operator interface, access to data or a mechanical component. Start by defining the result you need and how it can be demonstrated.
Before requesting quotations, describe the current limitation: stoppages associated with a component, unavailable spare parts, manual adjustments or missing information. Connecting the limitation to an observable production effect helps prevent a technology upgrade that leaves the underlying problem untouched.
Establish the machine’s actual configuration
Collect component models and versions, current electrical drawings, program copies, licences, manuals and records of previous modifications. An available file is not automatically a usable backup: establish which configuration it belongs to and who can restore it. Record missing information as well as available documents.
Observe a complete cycle with the machine operators. Important dependencies may sit outside the control cabinet: material supply, tooling, production software or operator procedures. The initial assessment should identify these connections and the people responsible for them.
- Critical components, spare parts and interface compatibility.
- Source software, company-controlled credentials and usage rights.
- Connections to other equipment and conditions required for restart.
Compare integration, retrofit and replacement
Compare options against the same requirements. An external sensor might solve a monitoring problem without correcting unstable machine control. A new operator interface may improve usability without increasing mechanical capacity. State which limitations each option would leave in place.
Scroll horizontally to see every column.
| Option | When to investigate | Questions to resolve |
|---|---|---|
| Add measurement | Useful data is missing but the cycle is suitable | Signal quality and access to measurement points |
| Control retrofit | Controls limit reliability or functionality | Compatibility, source files and recovery route |
| Replacement | The limitation concerns capacity or structure | Installation, tooling and transition costs |
References: Siemens — Migration for Automation Systems
Plan downtime and the return to production
The intervention cost includes preparation, installation, testing, training and restart support. Ask which tests can be completed before shutdown and which require the real machine. Agree a window that works for production, maintenance and the suppliers involved.
Define a decision point beyond which work cannot continue without reassessment. A recovery plan should specify the conditions, resources and estimated time needed to return to a usable configuration. Competent personnel must plan modifications and testing around safety and operational continuity; migration alone does not establish machine compliance.
References: NIST — Guide to Operational Technology Security
Agree acceptance criteria before placing the order
Turn the objective into agreed tests. For monitoring, compare a captured signal with a reference measurement. For recipes, test selection, permissions and recovery after an interruption. For control changes, identify the cycles and operating conditions that need verification.
The quotation should distinguish supplier deliverables from company responsibilities: software versions, revised drawings, training, spare parts, documentation and defect handling. Explicit exclusions make competing proposals more comparable than a total price alone.
Prepare for an initial assessment
- Describe the production problem and intended result with observable examples.
- List components, versions and documents that are actually available.
- Identify shutdown constraints, shifts and the people needed for restart.
- Record alternatives and technical limitations that may remain.
- Agree acceptance tests, responsibilities and recovery arrangements.
Common questions
Does a retrofit always make retaining the machine worthwhile?
No. Compare remaining mechanical limitations, expected usable life, support, downtime and transition costs. If the principal constraint survives, a limited modification may postpone the problem rather than solve it.
Can the entire scope be agreed without a site visit?
Documents support an initial assessment. Real interfaces, undocumented changes and operating practices may require checks on the equipment before the scope, timing and delivery commitments can be defined.
Sources and further reading
Apply this to your project.
Describe the machine, the limitation you want to overcome and the documentation available. These details provide a starting point for discussing the automation scope and investment.
Discuss your automation project