SCADA and PLC modernisation: how to migrate an obsolete system using planned shutdowns

On this page

• How to tell whether your PLC or SCADA is obsolete
• Inventory before deciding: spare parts, know-how and cybersecurity
• PLC and SCADA migration strategies, and when each one fits
• What is reused from the installation and what is replaced
• Phased migration plan, with FAT and SAT
• What changes in an agri-food factory
• How we do it at ER Ingeniería
• Frequently asked questions
• Sources

How to tell whether your PLC or SCADA is obsolete

The lifecycle status of a PLC is published by the manufacturer for each part number. While the product is active, it is sold and still being developed. When it enters phase-out, the manufacturer announces it with a timetable: for a while it is still sold as new equipment, and then it moves into a phase in which it is only supplied as a spare part [1]. When that period ends, the equipment is discontinued and what remains is repair, sometimes by the manufacturer itself [2], and the second-hand market.

The check is made with the exact part number shown on the rating plate of the CPU and of each module, in the manufacturer's own lifecycle tool. Here are some examples, with the status each manufacturer publishes:

SCADA systems usually age from underneath: the application keeps working, but the Windows it runs on no longer receives patches, and it cannot always be moved to a current one without changing the SCADA version and licence. Something similar happens with the programming software for older PLCs: it often only runs on an old laptop, and if that laptop fails nobody can connect to the PLC.

Inventory before deciding: spare parts, know-how and cybersecurity

No migration should be decided without knowing what is there. The inventory we carry out before proposing anything records, for each line:

• Every CPU, input and output module, communications card, screen and drive, with its part number and lifecycle status.
• The program in each PLC: whether there is a backup copy, what date it is from and whether anyone has compared it with what is loaded in the CPU.
• The programming software with its licence, the signal list and the wiring diagrams, and whether they reflect what is actually wired.
• The communication networks (Profibus, MPI, DH+, serial links) and which devices hang off each one.
• The SCADA: version, licences, number of tags and where it stores historical data, alarms and recipes.
• Who knows the system. If the answer is a single person, that goes into the inventory as one more risk.

Spare parts

The spare-parts store runs down without anyone keeping track: a card is used, it is not replaced because it is no longer made, and the next breakdown finds the shelf empty. The typical scenario is an output card that fails in the middle of the season and whose second-hand replacement takes days to arrive. Counting how many units of each part number are installed and how many are held in reserve shows which line is most exposed.

Know-how

Programs from twenty years ago are rarely commented, and changes made on the fly did not always make it into the drawings. When the person who made them retires, the plant loses the only real documentation. That is why, before touching anything, we write down what each machine does today and go over it with production and maintenance: that document is the basis of the migration.

Cybersecurity

The NIST guide to control system security, revised in 2023, points out that these systems were born isolated and that many older devices have no password protection and use protocols without authentication [7]. That stops being harmless as soon as the SCADA is connected to the office network, to the ERP or to remote maintenance access. That same guide and the ISA/IEC 62443 series of standards [8] are the references for putting it in order: dividing the control network into zones and controlling where each communication comes in and who has access. A migration is the moment to get it sorted, because the network is going to be rebuilt anyway.

PLC and SCADA migration strategies, and when each one fits

The choice depends on how long the plant can be stopped, how many lines there are, whether spare parts are still available for the hardware and how well documented the program is. These are the five options we work with:

When the plant runs all year round, we usually propose a phased migration that starts with the SCADA in parallel, which is what least affects production and what most quickly resolves the security of the computer. The CPUs and the inputs and outputs are replaced afterwards, area by area, during the stoppages the plant already has. Whether the program is converted or rewritten is decided after we have reviewed the code. Siemens names the S7-1500 and ET 200MP I/O as the successors to the S7-300 and ET 200M, with hardware migration supported in its configuration tools [1], and Rockwell has its own migration programme for the PLC-5, with tools for automatic code conversion [2]. Whatever is converted is reviewed and tested in full, just like a new program.

What is reused from the installation and what is replaced

In a migration, what has become obsolete is almost never the field level.

Phased migration plan, with FAT and SAT

The IEC 62381 standard, revised in 2024, sets out requirements and checklists for the factory acceptance test (FAT), factory integration test (FIT), site acceptance test (SAT) and site integration test (SIT) of process automation systems [9]. These tests are what make it possible to reach the stoppage with the new system already tested. This is how we organise a migration:

1. Diagnosis and inventory. Everything in the previous section, with a map of which line carries the most risk and in what order it makes sense to act.
2. Functional specification. What each machine does today and what it should do afterwards, written down and validated with production, quality and maintenance.
3. Engineering and programming. Control and network architecture, PLC programs and SCADA screens.
4. FAT before going on site. The program and the SCADA are tested with the panels assembled or with simulated signals, without touching the installation.
5. On-site preparation without stopping. Panels and network are installed, the new cable is laid and the SCADA is installed in parallel while the plant keeps producing.
6. Changeover during the stoppage. Connection, signal-by-signal checks and a no-load start-up. A written plan for reverting to the old system if something does not respond is brought along; it works as long as the old system has not been dismantled.
7. SAT and commissioning with product. Validation with real production: recipes, alarms, interlocks and batch records.
8. Training and documentation. Commented programs, updated diagrams and backup copies handed over, plus training for operators and maintenance staff.

What shortens the stoppage most is step 4: a fault that shows up in the FAT is fixed with time to spare and with the plant producing; the same fault on changeover day is fixed with the line stopped. To put figures on that stoppage, see our technical document for calculating the real cost of a production stoppage.

What changes in an agri-food factory

The method is the same as in any industry, but in food production there are three things that shape the timetable and the scope.

Intervention windows

A winery does not touch its control system during the grape harvest, but it has quiet months afterwards. A feed mill usually runs all year round with short weekend stoppages, so it goes in phases. Seasonal industries have their window when the season ends. The plan is adjusted to that calendar.

Batch data

The old SCADA stores batch, recipe and consumption history that the quality department uses to respond to an audit or a product recall. Before the changeover, it is exported and kept accessible, and the new system has to record from the very first batch it produces. If that traceability is currently completed by hand, now is the time to fix it; we cover this in the article on traceability failures caused by manual data.

Symptoms that are not obsolescence

Intermittent dropouts between the SCADA and the PLCs, or frozen tags, do not always call for a new system: sometimes they come down to a cable or a connector. Before migrating, it is worth ruling that out with our article on PLC–SCADA communication failures.

How we do it at ER Ingeniería

We have automated 45 agri-food plants, including plants featured on our website such as those of Inalsa, Agrocantabria and Casaseca. In a migration we do the complete job: inventory, specification, electrical and control engineering, programming, FAT and SAT testing, installation, commissioning and ongoing support through our SatER technical service.

For the supervision and data layer we have SuitER, our industrial software, which integrates with the plant's PLCs and SCADA for production, energy and traceability control, with modules such as SuitER Tracer for batch traceability. What we do in the sector is on our agri-food industry page and on our winery and feed mill automation page.

Sources

1. Pirttijoki, T. (2023, August). The Simatic S7 product family is getting upgrades. Siemens Blog. https://blog.siemens.com/en/2023/08/simatic-s7-product-family-is-renewing-rapidly/
2. Rockwell Automation. (n.d.). PLC-5 Migration. https://www.rockwellautomation.com/en-us/capabilities/industrial-maintenance-support/modernization/plc-5-migration.html
3. Microsoft. (2016). Support for Windows XP, Office 2003 and Exchange 2003 ended. Microsoft Learn. https://learn.microsoft.com/en-us/lifecycle/announcements/windows-xp-office-exchange-2003-end-of-support
4. Microsoft. (n.d.). What does it mean if Windows isn't supported? Microsoft Support. https://support.microsoft.com/en-us/windows/deployment/updates-lifecycle/what-does-it-mean-if-windows-isn-t-supported
5. Microsoft. (n.d.). Windows 10 support has ended on October 14, 2025. Microsoft Support. https://support.microsoft.com/en-us/windows/deployment/updates-lifecycle/windows-10-support-has-ended-on-october-14-2025
6. Microsoft. (n.d.). Windows 10 IoT Enterprise LTSC 2021. Microsoft Lifecycle. https://learn.microsoft.com/en-us/lifecycle/products/windows-10-iot-enterprise-ltsc-2021
7. Stouffer, K., Pease, M., Tang, C., Zimmerman, T., Pillitteri, V., Lightman, S., Hahn, A., Saravia, S., Sherule, A., & Thompson, M. (2023). Guide to Operational Technology (OT) Security (NIST SP 800-82 Rev. 3). National Institute of Standards and Technology. https://csrc.nist.gov/pubs/sp/800/82/r3/final
8. International Society of Automation. (n.d.). ISA/IEC 62443 Series of Standards. https://www.isa.org/standards-and-publications/isa-standards/isa-iec-62443-series-of-standards
9. International Electrotechnical Commission. (2024). IEC 62381:2024 Automation systems in the process industry: Factory acceptance test (FAT), site acceptance test (SAT), and site integration test (SIT). https://webstore.iec.ch/en/publication/67572