How to document a legacy PLC or SCADA system with AI

Updated: 24 October 2026 · SIM Solutions

Short answer

Back up the PLC and SCADA projects, let an AI read a copy through read-only connectors, and have it produce a tag list, an alarm list, a description of every PLC block and a cross-reference of where each tag is used. An engineer who knows the plant checks the output against the running process. The result is a documentation set you can use for maintenance, a migration or a NIS2 asset inventory.

Why plants end up undocumented

Projects change for years after commissioning: quick fixes during a stop, contractors who leave, comments in another language or none at all. The result is a system that works but that only one or two people can change with confidence. That is a risk for maintenance, for cybersecurity and for any future migration, such as the move away from S7-300 or a WinCC to Ignition migration.

The method, step by step

Back up first. Take a fresh backup of every PLC and SCADA project, and record which version runs where. The AI only ever works on copies.
Scan read-only. Connect the AI to the copies through read-only tools. For Siemens that means TIA Portal (or a migrated STEP 7 project) and the WinCC V7 project files; see our AI toolkit.
Generate the lists. Tag list with addresses and data types, alarm list with texts and classes, archive list, screen list, and a list of connections to other systems.
Describe the logic. A plain-language description of every block: what it controls, its inputs and outputs, interlocks and modes. Ask the AI to flag what it is unsure about rather than guess.
Cross-reference. Where each tag is read and written, from PLC block to SCADA screen and alarm. This is the part that saves the most time later.
Review on site. An engineer who knows the plant checks the descriptions against the real process and corrects them. Unclear points become questions for operators.
Keep it alive. Store the documentation with the project backups and update it with every change. Version control for PLC and SCADA projects helps here.

What you get

Document	Used for
Tag and I/O list	Maintenance, loop checks, migration mapping
Alarm list	Alarm rationalisation, operator training
Block descriptions	Troubleshooting, changes, onboarding new engineers
Cross-reference	Impact analysis before any change
System overview (controllers, networks, versions)	NIS2 asset inventory, backup plan, lifecycle planning

Where AI helps, and where it does not

Good: reading thousands of lines of code and screens quickly, consistent lists, first-draft descriptions, finding unused tags and broken references.
Needs a person: the purpose behind a strange interlock, process knowledge, and anything to do with safety functions.
Never: changing the running system as part of documentation. Documentation is read-only work.

This is also the first step of every migration we do. See legacy PLC and SCADA migration.

Questions we get

Does the AI need access to the running PLC?

No. It works on project backups. Online values can be added later through a read-only connection if they help explain the logic.

Can AI document STEP 7 Classic projects?

Yes, once the project is opened or migrated in TIA Portal, or from exported sources. The descriptions are a first draft for an engineer to check.

Does this help with NIS2?

Yes. NIS2 asks for an inventory of assets and backups; a documented system with known versions is the base for both. See NIS2 and OT in Belgium.