How to Write Documentation for SCADA, DCS, and PLC

Published: September 7, 2026

• What Are SCADA, DCS, and PLC?
• Key Differences
• Standards and Normative Framework
• Document Inventory: What You Need for Each System
• Document Structure: What to Write in Each Section
• Hidden Complexities
• Practical Checklist

Are there any differences or difficulties iun writing user guides for SCADA, DCS, and PLC? Well, there are. This post is about such pecularities.

A technical writer gets handed a specification for a SCADA documentation package. Forty-seven deliverables. Two weeks to deliver. The engineers go quiet — radio silence. The client keeps pushing on "compliance with industry standards." And the standards? ISA-101, IEC 61131-3, IEC 62443. Hundreds of pages, a lot of very abstract things. You're supposed to just... absorb it somehow. Good luck with that. And here's the fun part — nobody tells you which documents you actually need. Which ones you can shove into one volume. Or what's even supposed to be inside them. Just figure it out, right?

In this guide we'll try to answer those questions. We are going to consider the three parts of industrial automation — SCADA, DCS, and PLC — and describe a practical framework for documentation that operators, engineers, and maintenance crews will actually bother to open.

So What Exactly Are SCADA, DCS, and PLC?

These three systems do rather different jobs.

PLC (Programmable Logic Controller) — okay, picture a rugged little computer bolted inside a control cabinet. That's basically it. It sits right next to the machinery and runs the show at the device level. Feed it a logic program — ladder, function blocks, structured text, whatever — and it'll read your sensors, open your valves, spin your motors. Everything wired to it, it handles.

SCADA (Supervisory Control and Data Acquisition) — now we're talking about stuff spread all over the map. Pipelines. Pump stations. Substations. You can't stand next to all of it. So SCADA grabs data from remote PLCs and RTUs over industrial networks, throws it onto HMIs for the operators, and lets you run the whole thing from one room. One screen. That's the point.

DCS (Distributed Control System) — different beast. Here everything lives inside one plant. Chemical, refining, power gen. And the process never stops — it's continuous, it's fussy, it needs precision. So a DCS ties controllers, HMIs, and historians together into one tight package. Not scattered. Not remote.

Key distinction: PLCs control individual machines. SCADA supervises remote assets. DCS controls entire continuous processes. In practice, a SCADA system may include PLCs, and a DCS may include SCADA-like supervision.

Key Differences: SCADA vs. DCS vs. PLC

Standards and the Normative Mess Around Them

Documentation in industrial automation is tangled up in a rather dense web of standards.

IEC 61131-3 — The PLC Programming Languages

This one's the foundation, really. Everything else in PLC programming kind of leans on it. Five languages live in here. Ladder Diagram — LD. Function Block Diagram, FBD. Structured Text, ST. Instruction List, IL. And Sequential Function Chart, SFC. Yeah, five, I know, it's a lot to keep straight.

Now, documenting a PLC program? You can't just make it up as you go. There's this thing called IEC 61131‑3 documentation requirements, and it actually has opinions. Variable names that mean something. Comments with some structure to them. A program layout a human can follow. And a tag naming convention — one you sat down and thought about, not something you slapped together at 2 a.m. the night before a FAT.

ISA-101 — HMI Design and Documentation

ANSI/ISA-101.01-2015. Bit of a mouthful, I know. What it gives you is a full framework for designing and running HMIs in process automation. And not just designing — the whole lifecycle. From the first sketch all the way to decommissioning. For a technical writer, the ISA‑101 HMI documentation standard spells out how to document screen hierarchies, color schemes, alarm presentations, navigation, operator interaction sequences. The works.

IEC 62443 — Cybersecurity for Industrial Automation and Control Systems (IACS)

It's the only consensus-based industrial cybersecurity standard out there. That's not nothing. What does it want documented? Asset inventories. Zone and conduit models. Risk assessments. Security design specs. And if your SCADA or DCS project touches critical infrastructure? You're dealing with IEC 62443 documentation. No way around it.

ISA-88 and ISA-106 — Batch and Continuous Control

Two standards, two different worlds. ISA-88 handles batch control systems. ISA-106? That's procedural automation for continuous processes. They lay out how documentation for control recipes, phases, and state-based control should look.

NAMUR NE 22 — Manufacturer Documentation for Process Control Systems

The NAMUR NE 22 document list provides a breakdown of technical documents that DCS manufacturers should supply – covering engineering, operating, and maintenance instructions – and serves as a useful checklist for technical writers planning their deliverable set.

IEC 62682 — Alarm Management

This standard defines requirements for alarm system design, implementation, and documentation.

Document Inventory: What You Need for Each System

The exact document set depends on the project scope, client requirements, and applicable regulations. However, the following documents appear in nearly every project.

For PLC Systems

PLC work generally spawns four documents, though the exact count wobbles from project to project. First off, you've got PLC Program Documentation — it covers program structure, tag naming conventions, and the logic flow, plus the exported code with its comments and cross-references bundled in. Then there's the I/O List: every input and output mapped out, complete with tag names, descriptions, signal types, and scaling. Hardware Configuration is exactly what it sounds like — model, firmware, modules, power supply, network interfaces, the whole kit. And rounding things out, a Maintenance and Troubleshooting Guide that spells out diagnostics, fault finding, and how to swap a dead component without losing your mind.

For SCADA Systems

SCADA jobs tend to want an Operator Manual, and this one's pedantic by nature — each HMI screen, button function, data display, alarm handling. Screen-by-screen, screenshots mandatory, no shortcuts. A System Architecture Document then handles the plumbing: network topology, server configuration, communication protocols, data flow. The Alarm Management Document is where the alarm philosophy, prioritization, and operator response procedures live, and it's arguably the linchpin of your alarm management documentation — IEC 62682 alignment isn't optional here. Backup and Recovery Procedures round things off, explaining how to back up the SCADA server config and how to resurrect it after something goes sideways.

For DCS Systems

DCS documentation is a heavier beast altogether. A Control Philosophy opens the proceedings with a high-level account of how the process is governed — control loops, interlocks, safety systems, that sort of thing. The Functional Design Specification (FDS) gets granular, laying out the detailed functional requirements of the control system. Cause and Effect Diagrams come as a matrix, showing how alarms and trips trigger specific actions. Loop Diagrams trace the wiring and signal flow for each control loop. And I/O Assignment Tables map every field device back to its DCS I/O module.

Document Structure: What to Write in Each Section

Operator Manual (SCADA / DCS)

This is the one operators actually crack open every shift. So it has to be clear. Actionable. Screen-by-screen, no hand-waving.

Starts off with an intro. What the system is for. What it covers. A rough sketch, nothing fancy. After that — the screens. All of them. And for each screen you say what it's for, what the buttons do, what data pops up, which alarm lights sit there, how you get in and out. Then the procedures. Startup, running normally, switching modes, shutting down. Emergency shutdown too. Step by step, don't skip anything. Alarms get their own section. Each one listed. What it means. What the operator does about it. After that, troubleshooting. Operator mistakes, error messages, when it's time to call someone higher up. Last part is system stuff. Architecture, network layout, credentials, backup and restore notes.

Imperative mood is usuall used — "Click," "Select," "Verify." One action, one step. Screenshots aren't optional. [IMAGE: Example SCADA HMI screen with annotations | alt="SCADA HMI screen with labeled buttons and indicators"] Alarms go in tables — alarm, meaning, operator action. And please, steer clear of mush like "In the event of an alarm, the operator should take appropriate action." That tells nobody anything. You'd better write: "When the red alarm indicator flashes, press 'Acknowledge,' then check the process value."

PLC Program Documentation

Different audience here. This one's for maintenance engineers and programmers, and it has to be technical enough that somebody else can pick up the code and actually modify it without a séance.

A program overview may contain a high-level account of what the PLC controls, its modes of operation, the major functions. Then different tag naming conventions — how tags are named, what the prefixes mean, where the tag database lives. Routine descriptions walk through each routine or function block: purpose, inputs, outputs, the logic that matters. I/O mapping is a full table of every input and output with tag names, descriptions, physical addresses. Comments in code — and I mean every rung or block should carry one explaining why it's there. Version history records the changes of dates, authors, reasons.

On the writing side: stick to consistent naming, and for the love of it, avoid cryptic tags like "M123" or "B17." Comment with purpose — explain why, not just what. And keep your PDFs and tag lists under version control, otherwise you'll be chasing ghosts later.

DCS Functional Design Specification (FDS)

The FDS translates process requirements into control system requirements.

It begins with the control philosophy: how the process is controlled, including control strategies, setpoints, and constraints. Control loop descriptions then cover each loop — measured variable, setpoint, controller type, and actuator. Interlock and trip logic explains when interlocks and trips are triggered and what actions result. Alarm and event management covers alarm philosophy, prioritization, and handling. Cause and effect diagrams close it out with a matrix showing the relationship between process conditions and control actions.

Hidden Complexities: What the Standards Don't Tell You

Here's what you'll face in the real world that the standards gloss over.

Engineers Don't Document Their Work

Industrial automation engineers are focused on making the system work. Documentation is often an afterthought. What to do:

• Ask specific, answerable questions: "What is the tag name for the flow transmitter on line 101?" not "Can you explain the whole system?"
• Document as you go. Don't wait for the end of the project.
• Use the specification and design documents — they contain 70% of the answers.

Documentation Becomes Obsolete Fast

PLC code changes. HMI screens are updated. Field devices are replaced. Documentation has to keep pace. Store it in version control, the same way you would treat code. Keep a change log in every document. And synchronize document versions with software versions.

Terminology Chaos

The same object may be called "valve," "actuator," or "control element" in different documents. Create a glossary at the start of the project and enforce its use.

Regulatory and Compliance Documentation

For projects subject to IEC 62443, you'll need additional documentation: asset inventories, zone and conduit models, risk assessments, and security design specifications. Start this early, because it takes time.

The Three-Tier Documentation Model

Many automation engineers use a three-tier model that technical writers should understand. Tier 1 is the narrative log — a chronological record of decisions, rationale, and dead ends, the thing engineers skip and later regret skipping. Tier 2 is the formal deliverables: issued-for-review drawings, datasheets, test records, the official documentation. Tier 3 is the working and source files: PLC code, HMI projects, CAD files, the actual engineering artifacts. Your job as a technical writer is to extract Tier 1 and Tier 3 information and transform it into Tier 2 deliverables.

Practical Checklist for Technical Writers

Before you start, review the project specification and design documents. Identify which standards apply — IEC 61131-3, ISA-101, IEC 62443, and so on. Agree on the document list with the client and engineers. Create a glossary of terms. Establish a document numbering and versioning scheme.

During the work, write for a specific audience: operator, programmer, or administrator. Use active voice and imperative mood for procedures, and give each action its own step. Include screenshots and diagrams — they are not optional. Warn about risks and consequences. Maintain a change log.

Before delivery, verify that the document version matches the system version. Check all cross-references, figure numbers, and table numbers. Make sure each document is signed and approved if required. Deliver both PDFs and source files — Word, InDesign, or whatever specialized documentation tool you use.

Interactive Checklist: Is Your Documentation Ready?

Conclusion

Documentation for SCADA, DCS, and PLC systems is not a bureaucratic formality. It is a critical deliverable that affects operator safety, system reliability, and maintenance efficiency.

The technical writer in this domain is a translator — between engineers who build the system, operators who run it, and regulators who audit it. The quality of the operator manual determines whether an operator can respond correctly to an alarm. The quality of the PLC documentation determines how long it takes to troubleshoot a fault.

Standards provide the framework, but they don't provide all the answers. The real structure of documentation, the real content of each document, comes from understanding the specific needs of the project, the client, and the end users.

Good documentation is not about formatting and numbering. It is about making sure that the person holding the document can perform the right action at the right moment.

Start small. Take one document — say, the operator manual — and rewrite it so that someone unfamiliar with the project can understand it. Show it to an operator. Ask if it makes sense. Fix it. That is the right path.

Documentation for industrial automation is not a static set of files. It is a living system that evolves with the system itself. The technical writer's job is not just to create this system, but to ensure it remains accurate and useful throughout the entire lifecycle.