IEC 61131 Parts Overview: Beyond 61131-3
Almost everyone who knows IEC 61131 knows only part 3, the programming languages, and assumes that is the whole standard. It is not. IEC 61131 is a multi-part series covering the PLC as a product, from its hardware and environmental requirements to communication and user guidelines. This page maps the parts so you know what each covers and when you would ever open one other than part 3. It is a series overview, not a tutorial on the languages themselves.
IEC 61131 parts overview in one line: IEC 61131 is a series about programmable controllers, not a single document. Part 1 gives general definitions, part 2 covers equipment requirements and hardware tests, part 3 defines the five programming languages everyone cites, part 4 offers user guidelines, part 5 covers communications, part 6 addresses functional safety, part 7 adds fuzzy control programming, and part 8 gives guidelines for applying and implementing the languages of part 3.
Why Part 3 Overshadows the Rest
Part 3, IEC 61131-3, defines the programming languages, and it earned its fame because it did something no other part did: it standardized how PLCs are programmed across vendors. The site's page on IEC 61131-3 covers those five languages in detail. Because programming is the daily work of most automation engineers, part 3 is the only part they ever cite, and the series name and the part become synonyms in casual speech.
That shorthand hides the fact that a PLC is a product with hardware, environmental limits, communication interfaces, and safety considerations, and the other parts exist precisely to cover those. You will rarely open them, but knowing they exist tells you where to look when a question is not about logic at all but about how the controller must behave as a piece of equipment.
What Each Part Covers
The table below maps the series. Treat it as a lookup for when a PLC question falls outside programming.
| Part | Covers |
|---|---|
| 61131-1 | General information, definitions, and functional characteristics |
| 61131-2 | Equipment requirements and tests, including environmental and electrical |
| 61131-3 | Programming languages: LD, FBD, ST, IL, and SFC |
| 61131-4 | User guidelines for selecting and applying PLCs |
| 61131-5 | Communications between PLCs and other systems |
| 61131-6 | Functional safety aspects of programmable controllers |
| 61131-7 | Fuzzy control programming |
| 61131-8 | Guidelines for the application and implementation of programming languages |
Part 2 is the one hardware and commissioning engineers most often need outside part 3, because it sets the equipment requirements and the environmental and electrical tests a controller must pass. Part 6 matters when a controller is used in a safety context, where it connects to the functional-safety world and the site's page on the safety PLC becomes relevant.
The languages of part 3 themselves, structured text, the function block diagram, and the sequential function chart, each have their own site pages if you want the language detail rather than the series map.
When You Would Actually Open Another Part
In practice, most engineers spend a career touching only part 3. You would open part 2 when specifying or qualifying controller hardware, particularly its immunity, temperature range, and test regime, because that is where the equipment requirements live. You would reach for part 6 when the controller sits inside a safety function and you need to understand what the standard expects of it as a safety-related device.
Part 5 becomes relevant when you are defining how controllers communicate as peers rather than through a SCADA layer, and part 8 is a useful reference when you are setting programming conventions for a team and want a standard basis for portability and good practice.
The pattern to remember is simple: part 3 is the logic, part 2 is the box, and the remaining parts fill in communication, safety, and guidance. Knowing which part answers a non-programming question is the whole value of the series map, and it keeps you from assuming the standard is silent on something it actually addresses in a part you never knew existed.
Frequently Asked Questions
Is IEC 61131 the same as IEC 61131-3?
No. IEC 61131-3 is one part of a larger series. Part 3 defines the PLC programming languages and is the part almost everyone cites, but IEC 61131 as a whole also covers general definitions, equipment and test requirements, user guidelines, communications, functional safety, fuzzy control, and application guidelines. Using the series name when you mean only the languages is common shorthand but technically imprecise.
Which IEC 61131 part covers PLC hardware requirements?
Part 2, IEC 61131-2, covers equipment requirements and tests: the electrical, mechanical, and environmental requirements a programmable controller must meet, along with the type tests that verify them. If your question is about a controller's temperature range, immunity, or physical qualification rather than its logic, part 2 is the document to consult, not part 3.
Sources and verification
This page references the vendor products and their official documentation published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
- Rockwell Automation Literature Library (Allen-Bradley, Studio 5000) - Rockwell Automation
- Siemens SIMATIC and TIA Portal documentation - Siemens
Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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