IEC 61131-3

Key takeaways

IEC 61131-3 belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect IEC 61131-3 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of IEC 61131-3 from memory before moving on to harder problems.

Reference excerpt

IEC 61131-3 is the third part (of 10) of the international standard IEC 61131 for programmable logic controllers. It was first published in December 1993 by the IEC; the current (fourth) edition was published in May 2025. Part 3 of IEC 61131 deals with basic software architecture and programming languages of the control program within PLC. The current edition defines three graphical and one textual programming language standards:

Ladder diagram (LD), graphical Function block diagram (FBD), graphical Structured text (ST), textual Sequential function chart (SFC), has elements to organize programs for sequential and parallel control processing, graphical. Previous editions also included:

Instruction list (IL), textual. Deprecated in the third edition (2013) and removed in the fourth edition (2025).

Data types

Elementary Data Types (basic) Bit Strings – groups of on/off values BOOL - 1 bit (0,1) BYTE – 8 bit (1 byte) WORD – 16 bit (2 byte) DWORD – 32 bit (4 byte) LWORD – 64 bit (8 byte) INTEGER – whole numbers (Considering byte size 8 bits) SINT – signed short integer (1 byte) INT – signed integer (2 byte) DINT – signed double integer (4 byte) LINT – signed long integer (8 byte) USINT – Unsigned short integer (1 byte) UINT – Unsigned integer (2 byte) UDINT – Unsigned double integer (4 byte) ULINT – Unsigned long integer (8 byte) REAL – floating point IEC 60559 (same as IEEE 754-2008) REAL – (4 byte) LREAL – (8 byte)

Elementary Data Types (Date and Time)

Duration TIME – (implementer specific). Literals in the form of T#5m90s15ms LTIME – (8 byte). Literals extend to nanoseconds in the form of T#5m90s15ms542us15ns Date DATE – calendar date (implementer specific) LDATE – calendar date (8 byte, nanoseconds since 1970-01-01, restricted to multiple of one day) Time of day TIME_OF_DAY / TOD – clock time (implementer specific) LTIME_OF_DAY / LTOD – clock time (8 byte) Date and time of Day DATE_AND_TIME / DT – time and date (implementer specific) LDATE_AND_TIME / LDT – time and date (8 byte, nanoseconds since 1970-01-01)

Elementary Data Types (Character / Character string)

Existing CHAR – Single-byte character (1 byte, limited to characters 0 to 255 of ISO/IEC 10646) WCHAR – Double-byte character (2 byte, limited to characters 0 to 65535 of ISO/IEC 10646) STRING – Variable-length single-byte character string. Literals specified with single quote, 'This is a STRING Literal' WSTRING – Variable-length double-byte character string. Literals specified with a double quote, "This is a WSTRING Literal" New in version 4 of the standard UCHAR – Single character encoded according to UTF-8 USTRING - Variable-length character string in which each character is encoded according to UTF-8

Generic Data Types Only available for the input / output/ in-out variables of system-defined Program Organization Units (POUs, see below)

ANY ANY_DERIVED ANY_ELEMENTARY ANY_MAGNITUDE ANY_NUM ANY_REAL: LREAL, REAL ANY_INT ANY_UNSIGNED: ULINT, UDINT, UINT, USINT ANY_SIGNED: LINT, DINT, INT, SINT ANY_DURATION: TIME, LTIME ANY_BIT: LWORD, DWORD, WORD, BYTE, BOOL ANY_CHARS ANY_STRING: STRING, WSTRING ANY_CHAR: CHAR, WCHAR ANY_DATE: DATE_AND_TIME (DT), DATE_AND_TIME(LDT), DATE, TIME_OF_DAY (TOD), LTIME_OF_DAY(LTOD)

User-defined Data Types Enumerated data type Enumerated data type with named value Subrange data type – puts limits on value i.e., INT(4 .. 20) for current Array data type – multiple values stored in the same variable. Structured data type – composite of several variables and types. Directly derived data type – type derived from one of the above types to give new name and initial value as a type. References – a kind of strongly typed pointer. Arithmetic operation of the value of this type is prohibited.

Variables Variable attributes: RETAIN, CONSTANT, AT

Global Direct (local) I/O Mapping – Input, Output, I/O External Temporary

Configuration Resource – Like a CPU Tasks – Can be multiple per CPU. Programs – Can be executed once, on a timer, on an event.

Program organization unit (POU) Functions Standard: ADD, SQRT, SIN, COS, GT, MIN, MAX, AND, OR, etc. Custom Function Blocks Standard: Custom – Libraries of functions can be supplied by a vendor or third party. Programs

Configuration, resources, tasks Configuration – processing resources, memory for IO, execution rates, number of tasks.

Object oriented programming (OOP) The 3rd revision of the standard describes how to implement OOP within the application programming

References

External links PLCopen [1] IEC 61131-3:2025 Programmable controllers – Part 3: Programming languages CODESYS – an implementation independent of device manufacturers, includes object oriented programming (OOP) as an option de:STEP 7 implementation for Siemens devices, only

Worked examples

Example 1 — a first encounter with IEC 61131-3

Start with the simplest possible case. Write down what IEC 61131-3 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to IEC 61131-3 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about IEC 61131-3 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of IEC 61131-3

In research: IEC 61131-3 appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses IEC 61131-3 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: IEC 61131-3 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electronic design automation, IEC 61131, IEC 61131-3, so understanding it makes those chapters shorter.
In everyday life: Look for IEC 61131-3 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study IEC 61131-3 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what IEC 61131-3 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain IEC 61131-3 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is IEC 61131-3 in simple terms?

IEC 61131-3 is the third part (of 10) of the international standard IEC 61131 for programmable logic controllers. It was first published in December 1993 by the IEC; the current (fourth) edition was published in May 2025.

Why does IEC 61131-3 matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study IEC 61131-3?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on IEC 61131-3.