IEC61131 Common Elements

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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[1] by the IEC; the current (fourth) edition was published in May 2025.[2]

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:

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 literals
Unit Description
d Day
h Hour
m Minute
s Second
ms Millisecond
us Microsecond
ns Nanosecond
  • Duration [3]
    • 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)

STRING escape sequences
Escape sequence Produces
$$ $
$' '
$L linefeed
$N newline
$P page (form feed)
$R return
$T tab
$xx hex value
  • 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

  1. Stevic, Tom (5 May 2017). "A very short history of PLC programming platforms". Control Design. Retrieved 11 September 2021.
  2. "IEC 61131-3:2025". International Electrotechnical Commission (IEC). Retrieved 21 February 2026.
  3. "Time Duration Literals (IEC 61131-3)".
  • PLCopen
  • 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