IEC 61131-3

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. It defines three graphical and two textual programming language standards:

• Ladder diagram (LD), graphical
• Function block diagram (FBD), graphical
• Structured text (ST), textual
• Instruction list (IL), textual deprecated. Per IEC 61131-3-2025, chapter 7.2 Instruction List (IL) is no longer included in Edition 4. Thus, IL (AWL) is no longer part of IEC 61131-3.
• Sequential function chart (SFC), has elements to organize programs for sequential and parallel control processing, graphical.

Data types

• Elementary Data Type
  • 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)

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)
  • Character / Character string
    • 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"

STRING escape sequences

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

• 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 61131-3:2013". International Electrotechnical Commission (IEC). Retrieved 11 September 2021.
3. "Time Duration Literals (IEC 61131-3)".

External links

• 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