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IEC 61131-3is the third part (of 10) of theinternational standardIEC 61131 forprogrammable logic controllers. It was first published in December 1993^[1] by theIEC; the current (fourth) edition was published in May 2025.^[2]

Part 3 ofIEC 61131 deals with basic software architecture andprogramming languages of the control program withinPLC. 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), textualdeprecated. 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 andparallel control processing, graphical.

• 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 asIEEE 754-2008)
  • REAL – (4 byte)
  • LREAL – (8 byte)

• 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)

• 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

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)

• 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 samevariable.
• 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.

Variable attributes: RETAIN, CONSTANT, AT

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

• Resource – Like aCPU
• Tasks – Can be multiple perCPU.
• Programs – Can be executed once, on a timer, on an event.

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

• The 3rd revision of the standard describes how to implement OOP within the application programming

• 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

• IEC 61131-3
• Electronic design automation
• IEC 61131

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• Short description matches Wikidata
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