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 (third) edition was published in February 2013.^[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 in 3rd edition of the standard^[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 ^[4]
    • 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"

• 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