Glossary Updates12 new terms added to the glossaries · October 2, 2026, 22:44 CEST
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Glossary · PLC and control programming

Function block (FB)

Also known as: FUNCTION_BLOCK, FB

German: Funktionsbaustein

In IEC 61131-3, a function block is a program organization unit with inputs, outputs and internal variables whose values persist between calls; each use requires its own instance with its own data.

  • PLC programming
  • Standards

In one sentence

An IEC 61131-3 function block is a reusable program unit with inputs, outputs and internal memory, used through instances.

Example

A function block FB_Motor with inputs Start, Stop and Feedback and outputs Run and Fault is instantiated once for each of the twelve conveyor motors.

How it applies

  • Engineering: Function blocks are the main unit of reuse in PLC programs. One tested block type for a motor, valve or axis is instantiated many times, each instance with its own data. IEC 61131-3 standard blocks such as timers, counters and edge detectors are function blocks, and the 2013 edition added object-oriented features such as methods and interfaces.
  • Testing: A block type is tested once in depth; each instance is then checked for correct parameters and wiring.
  • Maintenance: Changing a block type affects every instance. Library versioning and Change control keep this under control.
  • Documentation: Library blocks need reference documentation: interface table, behavior, states, alarms and parameter defaults. Keep it with the library version, not as a separate document that drifts.

Function block vs. function

A Function (IEC 61131-3) returns a result and forgets everything after the call. A function block remembers its internal state between calls. In IEC 61499, the term function block has a related but different meaning: an event-driven unit with event and data interfaces, see Function block type.

By knowledge.aitechdoc.world · Published September 26, 2026 · Last reviewed

Source: IEC 61131-3, Programmable controllers — Part 3: Programming languages

Definitions follow the cited standards and specifications. Where a source is a copyrighted publication, such as an ISO, IEC or EN standard, the definition is a close paraphrase, not a verbatim quotation, so as not to infringe copyright. We recommend reading the original publication. The sections “How it applies” are editorial commentary by AI TechDoc Knowledge and are not part of any standard.

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