page_title: Functional flow block diagram
section_title: Key attributes
hierarchical_section_title: Functional flow block diagram / Building blocks / Key attributes
caption_attribution_description: English: Function block
context_page_description: A functional flow block diagram is a multi-tier, time-sequenced, step-by-step flow diagram of a system’s functional flow. The term "functional" in this context is different from its use in functional programming or in mathematics, where pairing "functional" with "flow" would be ambiguous. Here, "functional flow" pertains to the sequencing of operations, with "flow" arrows expressing dependence on the success of prior operations. FFBDs may also express input and output data dependencies between functional blocks, as shown in figures below, but FFBDs primarily focus on sequencing.
  The FFBD notation was developed in the 1950s, and is widely used in classical systems engineering. FFBDs are one of the classic business process modeling methodologies, along with flow charts, data flow diagrams, control flow diagrams, Gantt charts, PERT diagrams, and IDEF.
  FFBDs are also referred to as functional flow diagrams, functional block diagrams, and functional flows.
context_section_description: An overview of the key FFBD attributes: 
   
  Function block: Each function on an FFBD should be separate and be represented by single box (solid line). Each function needs to stand for definite, finite, discrete action to be accomplished by system elements.
  Function numbering: Each level should have a consistent number scheme and provide information concerning function origin. These numbers establish identification and relationships that will carry through all Functional Analysis and Allocation activities and facilitate traceability from lower to top levels.
  Functional reference: Each diagram should contain a reference to other functional diagrams by using a functional reference (box in brackets).
  Flow connection: Lines connecting functions should only indicate function flow and not a lapse in time or intermediate activity.
  Flow direction: Diagrams should be laid out so that the flow direction is generally from left to right. Arrows are often used to indicate functional flows.
  Summing gates: A circle is used to denote a summing gate and is used when AND/OR is present. AND is used to indicate parallel functions and all conditions must be satisfied to proceed. OR is used to indicate that alternative paths can be satisfied to proceed.
  GO and NO-GO paths: “G” and “bar G” are used to denote “go” and “no-go” conditions. These symbols are placed adjacent to lines leaving a particular function to indicate alternative paths.
caption: Graphical explanation of a "function block" used in these diagrams.  Flow is from left to right.[4]
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