input and output elements of a block pictured with lines
the relationships between the functions, and
the functional sequences and paths for matter and or signals[2]
The block diagram can use additional schematic symbols to show particular properties.
Since the late 1950s, functional block diagrams have been used in a wide range applications, from systems engineering to software engineering. They became a necessity in complex systems design to "understand thoroughly from exterior design the operation of the present system and the relationship of each of the parts to the whole."[3]
Many specific types of functional block diagrams have emerged. For example, the functional flow block diagram is a combination of the functional block diagram and the flowchart. Many software development methodologies are built with specific functional block diagram techniques. An example from the field of industrial computing is the Function Block Diagram (FBD), a graphical language for the development of software applications for programmable logic controllers.
↑James Perozzo (1994) The complete guide to electronics troubleshooting. p. 72
↑William H. Von Alven (1964) Reliability engineering explains: "Functional block diagrams show functional sequences and signal paths, and items which are wired in parallel are drawn in parallel" (p. 286)
