See also function block adapter (FBA).

Consistency During the Design of FP Adapters (PDF)
The focus of this paper lies in a consistent design of Function Block Adapters (FBAs).

Function Block Programming (PDF)
Function Block Diagrams (FBDs) are another part of the IEC 61131-3 standard. The primary concept behind a FBD is data flow.

Function Blocks
In engineering disciplines, especially within time driven systems, software languages are often based on function blocks. By Torsten Heverhagen, Robert Hirschfeld, Rudolf Tracht.