blocks are black boxes that represent mathematical or logical operations that occur in sequence from left to right and top to bottom, but not the physical entities, such as processors or relays, that perform those operations. It is possible to create such block diagrams and implement their functionality with specialized programmable logic controller (PLC) programming languages. In biology there is an increasing use of engineering principles, techniques of analysis and methods of diagramming. There is some similarity between the block diagram and what is called Systems Biology Graphical Notation. As it is there is use made in systems biology of the block diagram technique harnessed by control engineering where the latter is itself an application of control theory. An example of this is the function block diagram, one of five programming languages defined in part 3 of the IEC 61131 (see IEC 61131-3) standard that is highly formalized (see formal system), with strict rules for how diagrams are to be built. Directed lines are used to connect input variables to block inputs, and block outputs to output variables and inputs of other blocks. See also Black box Bond graph Data flow diagram Functional flow block diagram One-line diagram Reliability block diagram Schematic diagram Signal-flow graph References External links Diagrams Modeling languages