However it can also be used to develop hardware

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Unformatted text preview: e analyzed to identify those functions which must be performed to satisfy the objectives of each functional area. Each function is identified and described in terms of inputs, outputs, and interface requirements from top down so that subfunctions are recognized as part of larger functional areas. Functions are arranged in a logical sequence so that any specified operational usage of the system can be traced in an end-to-end path. Although there are many tools available, functional identification is accomplished primarily through the use of 1) functional flow block diagrams (FFBDs) to depict task sequences and relationships, 2) N2 diagrams to develop data interfaces, and 3) time line analyses to depict the time sequence of time-critical functions. B.7.1 Functional Flow Block Diagrams The purpose of the FFBD is to indicate the sequential relationship of all functions that must be accomplished by a system. FFBDs depict the time sequence of functional events. That is, each function (represented by a block) occurs following the preceding function. Some functions may be performed in parallel, or alternate paths may be taken. The duration of the function and the time between functions is not shown, and may vary from a fraction of a second to many weeks. The FFBDs are function oriented, not equipment oriented. In other words, they identify "what" must happen and do not assume a particular answer to "how" a function will be performed. FFBDs are developed in a series of levels. FFBDs show the same tasks identified through functional decomposition and display them in their logical, sequential relationship. For example, the entire flight mission of a spacecraft can be defined in a top level FFBD, as shown in Figure B-6. Each block in the first level diagram can then be expanded to a series of functions, as shown in the second level diagram for "perform mission operations." Note that the diagram shows both input (transfer to operational orbit) and output (transfer to space transportation system orbit), thus initiating the interface identification and control process. Each block in the second level diagram can be progressively d...
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