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Unformatted text preview: eveloped into a series of functions, as
shown in the third level diagram on Figure B-6. These
diagrams are used both to develop requirements and to
identify profitable trade studies. For example, does the
spacecraft antenna acquire the tracking and data relay
satellite (TDRS) only when the payload data are to be
transmitted, or does it track TDRS continually to allow
for the reception of emergency commands or
transmission of emergency data? The FFBD also
incorporates alternate and contingency operations,
which improve the probability of mission success. The
flow diagram provides an understanding of total
operation of the system, serves as a basis for
development of operational and contingency procedures, and pinpoints areas where changes in operational
procedures could simplify the overall system operation.
In certain cases, alternate FFBDs may be used to
represent various means of satisfying a particular
function until data are acquired, which permits selection
among the alternatives.
B.7.2 N2 Diagrams The N2 diagram has been used extensively to
develop data interfaces, primarily in the software areas.
However, it can also be used to develop hardware interfaces. The basic N2 chart is shown in Figure B-7. The
system functions are placed on the diagonal; the
remainder of the squares in the N x N matrix represent
the interface inputs and outputs. Where a blank
appears, there is no interface between the respective
functions. Data flows in a clockwise direction between
functions (e.g., the symbol F1 F2 indicates data flowing
from function F1, to function F2). The data being
transmitted can be defined in the appropriate squares.
Alternatively, the use of circles and numbers permits a
separate listing of the data interfaces as shown in Figure
B-8. The clockwise flow of data between functions that
have a feedback loop can be illustrated by a larger circle
called a control loop. The identification of a critical
function is also shown in Figure B-8, where function F4
has a number of inputs and outputs to all other functions
in the upper module. A simp...
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