MIL-DTL-24784/20B (SH)
a.
Master FBD. The master FBD depicts the system functions and major signal paths within the
system.
1.
Each major block on the master FBD shall represent a major functional element and shall
be identified by functional and hardware boundary lines and descriptive names. These
functional elements shall be connected by signal path and control lines, also identified
with descriptive names.
2.
Functional boundaries shall be depicted with thin, solid lines; and hardware boundaries
shall be depicted with thin, short-dash and long-dash lines. Signal and control lines shall
be depicted with thin, solid lines; while data lines shall be depicted with thick, solid lines
(see figure 20-2).
3.
Each block on the master FBD shall contain a circled number. The circled number
corresponds to a brief paragraph (referred to as "keyed text") that describes the purpose
and function of that block. The circled numbers shall start with the circle 1 in the first
block at the top left of the FBD and progress in a logical manner (either by data flow, or
top to bottom and left to right) on the FBD until all blocks are numbered. This "keyed
text" paragraph will be located on a page facing the FBD to which it applies. In addition
to the circled number, each block on the master FBD shall contain a reference to a
primary FBD and the paragraph number of the paragraph that describes the block (not
the keyed text paragraph).
b.
Primary FBD. Primary FBDs shall contain system function and subfunction information. The
primary FBD shall illustrate the interconnection of subfunctional elements in order to describe
how the functions work within the system. They shall provide a reference to secondary
diagrams and the paragraph number of the paragraph that describes the block (not the keyed
text paragraph). The secondary diagrams may be FBDs or FCDs. If there are no secondary
diagrams for reference, the reference shall be made directly to the hardware by reference
designation.
1.
The primary FBD shall consist of the major functional divisions enclosed by the proper
boundary lines (see figure 20-3). Each functional block shall be identified by a
descriptive name and referenced to a keyed text paragraph on the facing page. The
functional divisions shall be interconnected by signal path and control lines, each
identified with a descriptive name and proper line weight (line weights shall be in
accordance with 3.5.1.1.3). Primary FBDs may show equipment boundaries if
appropriate to the function being shown. A direct relationship shall be maintained in
regard to the number and nomenclature of interface lines between a primary FBD and
other interfacing primary FBDs.
c.
Secondary FBD. When the functional divisions represented on the primary FBD are
sufficiently large, subfunctionalization is sometimes necessary. The secondary FBD provides
the transition between the major functional elements on the primary FBD and the FCDs (see
3.5.1.3), or to actual hardware by reference designator where the function depicted is
performed. The subfunctional segments illustrated on the secondary FBD must often be
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