MIL-DTL-24784/20B (SH)
3.6.4.4 Functional description level.
3.6.4.4.1 First level of functional description. The first level of the description shall be confined
to data, such as, origin of the function at an equipment control or sensor; transmission of the
signal via intermediate equipment such as switchboards, relay transmitters, coordinate converters,
and distribution boxes; and presentation of the function at terminal equipment. Control functions
essential to the development of a signal shall be introduced and briefly described in their
relationship to the signal. The description shall be supported by an FBD (see 3.5.1.1). Functions
involving computations may be explained in mathematical terms, but at a level no higher than high
school mathematics. The second level of analysis shall be supported by one or more diagrams.
3.6.4.4.2 Detailed (second) level of functional description. The detailed level description shall
explain the system functions in terms directly related to the diagrams in Chapter 5. Data and
control functions shall be described down to the level of an equipment or an equipment group.
3.6.4.5 Preparation of FBD associated keyed text. This subsection describes the requirements
and method of presentation of the keyed text associated with each FBD.
a. Keyed text presents brief descriptions of each block on the FBD. The short (generally one to
three sentences) paragraph is "keyed" to the FBD by a circled number. The first text
paragraph shall describe the total FBD sheet. Each subsequent paragraph (circle one, circle
two, circle three, and so forth) shall describe what that block does within the operation of the
system.
b. The keyed text shall discuss, at an overview level, the purpose of each block (why it exists,
and what it does). The keyed text shall not discuss circuit implementation, or how it works.
When the same block appears on many levels of the FBD, the keyed text relating to the block
shall be identical at all levels (see figures 20-35 through 20-37).
3.6.5 Chapter 4, Operations. Chapter 4 shall describe system operating situations, modes, and
procedures. The descriptions shall be detailed to the level required for an understanding of the
operational interfaces of the system equipments and associated systems. Illustrations shall be
included when necessary for clarity (see figure 20-38). The various operating modes shall first be
described to acquaint the operator with all equipment combinations that can be employed to effect
a given mode of operation.
3.6.5.1 Preoperational conditions and setup. Specific preoperational conditions presumed to be
in effect prior to system operation shall be established. A system readiness checkoff list of
significant switch positions and indicator status shall be tabulated. For indicators such as dials,
where a band of readings are possible, upper and lower limits shall be delineated. The initial
conditions of associated system equipment that directly affect system operation shall be treated in
a similar manner.
3.6.5.2 Operating modes. The primary operating mode shall be discussed in detail, and alternate
modes shall be treated as modifications of the primary mode. Operating procedures common to
all modes shall be detailed under the primary mode and referred to under the alternate modes,
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