Functional flow block diagram

The Functional Flow Block Diagram (FFBD) is a multi-tier, time-sequenced, step-by-step flow diagram of the system’s functional flow.^[2]

Overview

FFBDs usually define the detailed, step-by-step operational and support sequences for [[system]s, but they are also used effectively to define processes in developing and producing systems. The software development processes also use FFBDs extensively. In the system context, the functional flow steps may include combinations of hardware, software, personnel, facilities, and/or procedures. In the FFBD method, the functions are organized and depicted by their logical order of execution. Each function is shown with respect to its logical relationship to the execution and completion of other functions. A node labeled with the function name depicts each function. Arrows from left to right show the order of execution of the functions. Logic symbols represent sequential or parallel execution of functions.

A key concept in modeling functional flow is that for a function to begin, the preceding function or functions within the “control” flow must have finished. For example, an “eat food” function logically would not begin until a “cook food” function was completed. The logical sequence of functions (i.e., the functional flow) describes the “control” environment of the functional model. In addition to a function being enabled, it may also need to be triggered with an input. So, in the example, the “eat food” function is enabled once the “cook food” function is completed, and once it receives the “prepared food” as input. This second aspect—triggering a function speaks to the “data” environment, which the N2 diagram captures.

Most system functionality can be modeled using the standard symbols discussed below. If an extended set of symbols is required, then it should be defined in the resulting Functional Analysis Document (FAD) to ensure that all stakeholders are able to accurately interpret the diagrams.

Building blocks

Function symbology

A function shall be represented by a rectangle containing the title of the function (an action verb followed by a noun phrase) and its unique decimal delimited number. A horizontal line shall separate this number and the title, as shown in see Figure 3 above. The figure also depicts how to represent a reference function, which provides context within a specific FFBD. (See Figure 9 for an example regarding use of a reference function.)

Directed lines

A line with a single arrowhead shall depict functional flow from left to right, see Figure 4.

Logic Symbols

The following basic logic symbols shall be used.

AND: A condition in which all preceding or succeeding paths are required. The symbol may contain a single input with multiple outputs or multiple inputs with a single output, but not multiple inputs and outputs combined (Figure 5). Read the figure as follows: F2 AND F3 may begin in parallel after completion of F1. Likewise, F4 may begin after completion of F2 AND F3.
Exclusive OR: A condition in which one of multiple preceding or succeeding paths is required, but not all. The symbol may contain a single input with multiple outputs or multiple inputs with single output, but not multiple inputs and outputs combined (Figure 6). Read the figure as follows: F2 OR F3 may begin after completion of F1. Likewise, F4 may begin after completion of either F2 OR F3.
Inclusive OR: A condition in which one, some, or all of the multiple preceding or succeeding paths are required. Figure 7 depicts Inclusive OR logic using a combination of the AND symbol (Figure 5) and the Exclusive OR symbol (Figure 6). Read Figure 7 as follows: F2 OR F3 (exclusively) may begin after completion of F1, OR (again exclusive) F2 AND F3 may begin after completion of F1. Likewise, F4 may begin after completion of either F2 OR F3 (exclusively), OR (again exclusive) F4 may begin after completion of both F2 AND F3

Contextual and Administrative Data

Each FFBD shall contain the following contextual and administrative data:

Date the diagram was created
Name of the engineer, organization, or working group that created the diagram
Unique decimal delimited number of the function being diagrammed
Unique function name of the function being diagrammed

Figure 8 and Figure 9 present the data in an FFBD. Figure 4.4-9 is a decomposition of the function F2 contained in Figure 8 and illustrates the context between functions at different levels of the model.

References

^ Systems Engineering Fundamentals. Defense Acquisition University Press, 2001
^ The first version of this article is completely based on the NAS SYSTEM ENGINEERING MANUAL SECTION 4.4 VERSION 3.1 06/06/06.