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Tuesday, March 22, 2011

Tools for Defining and Verifying Functional Interfaces

6.5.3 Define and Verify Functional Interfaces (Internal/External)
Logical interfaces with external elements are defined in the context diagram and the FFBDs and internal interfaces are defined in the FFBDs. Both types of interfaces must be analyzed to verify that all interfaces are properly located and defined. Examine each external interface and verify that the information coming from or going to the interface matches the information being handled by the parent function in the chain of lower level child functions. Similarly examine each function and verify that all information coming from or going to the function is accounted for; that no function has an output that doesn’t go to either another function or to an external logical interface; and that no function requires information that is not coming to the function from another function or external interface. This task is made easier if the links in a process-oriented FFBD are labeled. An example of a simple process-oriented FFBD of a toaster with internal and external interfaces is shown in Figure 6-26.
(Apologies to experienced designers of toasters for any mistakes by the authors who have limited domain knowledge of toaster design. We use the example of a toaster because it is simple enough that diagrams and models fit on a page and everyone has  some idea of what a toaster does and how it might work. To those “virtuous and pure” engineers whose response is “toasters don’t apply to my work so these examples are useless to me” we remind you that the authors have used these same methods on systems costing hundreds of millions of dollars to develop. Learn the methodologies illustrated by these examples and don’t be put off by errors or incompleteness in these examples or the fact that your systems are much more complex.)

Figure 6-26 An example of a FFBD for a toaster showing the internal and external interfaces for each function of the Operational mode.

A “from” “to” matrix of functions in a particular mode is an alternate tool for defining interfaces for functions. An example is shown in Figure 6-27 for a toaster in its operational mode.

Figure 6-27 A Matrix of Functions to Functions is an alternate tool for defining internal and external interfaces among functions.

N-Squared diagrams are useful tools for analyzing interfaces for systems with functions having many internal interfaces. This tool also provides verification of the grouping and sequencing of lower level functions. It’s much easier to detect sequencing problems in an N-Squared diagram than on a FFBD. An example of an N-Squared diagram used for defining internal and external interfaces is shown in Figure 6-28. The advantages of the N-Squared diagram aren’t apparent in this simple case but imagine if the functions were more randomly sequenced along the diagonal. Then there would be arrows on the left of the diagonal indicating poor sequencing.

It is good practice to develop two different tools for defining internal and external interfaces; for example a FFBD and an N-Squared diagram. The two are then compared to verify that all interfaces are defined, grouped and sequenced correctly and consistent with the definitions of functions in the data dictionary. The small amount of time it takes to verify functional interfaces via two different tools is sound risk mitigation against making a mistake that isn’t discovered until system or subsystem testing when correcting the error is very costly.

Figure 6-28 An N-Squared diagram is an excellent tool for defining, grouping and sequencing interfaces.

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