6.3.2.2 Define Life Cycle System Modes

Most texts, e.g. the NASA SE Handbook, use the terms states and modes.  These terms are often confused because both have transition diagrams and are often used interchangeably. It is preferable that they are not the same thing and are not interchangeable. Both state and mode diagrams define a condition of the system identified as a box, with a transition between them. In both cases, the transition is labeled with a system event which triggers the transition. The difference between a state and a mode is the definition of the box. States define an exact operating condition of a system, where modes define the set of capabilities or functions which are valid for the current operating condition. Let's use a simple example of a diagram for a television with two conditions OFF and ON. There are two transitions between them, one pointing to ON labeled as remote control ON detected, and a second pointing to OFF labeled remote control OFF detected. This diagram can represent a State or a Mode diagram. So what's the difference? If it is a Mode diagram, there is a data dictionary defining what functions can be supported in each condition. The TV in OFF mode must perform the function to sense the IR signal from the remote control to turn on. The TV in the ON mode must perform functions to change channels, adjust volume, and sense the IR signal from the remote control to return the TV to off. Knowing the TV is in the ON mode does not provide knowledge of what exact function the TV is performing. If this is a state diagram, there is no understanding of what functions the TV can perform. The state diagram details a single flip flop that toggles state based on the inputs of remote control ON, or remote control OFF. The output of this flip flop probably is used to activate primary power. In a more complex state diagram, there are many more memory elements but the key point is, in a given state, the condition of every memory element is explicitly known.

System modes have a hierarchy similar to functions. Modes can be decomposed into sub modes, which are also called modes, just as functions can be decomposed into lower level functions. Modes and the transitions between modes can be shown in a hierarchy of diagrams and matrices. The top level diagram or matrix should contain all modes in the life cycle of the system from assembly to an end of life mode. An example of a simple life cycle mode diagram for a commercial product for home use is shown in Figure 6-10. In this figure the event that determines transitioning from one top level mode to another is included as a labeled arrow on the diagram. It's important to define and examine the modes for all of a systems life cycle rather than just it's "in Use" mode. Sometimes there are requirements that are necessary for integration and test, storage or end mode that are not included in the modes for the "in Use" mode and the life cycle mode diagram is useful in identifying such requirements.

Diagrams or matrices can be developed for each of the top level modes that has sub modes to define the functions required in each mode and the allowed transitions between functions. Thus the top level mode diagrams identify lower level modes that are then examined during functional analysis to assist in defining all of the functions required in every mode of the life cycle of the system. This topic is revisited after discussion of functions and functional decomposition.

Transitions between sub modes are usually more complicated than between the top level life cycle modes. A very simple example that illustrates this is shown in Figure 6-11 where the transitions among the three modes of the IN USE mode of the product having the life cycle modes shown in Figure 6-10. The transitions are not labeled in this diagram at this point because the events causing these transitions can be dependent upon the physical design. It is only necessary at this point to define the allowed and required transitions between modes.

Figure 6-10 An example top level life cycle mode diagram for a simple commercial product.

Figure 6-11 A mode transition diagram for a simple commercial product in its IN USE mode.

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