In some cases, it is desirable to further define the Active state of an Alarm by providing a sub-state machine for the Active State. For example, a multi-state level Alarm when in the Active state may be in one of the following sub-states: LowLow, Low, High or HighHigh. The state model is illustrated in Figure 7.
Figure 7 – Multiple active states example
With the multi-state Alarm model, state transitions among the sub-states of Active are allowed without causing a transition out of the Active state.
To accommodate different use cases both a (mutually) exclusive and a non-exclusive model are supported.
Exclusive means that the Alarm can only be in one sub-state at a time. If for example a temperature exceeds the HighHigh limit the associated exclusive level Alarm will be in the HighHigh sub-state and not in the High sub-state.
Some Alarm systems, however, allow multiple sub-states to exist in parallel. This is called non-exclusive. In the previous example where the temperature exceeds the HighHigh limit a non-exclusive level Alarm will be both in the High and the HighHigh sub-state.