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conclusion: prob of composition of standalone modules/{\fgs}

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Robin P. Clark 2013-09-17 09:55:35 +01:00
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submission_thesis/CH8_Conclusion/copy.tex
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@ -162,6 +162,50 @@ in a way that is compatible with FMEDA/EN61508.
\fmmdglossFIT
\subsection{Composition of {\fgs}.}
The choice of components for a {\fg} are that they are components that
work together to perform a pre-defined function.
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The choice for {\fg} membership is made by the analyst.
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The act of choosing component to comprise a {\fg}
raises questions about the circuit under investigation.
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Ideally {\fgs} will be able to act as standalone modules.
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An inverting amplifier configuration, or a low pass filter are good examples of these.
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These have clear inputs and outputs, and are resilient to what they are connected to at
the output (in electronics terms they have high output impedance).
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In defining members for {\fgs} he analyst is forced to consider the interfaces between elements
of circuitry to identify modules.
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Consider the resistor capacitor low pass stage first looked at in example~\ref{sec:lp}. %\label{sec:lp}
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This circuit element, while applying a filtering effect, has a low output impedance.
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With a simple OpAmp buffer amplifier it becomes an effective, high impedance output, standalone module.
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These two elements, the resistor capacitor low pass stage and the OpAmp
are good candidates therefore, for being considered as a standalone module, and thus a {\fg}.
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However, different analysts may choose different {\fgs}
when analysing the same circuit.
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This means that {\fgs} are not guaranteed to be unique.
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This apparent anomaly is explored in the examples~\ref{sec:invamp},~\ref{sec:bubba} were different
structure of the FMMD hierarchy are used to analyse the same circuitry.
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The same system level failure modes are obtained, but the more de-composed examples
offer better performance in terms of comparison complexity.
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Further work may be required to apply justification for the choice of the membership in {\fgs}.
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For software already written this problem does not exist as the choice of membership has already been made by the programmer.
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\subsection{Deriving FTA diagrams from FMMD models}
\label{sec:fta}