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Robin P. Clark 2013-09-17 10:45:01 +01:00
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@ -164,10 +164,11 @@ in a way that is compatible with FMEDA/EN61508.
\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.
%The choice of components for a {\fg} are that they are components that
%work together to perform a pre-defined function.work together to perform a pre-defined function.
The members of a {\fg} are chosen to be components that work together to perform a specific function.
%
The choice for {\fg} membership is made by the analyst.
The choice of {\fg} membership is made by the analyst.
%
The act of choosing components to form a {\fg}
raises questions about the circuit under investigation.
@ -178,19 +179,20 @@ Ideally {\fgs} will be able to act as standalone modules.
%
An inverting amplifier configuration, or a low pass filter are good examples of these:
they 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).
the output (in electronics terms they have low output impedance).
%
In defining members for {\fgs} the analyst is forced to consider the interfaces between elements
of circuitry to identify modules.
%
This is to prevent undue influence on them from circuitry
they may be connected to.
The aim is to prevent undue influence on modules identified from circuitry
they are/may be connected to.
%
Consider the resistor capacitor low pass stage first looked at in example~\ref{sec:lp}. %\label{sec:lp}
%
This circuit element, while applying a filtering effect, has a low output impedance.
This circuit element, while applying a filtering effect, has a high output impedance.
%
With a simple OpAmp buffer amplifier on its output stage, it becomes an effective high impedance output standalone module.
With a simple OpAmp buffer amplifier on its output stage, it becomes an effective low impedance output standalone module\footnote{A well behaved, or ideal electronics `module' will
have a high impedance input (i.e. it will not overload and affect any driving stages) and a low output impedance (i.e. it will drive an electrical load at the output without being affected its-self).}.
%
The resistor/capacitor low pass stage and the OpAmp
are good candidates therefore for being considered as a standalone module, and thus a {\fg}.
@ -206,7 +208,7 @@ structures of the FMMD hierarchy were used to analyse the same circuitry.
The same system level failure modes were obtained, but the more de-composed examples
offered better performance in terms of comparison complexity.
%
Further work may be required to apply justification for the choice of the membership in {\fgs}.
Further work may be required to apply justification for the choice of membership in {\fgs}.
%
For software already written this problem does not exist as the choice of membership has already been made by the programmer.