75 takes us to the structure of our
on-board computer, which is described in the internal block diagram in Figure
2.76 . The picture will naturally change as we look at more fl ows. There will be
new blocks, ports, and connections to be added, and existing ones will be modi-
fi ed or removed.
The resulting picture has to make sense, of course. For example, may block X
communicate with block Y ? If we fi nd an undesirable connection we must search
for the reason why it ??™ s there. Is the exchange of messages between the blocks
concerned really necessary? Where does the requirement come from? Are all necessary
infrastructure elements, e.g., power supply, available?
These decisions are not hewn in stone. Considerations like the ones mentioned
above form a good starting point that, in turn, can change later on. It may
well happen that blocks are merged, removed, or newly shaped. You can also
experiment with alternative designs and compare them. Models provide an excellent
vehicle for this sort of studies.
We can derive the defi nitions of the blocks used from the internal structure of
our on-board computer and describe them in a block defi nition diagram ( Figure
2.
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