ii14arcs.htm
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<title>II.1.4. Arcs</title>
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<p class=heading-2>II.1.4. Arcs</p>
<p class=heading-2-p> </p>
<p align="justify" class=normal style="text-align: center;"> </p>
<p align="justify" class=normal style="text-align: justify;">To draw an arc in
the <b><i><a href="i4drawingarea.htm">Drawing Area</a></i></b>, the user must
press the <b><i><a href="i5drawingpanel.htm">Add Arc</a> </i></b>button from
the <b><i><a href="i5drawingpanel.htm">Drawing Panel</a></i></b> or select the
<b><i><a href="i22modelingmenu.htm#AddArc">Add Arc</a> </i></b>command from
the <b><i><a href="i22modelingmenu.htm">Modeling</a> </i></b>menu. Then, the
user must click on the start node and then on the end node. The implementation
in the <b><i>PN Toolbox</i></b> of PN models complies with the basic rule: <i>an
arc of a PN can only connect a place to a transition </i>(pre-arc)<i> or a transition
to a place </i>(post-arc)<i>, but never two nodes of the same kind</i>. The
role of splitting arcs in two categories (pre- and post-arcs) will become apparent
below, when talking about inhibitor arcs.</p>
<p align="justify" class=normal style="text-align: justify;"> </p>
<p class=normal style="text-align: center;"><img ID="FigII6" src="fig.II.6.JPG" border=2></p>
<p align="center" class=figura>Fig. II.6. <i>The uicontext menu of an arc</i><i>.</i></p>
<p class=figura-p> </p>
<p align="justify" class=normal style="text-align: justify;" id="cubic">By default, an arc
is represented as a straight arrow between the two selected nodes of the net.
While in <b><i><a href="petrinettoolboxatafirstglance.htm#Fig1">Draw Mode</a>
</i></b>and no button from the <b><i><a href="i5drawingpanel.htm">Drawing Panel</a></i></b>
is pressed, a right click on an arc of the net opens a MATLAB uicontext menu
(<a href="#FigII6">fig. II.6</a>) that allows: (i) modifying the graphical representation
in the <b><i><a href="i4drawingarea.htm">Drawing Area</a></i></b> (<b><i>Line</i></b>
command for a straight line or <b><i>Cubic Spline </i></b>command for a curve),
(ii) deleting the arc (<b><i>Delete</i></b> command) and (iii) opening the <b><i>Edit Arc </i></b>dialogue box (fig. II.7) that lets the user modify the properties
of the arc as a MATLAB object (<b><i>Properties</i></b> command).</p>
<p align="justify" class=normal style="text-align: justify;"> </p>
<p align="center" class=figura-p><img ID="FigII7" src="fig.II.7.JPG" alt="Fig. II.7. The Edit Arc dialogue box for modifying the properties of an arc." ></p>
<p align="center" class=figura-p>Fig. II.7. <i>The <b>Edit Arc</b> dialogue box for modifying
the properties of an arc</i>.</p>
<p class=figura-p> </p>
<p align="justify" class=normal> The <b><i>Edit Arc</i></b> dialogue box may be
also opened by one of the procedures (EA1) or (EA2) described below, followed
by a click on the desired arc. (EA1) consists in selecting the <b><i><a href="i22modelingmenu.htm#EditObjects">Edit
Objects</a></i></b> command from the <b><i><a href="i22modelingmenu.htm">Modeling</a></i></b>
menu; (EA2) consists in pressing the <b><i><a href="i5drawingpanel.htm">Edit
Objects</a></i></b> button from the <b><i><a href="i5drawingpanel.htm">Drawing
Panel</a></i></b>.</p>
<p class=normal> </p>
<p align="justify" class=normal>Each arc is uniquely identified with an id that
is automatically assigned by the <b><i>PN Toolbox</i></b> and cannot be changed
by the user. This id appears in the title bar of the <b><i>Edit Arc</i></b>
dialogue box.</p>
<p class=normal> </p>
<p align="justify" class=normal id="color">The option <strong><em>Color</em></strong> displays
the color used for drawing the arc. By default, this color is black, but the
user can select another one from a list of eight predefined colors. Using different
colors for the arcs of a net might be helpful for complex topologies (e.g. for
highlighting different phases requested by a multi-step design procedure).</p>
<p class=normal> </p>
<p align="justify" class=normal id="type">The option <em><strong>Type</strong></em> displays
the type of an arc. By default, the type of an arc of a PN model is regular,
but the user can change it into bidirectional or inhibitor, if necessary. A
regular arc represents the standard connection between two nodes of different
types. A bidirectional arc is equivalent to a pair of arcs with the same weight,
one connecting a place to a transition and the other one connecting the same
transition to the same place. The graphical representation of a bidirectional
arc is a line with arrows at both ends.</p>
<p align="justify" class=normal> </p>
<p align="justify" class=normal>An <i>inhibitor</i> arc can connect only a place
to a transition. The transition is enabled only if the number of tokens in the
input place is strictly smaller than the weight of the inhibitor arc. The graphic
representation of an inhibitor arc is a line between the two nodes ending with
a small circle (near the inhibited transition).</p>
<p align="justify" class=normal> </p>
<p class=normal style="text-align: center;"><img ID="FigII8" src="fig.II.8.JPG" width="168" height="107"></p>
<p class=normal style="text-align: center;">Fig. II.8. <i>The <b>View Arc Weights</b> dialogue box.</i></p>
<br>
<p align="justify" class=figura id = "weight">The option <b><i>Weight</i></b> displays the weight
(multiplicity) of the arc. By default, the weight of an arc is equal to 1, but
the user can set this field to a positive integer value.</p>
<p class=figura-p> </p>
<p align="justify" class=figura> By default, the weights of the arcs in a net
are not shown in the <b><i><a href="i4drawingarea.htm">Drawing Area</a></i></b>.
At any stage of PN drawing, the user can visualize the current values of the
weights for all the arcs in the net by selecting the <b><i><a href="i23viewmenu.htm#ArcWeights">Arc
Weights</a></i></b> command from the <b><i><a href="i23viewmenu.htm">View</a></i></b>
menu. This command opens the dialogue box presented in fig. II.8 and the user
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