The following example shows the rules that are set for the lengths of individual spur cables connecting a junction box to instruments in a variety of fieldbus topologies. For example, as shown in row 3, where 15 to 18 devices are associated per segment, and one instrument is associated per spur, the cable can be up to 60 meters long. If we allow 2 instruments per spur with the same number of associated devices per segment, each cable can run up to 30 meters, and so forth.
In this example, the recommended spur cable lengths are:
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120 meters for 1 to 12 devices per segment and one device per spur;
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90 meters for 1 to 12 devices per segment and two devices per spur;
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60 meters for 1 to 12 devices per segment and three devices per spur;
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30 meters for 1 to 12 devices per segment and four devices per spur;
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90 meters for 13 to 14 devices per segment and one device per spur; and so forth.