ABB AC 800M User Manual page 78

PROFIBUS PA junctions and connectors
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Figure 28. Junction box (T-junction)
The junction box retains its role as a central connection unit to build up a tree
topology, where all field devices are connected in parallel.
The line topology offers connection points along the field bus cable. The cable can
be looped through the individual field devices. The combination of tree and line
topology permits the optimization of the fieldbus length and the adaption to existing
system structures. It must be taken into account, that the limitation of the spur length
for intrinsically safe installations according to FISCO bases on pure tree- or line
topology. If a combination is used in a hazardous area, then the limit has to be
applied to each connection between a field device and the trunk cable (via junction
box). As an example, if the cable length between the trunk and the junction box is
20 m, then the cable length between the junction box any device connected to it
must not exceed 10m (see also next topic "Bus spur lines").
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Usage of junction box (T-junction):
Section 3 Fieldbus Topologies
3BDS009029R5001 B