Fieldbus Topologies

Line Topology

With this topology, the fieldbus cable is routed from device to device on this segment, and is interconnected at the terminals of each fieldbus device. Installations using this topology should use connectors or wiring practices such that disconnection of a single device is possible without disrupting the continuity of the whole segment.

Line Topology with spurs

With this topology, the fieldbus devices are connected to the bus segment through a length of cable called a spur. A spur can vary in length from 1 m to 120 m. A spur that is less than 1 m in length is considered a splice.

PROFIBUS provides a wide range of possible transmission rates. The permissible lengths of possible spur cables depend upon the transmission rate used. No spurs are permitted for transmission rates over 1,5 Mbit/s. With transmission rates less than 1.5 Mbit/s the total length of all spur cables should be less than 6,6 m. When using spurs with low transmission rates (93.75 kbit/s), the ratio between spur cable length and next bus termination should be at least 1:20.

NOTE: There is always a cable of several centimeters between a PROFIBUS connector and the transceiver of a node. When connecting 32 nodes to the bus, an important total spur cable length will result from this. If there should encounter transmission problems in the installation although everything works reliably at a low transmission rate, it is recommended to perform a dynamic bus analysis. This analysis will reliably detect mismatches.

Tree Topology

With this topology, devices on a single fieldbus segment are connected via individual twisted wire pairs to a common junction box, terminal, or I/O card. This topology can be used at the end of a home run cable as well as in between. It is practical if devices on the same segment are well separated, but in the general area of the same junction box. When using this topology, the maximum spur length must be taken into consideration. Maximum spur lengths are discussed in Line Topology with spurs.

NOTE: Combinations of the topologies above are possible. However, all rules for maximum fieldbus segments length, including spurs, must be taken into account for the calculation of the total bus length.