Siemens SIMATIC NET PROFIBUS, Owner'S Manual, Page: 20 / 490

PROFIBUS NETWORKS
1-10
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
1.2.4 Transmission Techniques According to EIA Standard RS-485
EIA Standard RS-485
The RS-485 transmission technique corresponds to balanced data transmission as
specified in the EIA Standard RS-485 /4/. This transmission technique is
mandatory in the PROFIBUS standard EN 50170 for data transmission on twisted
pair cables.
The medium is a shielded, twisted pair cable.
The bus cable is terminated at both ends with the characteristic impedance. Such
a terminated bus cable is known as a segment.
The attachment of the node to the bus is via a bus terminal with a tap line or a bus
connector (maximum 32 nodes per segment). The individual segments are
interconnected by repeaters.
The maximum length of a segment depends on the following:
S
The transmission rate
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The type of cable being used
Advantages:
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Flexible bus or tree structure with repeaters, bus terminals, and bus connectors
for attaching PROFIBUS nodes
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Purely passive passing on of signals allows nodes to be deactivated without
affecting the network (except for the nodes that supply power to the terminating
resistors)
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Simple installation of the bus cable without specialized experience.