Technical data

PROFIBUS Network Manual
System Manual, Edition 04/2009, C79000-G8976-C124-03
25
Topologies of SIMATIC NET PROFIBUS networks
2
2.1 Topologies of RS485 networks
2.1.1 Overview
Transmission speed
When operating SIMATIC NET PROFIBUS in the RS485 transmission technique, the user
can select one of the transmission rates below:
9.6 kbps, 19.2 kbps, 45.45 kbps, 93.75 kbps, 187.5 kbps, 500 kbps,
1.5 Mbps, 3 Mbps, 6 Mbps or 12 Mbps
Depending on the transmission rate, transmission medium, and network components
different segment lengths and therefore different network spans can be implemented.
The bus attachment components can be divided into two groups:
● Components for transmission rates from 9.6 kbps to a maximum of 1.5 Mbps
● Components for transmission rates from 9.6 kbps to a maximum of 12 Mbps
Bus cable
The transmission media used are the SIMATIC NET PROFIBUS cables described in
Chapter 4. The technical information below applies only to networks implemented with these
cables and SIMATIC NET PROFIBUS components.
Node attachment
The nodes are attached to the bus cables via bus connectors, bus terminals or RS485
repeaters.
Terminator
Each bus segment must be terminated at both ends with its characteristic impedance. This
cable terminator is integrated in the RS485 repeaters, the bus terminals and the bus
connectors and can be activated if required.
This line terminator can only be effective if the relevant connection element is supplied with
power. With the bus terminals and the bus connectors, this power is supplied by the
connected end device, whereas the RS485 repeater and the terminator have their own
power supply.
The RS485 transmission technique allows the attachment of a maximum of 32 devices (end
devices and repeaters) per bus segment. The maximum permitted cable length of a segment
depends on the transmission rate and the bus cable used.