Siemens SIMATIC NET PROFIBUS - Manual (Page 28)

Topologies of SIMATIC NET PROFIBUS Networks
2-2
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
2.1 Topologies of RS-485 Networks
Transmission Rate
When operating SIMATIC NET PROFIBUS in the RS-485 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:
S
Components for transmission rates from 9.6 Kbps to a maximum of 1.5 Mbps
S
Components for transmission rates from 9.6 Kbps to a maximum of 12 Mbps
LAN 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 LAN cables via bus connectors, bus terminals or
RS-485 repeaters.
Cable Termination
Each bus segment must be terminated at both ends with its characteristic
impedance. This cable terminator is integrated in the RS-485 repeaters, the bus
terminals, the ILM and the bus connectors and can be activated if required.
Before the cable terminator can be activated, the component must be supplied with
power. With the bus terminals and the bus connectors, this power is supplied by
the connected DTE, whereas the RS-485 repeater, the ILM, and the terminator
have their own power supply.
The RS-485 transmission technique allows the attachment of a maximum of 32
devices (DTEs and repeaters) per bus segment. The maximum permitted cable
length of a segment depends on the transmission rate and the LAN cable used.