Siemens SIMATIC S7-400H Manual page 177

Electrical network
The transmission medium of the electrical network is a shielded, twisted pair.
The RS 485 interface operates with voltage differences. It is therefore less
sensitive to interference than a voltage or current interface. In the case of
PROFIBUS the nodes are connected through a bus terminal or a bus connector to
the bus (up to 32 nodes per segment). The different segments are interconnected
by means of repeaters.
The maximum segment size depends on the transmission rate.
Apart from RS-485 transmission technology, there is PROFIBUS PA to IEC 1158
for process automation. PROFIBUS PA transmission technology aims at the
intrinsically safe EX area and thus operates with a synchronous, low-energy
transmission method. Up to ten nodes can be operated in the intrinsically safe Exi
area on a single PROFIBUS PA segment, provided that the overall power
requirement never exceeds 100 mA. In the non-intrinsically safe area as many as
30 nodes can operated on one PROFIBUS PA segment. The transmission rate
used is then 31.25 kbytes.
Optical network
The optical PROFIBUS network uses fiber-optic cables as a transmission medium.
The fiber-optic cable variant is insensitive to electromagnetic interference, is
lightning-proof, does not require electrical equipotential bonding and is suitable for
long distances (glass fiber-optic cable).
The maximum segment length does not depend on the transmission rate (except
redundant optical rings). Optical rings can be configured as one- or two-fiber rings
(enhanced network availability).
Configuration of the fiber-optic cable networks is accomplished by means of optical
link modules (OLMs). A network can be configured with OLMs as a line-type, ring
or star network.
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Communication Functions
8-7