Siemens SIMATIC NET PROFIBUS User Manual

12

3 Network Topologies

Version 1.0 8/00

3.2 Star topology 

Terminal unit /

bus segment

Terminal unit /

bus segment

Terminal unit /

bus segment

Terminal unit /

bus segment

Terminal unit /

bus segment

CH  2

T

R

OLM/P11

CH  1

CH  2

T

R

OLM/P11

CH  1

CH  2

T

R

CH  3

T

R

OLM/G12

CH  1

CH  2

T

R

OLM/G11-1300

CH  1

OLM/P11

CH 2

T

R

CH  1

OLM/G11-1300

CH 2

T

R

CH 1

OLM/P11

CH  2

T

R

CH  1

OLM/G11

CH  2

T

R

CH  1

OLM/G11

CH  2

T

R

CH  1

Electrical star segment

S0 = 1

S0 = 1

S0 = 1

S0 = 1

RS 485 bus line

Optical fiber

Several modules are combined to form an active PROFIBUS star coupler. Other modules are connected to this by
dual-fiber optical fiber lines. The modules of the star coupler are connected to one another via the electrical port
(electrical star segment). 
All OLM types for different fiber types (plastic, PCF, glass) can be combined using the electrical star segment. 

CH1 in mode ”Monitor off“ (S0 = 1) must be activated on all OLMs which are connected to the electrical star
segment. This deactivates the segmenting function of the RS 485 port on these OLMs, providing a high degree
of availability of the electrical star segment.

Ensure that the electrical star segment is wired carefully. Keep it as small as possible to avoid interference
injection into the electrical star segment, and from here into the entire network. This can be achieved by laying
out the OLMs in the electrical star segment directly next to each other on a hat rail.