Siemens SIMATIC NET PROFIBUS Owner's Manual page 37

A break on a fiber-optic cable between two modules is detected by the modules
and the network is reconfigured to form an optical bus. The entire network remains
operational.
If a module fails, only the DTEs or electrical segments attached to the module are
separated from the ring; the remaining network remains operational as a bus.
The problem is indicated by LEDs on the modules involved and by their signaling
contacts.
After the problem is eliminated, the modules involved cancel the segmentation
automatically and the bus is once again closed to form a ring.
Note
To increase the availability, the duplex cables for the outgoing and incoming paths
in the ring should be routed separately.
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
Topologies of SIMATIC NET PROFIBUS Networks
2-11