Siemens SIMATIC NET PROFIBUS - Manual (Page 440)

Introduction
1-2
PROFIBUS Optical Bus Terminal (OBT)
C79000-G8976-C122-02
Connections
The connection between the individual nodes takes the form of an optical bus with
two-fiber plastic FO cables (plastic fiber-optic cables are also known as POF,
Polymer Optical Fiber) or PCF FO cables (PCF = Polymer Cladded Fiber,
corresponds to HCS
t 1) fiber-optic cable). Since fiber-optic cables are
completely insensitive to electromagnetic disturbance, no grounding concept
whatsoever is necessary. For the same reason, equipotential bonding is also not
necessary. The optoelectronic conversion provides automatic isolation so that
differences in potential as can occur in extensive systems have no effect.
1)
HCS t is a registered trademark of Ensign-Bickford Optics
Company and stands for “Hard Polymer Cladded Silica Fiber”.
Sensitivity
Just as fiber-optic cable is insensitive to electromagnetic noise, a fiber-optic cable
emits no electromagnetic noise into the environment. Sensitive electronic devices
close to the fiber-optic cable therefore need no additional protection or noise
suppression.
Power Supply
The OBT requires an operating power supply of 24 V direct voltage that is
connected via two terminal screws.
Operating Mode
LEDs signal the current mode and any problems in operation.
Mechanical Design
The optical bus terminal consists of a compact plastic casing which can be
installed either on a standard rail or on any flat surface.