Siemens SIMATIC NET PROFIBUS, Owner'S Manual, Page: 82 / 490

Configuring Networks
3-16
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
Link Power Margin
When calculating the power budget, a link power margin of at least 3 dB (at a
wavelength of 860 nm) or at least 2 dB (at a wavelength of 1300 nm) must be
maintained.
If the link power margin calculated is lower, the transmission path will not be
reliable in its currently planned form. This means that the transmission path may
well function when it is first started up since components are normally better than
their rated performance (particularly when brand new) but due to aging,
replacement of components as a result of repairs and changing environmental
conditions, the bit error rate will tend to rise to an unreliable level the longer the
equipment is in use.
Note
To avoid possible errors during the installation of the transmission path, when
installing glass fibers, the installed sections must be tested during installation and
the measured values logged (see Section A-2 “Testing FO Transmission Paths”).
Work Sheet
Section 3.2.4 of this manual contains a work sheet for calculating the power budget
of glass fiber-optical links.