Siemens SIMATIC NET PROFIBUS - Manual (Page 247)

Testing PROFIBUS
A-7
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
A.2 Testing FO Transmission Paths
A.2.1 Necessity of a Final Test
The total attenuation of an FO transmission path, particularly the influence of
splices, can only be estimated roughly during planning. As a result of inaccuracies
when creating the splices and subjecting cables to excessive stress during
installation, the actual attenuation may well be higher than the calculated values.
The only way to be sure that a fiber-optic link functions reliably and has an
adequate link power margin is to measure the attenuation following installation. It is
advisable to test every fiber-optic link in which the connectors were fitted on-site
and to document the results in an acceptance report.
Attenuation
Attenuation defined as the optical power loss in decibels (dB) is the decisive
criterion in optical networks. All system components such as the cable, connectors,
splices, couplers etc. add to the total attenuation of a link and with it to the
attenuation within the entire network. To measure this attenuation, fiber-optic test
units must be used following installation. During the measurements, light with the
same wavelength must be used as in the optical transmission system.
Test Methods
In the main, two test methods are used:
1. The Optical Power Source and Meter
2. Optical Time Domain Reflectometer (OTDR)
A valuable tool for every user is also the fiber-optic troubleshooter. This light
source sends visible light so that fiber breaks, bad splices etc. can be localized
visually.
Some new network components, such as the PROFIBUS OLM version 3 have
integrated diagnostic functions with which they can check the quality of the
received optical signal.
A.2.2 Optical Power Source and Meter
The attenuation on the fiber-optic link is first measured. All the link components
such as the fiber, connectors, couplers and splices contribute to the total
attenuation. The total attenuation must be below the available optical power budget
between the optical transmitter and receiver. Light sources are available for all
normal wavelengths (650 nm, 850 nm and 1300 nm). This means that this method
can be used for plastic, PCF, multimode and single-mode fiber-optic cables.