Siemens SIMATIC NET PROFIBUS - Manual (Page 81)

Configuring Networks
3-15
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
Power Budget
The power budget of an optical link not only takes into account the attenuation in
the fiber itself, temperature and aging effects but also the attenuation values of the
connectors and splices and therefore provides exact information about whether or
not an optical link can be implemented. The starting point for calculating the
maximum transmission path length is the minimum transmitter power that can be
coupled into the fiber type. To simplify matters, the budget is calculated in dBm and
dB.
The following is subtracted from the minimum transmitter power:
S
The attenuation of the fiber a
FOC
[in dB/km or dB/m] (see manufacturers data)
S
The input power required at the receiver
The coupling losses at the send and receive diodes are already taken into account
in the information about the transmitter power and receiver sensitivity.
Plastic and PCF FO Cables
Plastic and PCF FO cables can only be used on short distances due to their
relatively high fiber attenuation. They are installed in one piece. Fiber-optic
connections with couplers or splices should not be considered since they further
reduce the distance that can be covered.
The maximum permitted cable lengths are listed in Tables 3-7 and 3-8.
Glass FO Cables
Glass FO cables can span distances in the kilometer range. It is often not possible
to install cables over such distances in one piece. The fiber-optic path must then
be put together in cable sections.
The couplers or splices where the sections are joined always involve certain
attenuation losses.
With transmission paths using glass fibers, the following aspects must also be
taken into account:
S The attenuation of splices
S The attenuation of connectors
S 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.
Splices
Along with the splices, future repair splices must also be taken into account.
Depending on the route of the cables and the risk of mechanical damage, one or
more future repairs (approximately 1 per 500 m) should also be included in the
budget. A repair always means two splices since a new section of cable must be
inserted (the length depending on the accuracy of the test equipment).