Siemens SIMATIC NET PROFIBUS Power Budget, Plastic and PCF FO Cables, Glass FO Cables, Splices

Configuring Networks

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:

SThe attenuation of the fiber aFOC [in dB/km or dB/m] (see manufacturers data)

SThe 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:

SThe attenuation of splices

SThe attenuation of connectors

SWhen 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).

PROFIBUS Networks SIMATIC NET	3-15
6GK1970-5CA20-0AA1 Release 2 05/2000