Configuration Manual Owner manual

Publication 1757-PM002A-EN-E - June 2006
Set Up A System 2-13
Spur Wire Length Calculations
Spur lengths can vary from 1 meter to 200 meters. If you have a choice about
spur length, the general rule is that the shorter the spur, the better. A spur less
than 1 meter is considered a splice.
Table 2.1 below lists recommended spur
lengths for devices.
Note: These lengths are recommended, not required
Shielding
For optimal performance, fieldbus cables should be shielded. Common
multi-conductor (multi-core) “instrument” cable can be used. It has one or
more twisted pairs, an overall metallized shield, and a shield wire.
The instrument shield should be terminated at the host (power conditioner)
end of the network in a marshalling cabinet and should not be connected to
ground at any other place. If a multiple home run cable goes to a fieldbus
junction box, do not attach the cable shield wires from different networks
together. This creates ground loops and noise on the network. The most
common problems with fieldbus networks are usually due to noise, which can
occur if you have the wrong wiring, improper grounding, and/or bad
connections.
A cable signal encountering a discontinuity, such as a wire open or short,
produces a reflection. The reflection is a form of noise that distorts the
original signal. A terminator is used at the ends of a fieldbus cable to prevent a
reflection.
Table 2.1 Recommended Maximum Spur Length Table m(ft)
Total Devices 1 Device per Spur 2 Devices Per
Spur
3 Devices per
Spur
4 Devices per
Spur
25-32 1(3) 1(3) 1(3) 1(3)
19-24 30 (98) 1(3) 1(3) 1(3)
15-18 60 (197) 30 (98) 1(3) 1(3)
13-14 90 (295) 60 (197) 30 (98) 1(3)
1-12 120 (394) 90 (295) 60 (197) 30 (98)