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14. Appendix E
14.1. Fieldbus Wiring Considerations
Signal attenuation is calculated as 0.035 dB/nf.
For example, if the fieldbus topology includes five devices each on a 20 m (66 ft) spur,
the total cable length is 100 m (328 ft), equivalent capacitance is 3 nf (100 m x 0.03
nf/m), and spur distortion is 0.105 dB (3 nf x 0.035 dB/nf = 0.105 dB).
Another source of distortion is the fieldbus device's equivalent capacitance. Use the
following guidelines to calculate the distortion due to device capacitance.
Determine each device's input capacitance.
Assume all devices are at the extreme end of the trunk.
Use 0.035 dB/nf as signal attenuation calculation.
For example, if the fieldbus topology includes five devices each with a 3000 pF input
capacitance, the equivalent capacitance is 15 nf (5 x 300 pF) and the resulting
attenuation is 0.525 dB (15 nF x 0.035 dB/nF).
Attenuation calculation summary
Take the sum of the following calculations to determine if the attenuation will exceed the
allowed level of 14 dB.
Calculate the cable attenuation.
Calculate the attenuation due to spurs.
Calculate the attenuation due to device capacitance.
For example, the total attenuation for the fieldbus topology used in the previous
examples in this section would be 2.13 dB (1.5 dB (cable attenuation) + 0.105 (spur
attenuation) + 0.525 (device capacitance attenuation) = 2.13 dB).
Cable test
Use a fieldbus tester and signal generator to determine if existing cable is suitable for
fieldbus use or not. The tester and signal generator work together to determine the
resistance of the wires and measure the signal quality of the transmission. This testing
also determines if the wiring polarity is consistent.
516
Experion PKS Series A Fieldbus Interface Module User's Guide
Honeywell
R400
July 2010
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