Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 242)

Publication 1757-UM006A-EN-P - May 2002
D-6 Fieldbus Wiring Considerations
Signal Distortion vs
Capacitance Other factors such as varying characteristic impedance, spur
connection reflections, and capacitive unbalance between the wires
and shield can distort signals as they travel though the cable. This
means an ideal signal transmission that is within fieldbus
specifications can arrive at the other end of the cable as a distorted
signal.
In addition to cable attenuation, calculations for determining the
maximum fieldbus cable length must also use the following guidelines
to calculate signal distortion due to spurs.
• Each spur is converted to an equivalent capacitance.
• All spurs are assumed to be at the extreme end of the trunk.
• 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.
EXAMPLE
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).