Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 243)

Publication 1757-UM006A-EN-P - May 2002
Fieldbus Wiring Considerations D-7
Calculating Attenuation 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.
Testing the Cable 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.
Repeaters Repeaters can be used in a Fielbus cable system. There can be a
maximum of four repeaters between any two nodes on a Fieldbus. If
repeaters are used, there must be terminators on each side of the
repeaters. A Fieldbus with one repeater will use four terminators, you
terminate each Fieldbus segment.
EXAMPLE
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).