Appendix A: System calculations
EST3X Technical Reference Manual 233
Cable properties
Data and audio networks in an EST3X system do not require the use of shielded
cable. Networks designed with twisted-pair can be about twice as long as those
designed with shielded cable.
The maximum length of a data network varies with the properties of the wire
used. Wire manufacturers typically provide specifications for wire resistance and
capacitance.
Resistance is generally specified in ohms per 1,000 feet, and must be doubled
for 1,000 feet of a twisted-pair cable. Capacitance is specified in picofarads per
foot (pF/ft).
The capacitance between conductors of a twisted-pair is commonly referred too
as conductor-conductor or mutual capacitance. Shielded cable has an additional
capacitance between each conductor and the shield. The capacitance of either
conductor to shield is typically twice the value of mutual capacitance, and the
highest value of capacitance must be used when calculating the maximum length
of a data network.
The overall length of data networks designed with twisted-pair cable is about
twice as long as data networks designed with shielded cable due to the additional
capacitance resulting from the shield.
Calculating a maximum length
The maximum length of a data network can be calculated by dividing the
maximum cumulative capacitance allowed by the highest capacitance rating of
the selected cable.
For example, say you wanted to determine maximum length of a data network
using 18 AWG cable that is rated at 25 pF per foot. The network will
communicate at 38.4 Kbaud.
The maximum length equals the maximum cumulative capacitance divided by the
capacitance per foot. In equation form:
ML = MCC / CPF
Where,
ML = Maximum length
MCC = Maximum cumulative capacitance
CPF = Capacitance per foot
In our example, the calculation is as follows:
ML = 1.4 µF / 25 pF/ft
ML = 56,000 ft
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