Appendix A: System calculations
168 P/N 3101890-EN • REV 006 • ISS 21JUN18
Network data riser limits
Cumulative data network capacitance refers to the total capacitance of all copper wire used for the data riser. The
cumulative capacitance of data networks must be within certain limits to permit stable network communications.
Audio networks are not affected by cumulative capacitance, due to the method of retransmitting data. The audio
network retransmits data byte-by-byte, so the individual bit times of a byte are restored at each node in the
network.
The data network retransmits data bit-by-bit. This method of retransmitting data restores the amplitude of a bit at
each node, but any distortions in bit timing are passed through to the next node. Data network communication
faults begin to occur at about 23% distortion of bit timing.
Cumulative data network capacitance induces bit timing distortion.
A fiber link in a data network electrically isolates two nodes, but distortions in bit timing are not restored by the
fiber segment. Distortions in bit timing are passed through the fiber to the next node. The bit transition time of
model VM-NOCF fiber cards is fast enough to be neglected in determining the maximum wire length that can be
used in the data network.
Data network specifications
Here are the maximum allowed values between any three nodes of a network.
• Resistance: 90 ohms (Ω)
• Capacitance: 0.3 microfarads (µF)
• Distance: 5,000 feet
The following table lists the maximum cumulative capacitance for the entire data network given various wire sizes
and transmission rates. Maximum cumulative capacitance is the total capacitance of all installed copper wire used
in the data network.
Maximum cumulative capacitance in microfarads
At 38.4 Kbaud At 19.2 Kbaud
1.4 2.8
1.8 3.6
2.1 4.2
Cable properties
Data and audio networks in a VM-1 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 to 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.
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