System calculations
B.2 EST3 Installation and Service Manual
Network data riser limits
Overview
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 3-FIB 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
Wire size (AWG) At 38.4 Kbaud At 19.2 Kbaud
18 1.4 2.8
16 1.8 3.6
14 2.1 4.2
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