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AMB-5 / SMX-6 IQ Mixer/Multiplexer
6.5 A Closer Look at Crown Bus Wiring
The Crown Bus is a serial communication loop de-
signed to transmit IQ commands and data. As imple-
mented in the IQ mixer/multiplexer, it is a 20 mA current
loop operating at a baud rate of 38.4 K.
The Crown Bus connection can use inexpensive
twisted-pair wiring (shielded or unshielded) and it must
be unbroken. If fiber optic cabling is required, contact
the Crown Technical Support Group (see Section 8 for
contact information)
Here are some guidelines for twisted-pair wiring:
• When interference is a problem, use shielded
twisted-pair wire at least 26 AWG in size. The
wire should be of good quality and should have
low capacitance—30 pF/foot or less is good. The
shield serves two purposes: First, it helps prevent
the IQ data signal from transmitting to nearby au-
dio wiring. Second, it helps prevent outside RF
from interfering with the data signal. However, in
most cases interference is not a problem and,
since unshielded wire has lower capacitance, it is
a better choice.
• Minimize the total capacitance of a Crown Bus
loop. The total capacitance should be less than
30 nF. Allow approximately 60 pF for each IQ
component in a loop. This accounts for a slight
signal degradation which occurs as data signals
pass through a component.
• Add an IQ Repeater for very long loops—greater
than 1,000 feet (305 m)—or when required by
high-capacitance wire. Although we recommend
adding a repeater for loops longer than 1,000
feet, it is often possible to go 2,000 feet (610 m) or
more. The most significant characteristic of the
wire is its capacitance. Lower capacitance allows
longer loops. Unshielded wire usually has less
capacitance.
• Never use the ground wire in a mic snake line.
It may sometimes be convenient to run Crown
Bus data signals to and from stage monitor ampli-
fiers along unused wire pairs in a mic snake. Do
not use the ground wire which is normally con-
nected to pin 1 on an XLR connector or data
noise will be added to the audio lines. Use only
the signal lines which normally connect to pins 2
and 3 of the XLRs.*
* Because typical mic cables have high capacitance, the maximum possible
Crown Bus loop distance will be less.
IQ–AMB-5 / IQ–SMX-6 Reference Manual
Outside RF interference is seldom a problem for a
Crown Bus loop—especially if shielded twisted-pair
wire is used. However, there are extreme situations
when fiber optic wiring is recommended. For example,
locating a Crown Bus loop next to an AM radio trans-
mission line may require fiber optic cabling. An ex-
tremely long Crown Bus loop distance may also
require fiber optic cabling.
Page 29
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