Crown IQAMB-5, IQSMX-6 manual Closer Look at Crown Bus Wiring

AMB-5 /SMX-6IQ 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 lowcapacitance—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.*

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.

*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 Page 29