The IQ Bus
The IQ Bus is an integral part of
the overall IQ System hardware structure. Scroll down to read
through the whole page, or use the links on the right for quick
access to the area that interests you most...
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IQ System Communication Overview
There are two primary communication pathways
that may be employed in a given IQ System. The first type, between the
computer and the system interface, is RS-232 (or RS-422). The second is
called the IQ Bus.
Definition
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The IQ Bus a communication standard defined as the
carrier of IQ command protocol on a serial data loop. That loop
must be complete and unbroken from the system interface into and
out of each component, returning to the interface to form a complete
loop.
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Data Format
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Data on the IQ Bus is formatted for 8N1 at 38,400
baud. On twisted pair wire >12 mA is required for a logic high,
<4 mA is required for a logic low.
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Transmission Medium
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The IQ Bus normally operates on a simple two wire
(shield optional) 20 mA current loop, although fiber
optic cabling or other media may be used. It is important to
note that the IQ Bus, although normally a current loop, does not
have to be. The IQ Bus is a serial communications loop designed
to transmit and receive IQ protocol. It is a communications standard,
regardless of the type of wiring actually used. If a fiber optic
option is used then external fiber transceivers will be necessary
to make connections to the data ports of IQ System components. Operating
with either the typical twisted pair or fiber optic lines, the IQ
Bus is a highly flexible communication standard. Because the IQ
Bus is a serial loop, the loop must remain unbroken for proper operation.
A break is defined as any condition that prevents data flow coming
out of the interface from returning to the interface.
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Loop Component Identification
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All IQ components which
are capable of being controlled or monitored from software must be
identifiable by the software. Component identification is based on
the type of component, the interface loop into which the component
is wired, and an eight bit address. The software on the computer can
identify the type of component by a code embedded in the firmware
of the unit. The loop number is determined by which IQ Bus loop the
unit is physically wired into from the unit serving as interface.
The address itself is set by the installer. For example, an Crown
USP2 module wired into the IQ Bus on the second loop of an Crown IQ-INT
II interface may have its address DIP switch set to 004. The unit
is identified by the system as Loop 2 USP2 004. Loop 2 is one part,
USP2 is another part, and 004 is the last part.
Each IQ component has an assignable
IQ address. Valid addresses in an IQ System are 1 to 250. There
must be no duplications of component identification anywhere in
an IQ System. For practical purposes, this means that you cannot
have two of the same type of product on the same loop with the same
loop address. It is legal to have two like components with the same
address on different loops, however, because the overall identification
of the two components would be different. For example, you could
have a Loop 3 USP2 004 and a Loop 2 USP2 004. Likewise, you could
also have a Loop 2 USP2 004 and a Loop 2 SLM 004. Even though the
address number is the same, the type of component is different.
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Selecting Twisted Pair Cable
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There is no endorsement
for any cable in particular. Instead, we offer you the information
you need to select the cable that will best suit the needs of your
installation. When you do select cable for the IQ Bus, it must be
twisted pair, 26 AWG or larger, and have low capacitance. Generally,
30 pF or less per linear foot is satisfactory. Loop capacitance is
the primary factor limiting maximum loop length. Cable may be shielded,
however shielded twisted pair typically has higher capacitance than
unshielded cable. Shielded cable should be considered if there is
concern that the IQ Bus may emit noise into other sensitive cables,
or where the IQ Bus is located in a high RFI environment.
Never use the ground wire in a mic
snake for the IQ shield or ground wire. It may be convenient to
run IQ data lines along mic snakes to and from a stage, but if you
must do this, then do not use the ground wire normally connected
to the XLR pin 1 or you will probably get data noise in your audio.
You may use wire pairs normally connected to XLR pins 2 and 3. Because
mic cables tend to have high capacitance, the maximum length of
the IQ Bus loop will usually be less.
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Loop Capacitance
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The maximum loop capacitance
at which the IQ Bus can reasonably be expected to operate at is approximately
35 to 40 nF. It is generally recommended that you avoid exceeding
30 nF to ensure a margin of safety. Most cable carries a specification
for capacitance in picofarads (pF) per linear foot (or meter). When
calculating capacitance you should also allow for as much as 60 pF
per IQ component on the loop. Adding 60 pF per component allows for
signal degradation through the optic couplers and analog output line
drivers of IQ components.
To calculate capacitance start with
the IQ Bus output from the unit serving as interface and determine
the cable distance around the loop in feet (or meters) of wire.
Multiply this distance by the capacitance rating of the wire. Add
60 pF per component on the loop. If the total capacitance exceeds
30 nF add Crown
IQ-RPT Repeaters as necessary to extend the length. If a repeater
is used, restart the loop capacitance calculations again from the
output of the repeater. Remember that every site is different and
no two IQ Systems are exactly the same.
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IQ Bus Length
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In most applications the IQ Bus may operate at distances
of over 1000 feet (300 m), and sometimes over 2000 feet (600 m).
The published specification on IQ Bus transmission distance is 300
to 3000 feet (100 to 1000 meters) with 1000 feet as "typical." In
cases where equipment is located at great distances, fiber optic
cable is often a cost effective way to get the IQ Bus to and from
remote locations. Depending on the type and number of fiber transceivers,
repeaters may still be required.
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IQ Component Bus Ports
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All IQ components include a two-wire input and two-wire
output port. Connector styles vary, but the internal circuitry supporting
IQ Bus communication is essentially the same. At a IQ Bus input,
the 20 mA current is applied to an opto-isolator circuit. This allows
electrical isolation between components. To ensure isolation, connect
the shield (if used) to the input only. The optically isolated input
drives buffer circuitry for input to the component's processor.
The rest condition (between data strings) is a current high. Because
this is the expected condition, components can automatically sense
a loss of input continuity and report a break in the IQ Bus. A transistor
circuit is used to drive output current to the next loop component
at the IQ Bus output. Electrically, a current loop exists between
each output and input. Most IQ System components also include a
IQ Bus drop-out relay. This relay is normally closed. When a component
is not powered up, the closed relay allows the IQ Bus port to be
bypassed. When the component is powered, and therefore capable of
communicating, the relay is held open so that the component is in
the communication loop.
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IQ Bus Connectors
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There are several types of connectors used on IQ
System components. These connector styles include 4-pin and 5-pin
DIN connectors, removable barrier blocks, and RJ45 modular types.
If shielded cable is used, the shield must be connected to IQ Bus
inputs only. This practice prevents possible ground loops in data
cabling. The following diagrams show various IQ Bus interconnections
and proper wire termination:
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