Clear-Com PIC-4704 - Manual (Page 11)

P I C - 4 7 0 4 , M A - 7 0 4 & A X - 7 0 4 I F B S Y S T E M
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INSTALLATION
SYSTEM CAPACITY
A system may have up to fifty access locations. Cue audio from the MA-704 can
drive up to twenty-three AX-704 units, thus permitting a maximum of ninety-six
talent positions.
In order to use the IFB system at its maximum capacity, two factors must be
considered: system wiring (architecture) and power requirements. The MA-704
consumes a maximum of 180 mA (idle current 140 mA), and the AX-704
consumes a maximum of 150 mA (idle current 120 mA). Since the resistance of
the conductors in the interconnect cable may be on the order of five to ten Ohms
per 1000 feet, care must be taken to avoid having too many stations on one long
cable run.
For example, a system with two MA-704s on a 2000 foot cable which has 16
Ohms cumulative resistance in the power conductor, plus another 10 Ohms in
the common conductor, the voltage drop is a maximum of 9 volts. If another two
MA-704s were to be added for the same cable run, the voltage drop would be an
unacceptable 18 volts. Therefore, the other set of access stations would have to be
connected on separate cable run from the PIC-4704.
To determine the number and type of power supplies a system requires, add up
the number of Unit Loads (1 Unit Load=50 mA).
• PIC-4704=2 Unit Loads
• Four TR-50s=1 Unit Load
• MA-704=4 Unit Loads
• AX-704=3 Unit Loads
A PS-702 has enough capacity for 24 unit loads.
SYSTEM ARCHITECTURE
Two basic cabling methods for connecting the system may be used: “daisy-chain”
(or loop-through) and “hub.” Both methods may be combined in any system.
Since the PIC-4704 has only two IFB line connectors, a hub-type system is
limited to two branches unless a special splitter box is used. Generally,
resistance-buildup effects and resultant voltage drop are worse when using the
daisy-chain approach. The hub approach minimizes voltage-drop effects at the
expense of greater cumulative cable capacitance. Cable capacitance is not quite
the problem it is in regular intercom systems, because there is no sidetone null
change, only a degradation of high-frequency response.
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