Studio Technologies 76DA V.4.02 - Manual (Page 15)

Model 76DA/77 User Guide Issue 4, March 2013
Studio Technologies, Inc. Page 15
for Surround
can be wired in a one-to-one fashion sup-
porting all nine pins, only four connections
are required: pin 1 (data +), pin 6 (data –),
pin 4 (DC +), and pin 9 (DC –). The Model
76DA’s connector pin-out scheme was de-
signed to allow creation of an interconnect-
ing cable which uses commonly available
2-pair audio cable. This cable, consisting
of two twisted pairs each with an individual
shield, is typically sleek, flexible, and avail-
able in many colors. One pair and shield
can be used for the data connections while
the other pair and shield can be used for
the DC connections. This implementation
has the advantages of providing a shield
for the data path and a more robust com-
mon connection (two conductors including
the shield) for the DC power circuit.
A few simple calculations are required to
determine the maximum cable length when
connecting a Model 76DA to a Model 77.
The differential transmission scheme used
by the system’s RS-485 interface makes
an interconnection in excess of 1000 feet
(>300 meters) easily possible. The limiting
factor is typically the ability of the wiring to
pass the DC power supplied by the Model
76DA to a Model 77. The Model 76DA
supplies 12 volt DC with a maximum
current of 500 milliamperes.
The Model 77 requires a minimum of
9 volts DC, 100 milliamperes, for correct
operation. (The voltage must be measured
directly at the Model 77’s 9-pin connector.)
So the maximum interconnecting cable
length is directly related to the resistive
voltage losses associated with the two
DC-carrying conductors. As the Model
76DA supplies 12 volts and the Model
77 requires 9 volts minimum, this directly
leads to a 3 volt DC maximum drop due
to the interconnecting cable. Using Ohm’s
law it’s quite easy to determine whether
the selected cable will support the desired
interconnection length. Calculate the volt-
age drop by multiplying the total resistance
(in ohms) of the proposed cable by 0.1 (the
Model 77’s required current in amperes).
Remember to include the resistance in
both the DC + and DC – wires when calcu-
lating the voltage drop. If it’s greater than
3 volts your cable is too long or the wire
gauge is too small.
Additional Control Consoles
Some installations may benefit from the
Model 76DA’s ability to be controlled by
additional control consoles. At least one
Model 77 Control Console must be con-
nected to the Model 76DA Central Control-
ler. After this requirement has been met up
to three additional Model 77 or Model 71
Control Consoles can also be connected
and powered by the Model 76DA.
When connecting multiple control consoles
to a Model 76DA all nine pins of each
interconnecting cable can be connected in
parallel (“multed”). Using this arrangement
the data and 12 volt DC power signals be-
tween all the units will be multed. A custom
cable implementation requires just four
pins to be connected: pin 1 (data +), pin 6
(data –), pin 4 (DC +), and pin 9 (DC –).
To make installation simple, a “bus” cable
assembly can be created using a short
length of ribbon cable with one male and
multiple female 9-pin D-subminiature insu-
lation-displacement connectors attached.
Then standard 9-pin cables can link the
control consoles with the connectors on
the bus cable.
Refer to the previous paragraphs of this
user guide where the issues involving
Model 76DA to Model 77 cable length are
discussed. Note the required current for a