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Model 760-03/77 User Guide Issue 4, October 2016
Studio Technologies, Inc. Page 15
for Surround
commonly available 2-pair audio cable. This
cable, consisting of two twisted pairs each
with an individual shield, is typically sleek,
flexible, and available 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 pro-
viding a shield for the data path and a more
robust common connection (two conduc-
tors including the shield) for the DC power
circuit.
A few simple calculations are required to
determine the maximum cable length when
connecting a Model 760-03 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
760-03 to a Model 77. The Model 760-03
supplies 12 Vdc with a maximum current of
500 milliamperes (mA).
The Model 77 requires a minimum of 9 Vdc,
100 mA, 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 re-
lated to the resistive voltage losses associ-
ated with the two DC-carrying conductors.
As the Model 760-03 supplies 12 V and the
Model 77 requires 9 V minimum, this di-
rectly leads to a 3 Vdc 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 calculating
the voltage drop. If it’s greater than
3 V your cable is too long or the wire
gauge is too small.
Additional Control Consoles
Some installations may benefit from the
Model 760-03’s ability to be controlled by
additional control consoles. At least one
Model 77 Control Console must be con-
nected to the Model 760-03 Central Con-
troller. After this requirement has been met
up to three additional Model 77 or Model
71 Control Consoles can also be connect-
ed and to powered by the Model 760-03.
When connecting multiple control consoles
to a Model 760-03 all nine pins of each
interconnecting cable can be connected in
parallel (“multed”). Using this arrangement
the data and 12 Vdc 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 9-pin female 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 760-03 to Model 77 cable length
are discussed. Note the required current
for a Model 77 is 100 mA while a Model
71 requires only 35 mA. It’s important to
review this information prior to creating
the interconnection scheme to be used for
installing multiple Model 77 units.