Studio Technologies 76DA V.4.12 - Manual (Page 15)

Model 76DA/77 User Guide Issue 9, May 2014
Studio Technologies, Inc. Page 15
for Surround
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 sup-
plies 12 volts 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 inter-
connecting cable. Using Ohm’s law it’s quite
easy to determine whether the selected
cable will support the desired interconnec-
tion length. Calculate the voltage 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 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 to 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 volts 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 9-pin female D-subminiature insu-
lation-displacement connectors attached.
Signal Pin Direction
Data + (RS-485) 1 To/From Models 77/71
Data – (RS-485) 6 To/From Models 77/71
Data Shield 2 To/From Models 77/71
DC + (12 V) 4 To Models 77/71
DC – (12 V Return) 9 To Models 77/71
DC Power Shield 5 To/From Models 77/71
Note: Connector type on Model 76DA is 9-pin female
D-subminiature (DE-9F). Connector uses 4-40
threaded inserts for locking with mating plug.
Figure 7. Connections between Model 76DA and
Model 77 and Model 71