Studio Technologies 76D 2014 - Manual (Page 14)

Issue 11, May 2014 Model 76D/77 User Guide
Page 14 Studio Technologies, Inc.
for Surround
Should an interconnecting cable of a differ-
ent length be required there’s no problem
for one to be fabricated and used. While it
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
76D’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 76D 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
76D to a Model 77. The Model 76D supplies
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 76D
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 76D’s ability to be controlled by addi-
tional control consoles. At least one Model
77 Control Console must be connected
to the Model 76D Central Controller. After
this requirement has been met up to three
additional Model 77 or Model 71 Control
Consoles can also be connected to and
powered by the Model 76D.
When connecting multiple control consoles
to a Model 76D all nine pins of each in-
terconnecting 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
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 76D is 9-pin female
D-subminiature (DE-9F). Connector uses 4-40
threaded inserts for locking with mating plug.
Figure 6. Connections between Model 76D and
Model 77 and Model 71