Studio Technologies 76 V.1.04 - Manual (Page 11)

Model 76/77 User Guide Issue 4, September 2008
Studio Technologies, Inc. Page 11
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
covering all nine pins, a minimum of
only four connections are required: pin 1
(data+), pin 6 (data–), pin 4 (DC+), and
pin 9 (DC–). The Model 76’s connector
pin-out scheme was designed to allow
creation of an interconnecting cable which
uses commonly available 2-pair audio
cable. This cable, consisting of two twisted
pairs each with an individual shield, is typi-
cally sleek, fl exible, 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 connec-
tions. This implementation has the advan-
tages of providing a shield for the data
path and a more robust common connec-
tion (two conductors including the shield)
for the DC power circuit. Refer to Figure 6
for details.
A few simple calculations are required
to determine the maximum cable length
when connecting a Model 77 to a Model
76. 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 76 to the Model 77. The Mod-
el 76 supplies 12 volt DC nominal with a
maximum current of 200 milliamperes. The
Model 77 requires a minimum of 9 volts
DC, 100 milliamperes maximum 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 76
supplies 12 volts and the Model 77 re-
quires 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 0.01
(the square of the Model 77’s maximum
required current). Remember to include the
resistance in both the DC+ and DC– wires
in the fi gure for the total resistance.
AC Mains Power
The Model 76 is internally confi gured to
operate from nominal 100, 120, or 220/240
volts, 50/60 Hz. Units shipped to North
America are factory selected for 120 volt
operation. Units bound for Japan are
generally selected for 100 volts while our
friends “down under” and in Europe receive
units set for 220/240 volts. Before con-
necting the Model 76 to AC mains power,
check to be certain that it is confi gured to
match the local mains voltage. Look on the
back panel (adjacent to the power entry
connector) for an indication of the factory-
confi gured voltage. Note that an incorrect
Signal Pin Direction
Data+ 1 To/From Model 77
Data– 6 To/From Model 77
Data Shield 2 To/From Model 77
DC+ 4 To Model 77
DC– 9 To Model 77
DC Power Shield 5 To/From Model 77
Note: Connector type on Model 76 is 9-pin D-subminiature
female. Connector uses 4-40 threaded inserts for
locking with mating plug.
Figure 6. Connections between Model 76 and
Model 77