Studio Technologies 76 V.1.10 - Manual (Page 11)

Model 76/77 User Guide Issue 7, August 2012
Studio Technologies, Inc. Page 11
for Surround
Console. A cable with 9-pin male D-sub
connectors on each end is used to inter-
connect the Model 76 with the Model 77. A
20-foot (6.1-meter) cable is included in the
shipping carton. The cable implements all
nine connector pins in a one-to-one man-
ner.
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 inter-
connecting 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 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 implementa-
tion has the advantages of providing a
shield for the data path and a more robust
common connection (two conductors in-
cluding 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 Model 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 intercon-
necting 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 select-
ed cable will support the desired intercon-
nection length. Calculate the voltage 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 figure for the
total resistance.
AC Mains Power
The Model 76 is internally configured 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 gener-
ally selected for 100 volts while our friends
“down under” and in Europe receive units
set for 220/240 volts. Before connecting the
Model 76 to AC mains power, check to be
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