Studio Technologies 76D V.2.02 - Manual (Page 13)

Model 76D/77 User Guide Issue 2, June 2009
Studio Technologies, Inc. Page 13
for Surround
access to an RS-485 data interface. This
interface is not supported in the Model 76D
and, as such, pins 7 and 2 should remain
unterminated.
Connecting the Model 76D to
the Model 77
A 9-pin female D-subminiature connector,
labeled To/From Control Consoles, is pro-
vided on the back panel of the Model 76D
Central Controller. This is used to interface
the unit with Model 77 Control Consoles.
A 9-pin female D-sub connector, labeled
To/From Central Controller, is provided on
the back panel of each Model 77 Control
Console. A cable with 9-pin male D-sub con-
nectors on each end is used to interconnect
the Model 76D with the Model 77 units. A
cable is included in the shipping carton. The
cable implements all nine connector pins in
a one-to-one manner.
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 support-
ing 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 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, 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 connec-
tions. This implementation has the advan-
tages of providing a shield for the data path
and a more robust common connection (two
conductors including the shield) for the DC
power circuit. Refer to Figure 6 or Appendix
A for details.
A few simple calculations are required to
determine the maximum cable length when
connecting a Model 77 to a Model 76D.
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 the Model 77. The Model
76D supplies 12 volt DC with a maximum
current of 500 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 interconnect-
ing 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 interconnecting cable. Us-
ing Ohm’s law it’s quite easy to determine
whether the selected cable will support the
desired interconnection length. Calculate
the voltage drop by multiplying the total
resistance (in ohms) of the proposed cable
by 0.1 (the Model 77’s maximum required
current in amperes). Remember to include
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 D-subminiature
female (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