Studio Technologies 76DA V.3.01 - Manual (Page 14)

Issue 2, April 2011 Model 76DA/77 User Guide
Page 14 Studio Technologies, Inc.
for Surround
and 9). These are provided for future ap-
plications and should remain unconnected.
This connector also allows access to an
RS-485 data interface. This interface is
not supported in the Model 76DA and,
as such, pins 7 and 2 should remain
unterminated.
Connecting the Model 76DA
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 76DA
Central Controller. This is used to interface
the unit with Model 77 Control Consoles.
Refer to Figure 7 or Appendix A for details.
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
(DE-9M) connectors on each end is used
to interconnect the Model 76DA 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
supporting all nine pins, only four
connections are required: pin 1 (data +),
pin 6 (data –), pin 4 (DC +), and pin 9
(DC –). The Model 76DA’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 typically
sleek, flexible, and available in many col-
ors. 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.
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 volt 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 interconnecting cable. Using Ohm’s
law it’s quite easy to determine whether
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 D-subminiature
female (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