Studio Technologies 761 - Manual (Page 13)

Model 761/771 User Guide Issue 1, March 2008
Studio Technologies, Inc. Page 13
for Surround
mon connection as well as the Model 761’s
chassis and mains earth connection. The
two spare inputs are provided for future
special applications. As such, pins 8 and 9
should remain unconnected.
Connecting the Model 761
to the Model 771
A 9-pin female D-subminiature connector,
labeled To/From Control Console, is pro-
vided on the back panel of the Model 761
Central Controller. Another 9-pin D-sub
connector, labeled To/From Central Con-
troller, is provided on the back panel of
the Model 771 Control Console. A cable
with 9-pin male D-sub connectors on each
end is used to interconnect the two units. A
20-foot (6.1-meter) cable is included in the
shipping carton of each system. This cable
implements all nine connector pins in a
one-to-one manner.
Should an interconnecting cable of a
different length be required there’s no
problem for one to be fabricated and used.
While it can be wired in a one-to-one fash-
ion covering all nine pins, a minimum of
only four connections are required: data+,
data–, DC+, and DC–. The Model 761’s
connector pin-out scheme was designed
to allow creation of an interconnecting
cable which uses commonly available two-
pair audio cable. This cable, consisting of
two twisted pairs each with an individual
shield, is often sleek, fl exible, and available
in many colors. One pair and shield can
be used for the data connections and 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 common
connection (two conductors including the
shield) for the DC power circuit. Refer
to Figure 7 for details.
It’s hard to specify a maximum intercon-
necting cable length. The data connections
won’t be the limiting factor as the differ-
ential transmission scheme of an RS-485
interface makes an interconnection in
excess of 1000 feet (300+ meters) eas-
ily possible. The culprit is the DC current
supplied by the Model 761 to power the
Model 771. This nominal 12 volt DC, 100
milliamperes maximum signal is subject
to the resistive voltage losses associated
with the interconnecting cable. For correct
Model 771 performance the voltage sup-
plied to the Model 771, when measured
directly at the Model 771’s 9-pin connector
and at full load, must be 9 volts minimum.
This requires that the voltage drop due to
the interconnecting cable be no more than
3 volts DC. Using Ohm’s law, it’s quite easy
to determine whether the selected cable
will support the desired interconnection
length. To calculate the voltage drop, mul-
tiply the total resistance (in ohms) of the
proposed cable by 0.01 (the square of the
maximum current). Remember to include
the resistance in both the DC– and DC+
wires.
Signal Pin Direction
Data+ 1 To/From Model 771
Data– 6 To/From Model 771
Data Shield 2 To/From Model 771
DC+ 4 To Model 771
DC– 9 To Model 771
DC Power Shield 5 To/From Model 771
Note: Connector type on Model 761 is 9-pin D-subminiature
female. Connector uses 4-40 threaded inserts for
locking with mating plug.
Figure 7. Connections between Model 761 and
Model 771