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Model 45 User Guide Issue Preliminary 2, March 2009
Studio Technologies, Inc. Page 19
the Model 45’s 2-wire-to-4-wire converter
circuitry. These audio artifacts will not
cause any damage but will be sent out of
the 4-wire line outputs. Users of the 4-wire
equipment may be less than pleased with
what they hear!
4-Wire Mute during Auto Null
As part of the auto nulling process audio
tones are generated and sent out the
2-wire intercom interfaces. These tones,
depending on the nulled state of the
hybrid circuits, will also appear on the
outputs of the 4-wire interface circuits. To
prevent these unwanted tones from being
sent to the inputs of the connected 4-wire
devices by default the 4-wire outputs mute
when the auto nulling process is active.
There may be cases, primarily at the fac-
tory, where it’s desirable to have the null-
ing tones appear on the 4-wire outputs.
Configuration DIP switch 5 allows the mut-
ing of the 4-wire outputs to be disabled.
When switch 5 is in its off (down) posi-
tion the Model 45 operates in its standard
fashion. When switch 5 is in its on (up)
position the 4-wire outputs will not mute
during the auto nulling process. As ex-
pected, when the Model 45 is deployed
in the field switch 5 should remain in its
off (down) position. However, no damage
to the Model 45 or connected equipment
will occur when muting during auto null is
disabled.
Operation
Upon power-up the Model 45 will always
go through a short initialization sequence
before normal operation will begin. The
LEDs will light sequentially and, upon
completion, the applicable input power
LED will light. The null settings for the two
hybrid circuits are recalled and used, hav-
ing been previously stored in nonvolatile
memory.
Significant user intervention is typically not
required during normal Model 45 opera-
tion. The unit is designed for continuous
operation with no routine adjustment or
maintenance necessary. Assuming that
the desired configuration has been select-
ed using the DIP switches, only occasion-
ally activating the auto null function should
be necessary. The latter may be warranted
should connected user devices or wiring
associated with the 2-wire party-line inter-
com circuit be changed.
External 24 Volt DC Power
Source
In many applications an external source
of 24 volts DC will be connected to the
Model 45. Typically this will be in the form
of the external power supply provided
with the unit by Studio Technologies. The
24VDC input power status LED will light
whenever the external source is providing
power for the unit’s circuitry. In this sce-
nario the Model 45 can be configured as
to whether or not it acts as a power source
for the 2-wire party-line interface circuit.
When the Model 45 is not Serving as
the 2-Wire Intercom Power Source
When an external source of 24 volts DC is
connected and the Model 45 is configured
to not provide 2-wire party-line intercom
power (configuration DIP switch 2 set to
off), the unit will not provide power to, nor
take power from, pin 2 of the 2-wire (PL)
connectors. The 4-wire inputs and out-
puts, as well as the internal power supply,