Manual

Issue 3, August 2015 Model 45A User Guide
Page 20 Studio Technologies, Inc.
Operation
Upon power-up the Model 45A will always
go through a short initialization sequence
before normal operation will begin. The
LEDs will light sequentially and, upon com-
pletion, the applicable input power LED will
light. The null settings for the two hybrid
circuits are recalled and used, having been
previously stored in nonvolatile memory.
Significant user intervention is typically not
required during normal Model 45A opera-
tion. The unit is designed for continuous
operation with no routine adjustment or
maintenance necessary. Assuming that the
desired configuration has been selected
using the DIP switches, only occasionally
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 45A. 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 scenar-
io the Model 45A 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 45A is Not Serving as
the 2-Wire Intercom Power Source
When an external source of 24 volts DC is
connected and the Model 45A 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
,
are fully isolated from the 2-wire intercom
circuit. In this situation the Model 45A’s
only function is to route, by way of the hy-
brids, the 2-wire interface’s audio signals
to and from the 4-wire outputs and inputs.
In this mode of operation the Model 45A
acts as a typical user station on the con-
nected intercom circuit.
As a connection confirmation, hardware
and software in the Model 45A monitors
pins 2 and 3 of the 2-wire (PL) intercom
connectors for the presence of DC volt-
age. A level greater than approximately
20 volts DC on pin 2 will cause the status
LED labeled PIN 2 to light. A level greater
than 20 volts DC on pin 3 will cause the
status LED labeled PIN 3 to light.
The author is aware that user intercom
devices almost always draw power from
pin 2 rather than pin 3. However, in many
broadcast applications power is provided
on all intercom paths so that flexible chan-
nel assignments can be made. In the ex-
ternal 2-wire power mode the Model 45A
is simply an observer; whether intercom
power is present on pin 2, pin 3, or both
pins 2 and 3 is not significant.
In order that the Model 45A’s 2-wire-to-
4-wire interface circuits remain stable and
don’t generate audio artifacts an auto ter-
minate feature is implemented. This main-
tains a 200 ohm termination on both pins
2 and 3 as long as neither of the 2-wire
power status LEDs is lit. To clarify, if either
or both the LEDs are lit then both termi-
nations are removed. It was felt that this
method would provide a fairly accurate