Studio Technologies 45A - Manual (Page 20)

Issue 2, November 2011 Model 45A User Guide
Page 20 Studio Technologies, Inc.
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 outputs,
as well as the internal power supply, are
fully isolated from the 2-wire intercom cir-
cuit. In this situation the Model 45A’s only
function is to route, by way of the hybrids,
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 connected inter-
com 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 voltage.
A level greater than approximately 18 volts
DC on pin 2 will cause the status LED
labeled PIN 2 to light. A level greater than
18 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
indication that an active intercom circuit
is connected. In this case the 200 ohm
termination is expected to be provided
elsewhere, typically as part of the external
power supply, and the Model 45A’s termi-
nation is “lifted.”
When the Model 45A is Serving as the
2-Wire Intercom Power Source
When configuration DIP switch 2 is set
so that the Model 45A provides 2-wire
(PL) power its interface supplies 30 volts
DC on pin 2 of the 2-wire (PL) intercom
connectors. A maximum current draw of
300 milliamperes is available. This current
is sufficient to power various intercom
user devices such as small user stations
and belt packs. A common broadcast