Studio Technologies 47 - Manual (Page 23)

Model 47 User Guide Issue 1, August 2010
Studio Technologies, Inc. Page 23
External Power Source
When set for an external 2-wire power
source, the Model 47 will not provide
power on pins 2 and 3 of the 2-wire con-
nectors, nor will the Model 47 use any
power from a connected intercom circuit.
As expected, audio signals will be sent
from, and received by, each interface’s
two audio circuits. In this mode of opera-
tion the Model 47 acts as a typical user
station on the connected intercom circuit,
albeit without drawing any power.
As a connection confirmation, hardware
and software in the Model 47 monitors
pins 2 and 3 of the 2-wire PL intercom
connectors for the presence of DC volt-
age. A level greater than approximately
18 volts DC on pin 2 will cause the pin 2
status LED to light. A level greater than
18 volts DC on pin 3 will cause the pin 3
status LED 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
external 2-wire power mode the Model 47
is simply an observer; whether intercom
power is present on pin 2, pin 3, or both
pins 2 and 3 is not significant.
So that the Model 47’s 2-wire-to-4-wire
interface circuits remain stable and don’t
generate audio artifacts, an auto terminate
feature is implemented. This maintains a
200 ohm termination on both pins 2 and 3
so long as neither of the 2-wire power sta-
tus LEDs are lit. To clarify, if either, or both,
LEDs are lit then both terminations are re-
moved. 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 47’s termination is “lifted.”
Internal Power Sources
When an interface’s front-panel DIP switch
is set for internal 2-wire power sources
the Model 47 supplies 30 volt DC on pins
2 and 3 of the output connector. A maxi-
mum current draw of 300 milliamperes
is available on each pin. This current is
sufficient to power various intercom user
devices such as small user stations and
belt packs. A common broadcast appli-
cation would be to use RTS BP325 belt
packs. Select the connected devices so
that their total current doesn’t exceed 300
milliamperes. That’s not always the easi-
est figure to calculate but a web search
will generally find specifications for all
commonly used devices. For example, a
search finds that the original version of
the BP325 consumes a maximum of 85
milliamperes of current. According to this
figure up to three of these units can be
connected to each Model 47 interface.
Newer versions of the BP325 (production
date of 1992 and later) use surface mount
component technology and have a lower
maximum current draw of 65 milliamperes.
Four of these “modern” BP325 units can
easily be supported.
The Model 47’s intercom power supply
circuits operate under software control.
This allows detection of fault conditions
and protection of both the Model 47’s
circuitry and connected intercom user
devices. Upon initial Model 47 power-
up no monitoring of the intercom power
sources takes place for a period of three
seconds. This allows the Model 47’s
circuitry and the connected intercom user