Studio Technologies 47A - Manual (Page 22)

Issue 1, September 2014 Model 47A User Guide
Page 22 Studio Technologies, Inc.
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 47A 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 47A 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 ex-
ternal 2-wire power mode the Model 47A
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 47A’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 47A’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 47A supplies 30 volt DC on pins 2
and 3 of the output connector. A maximum
current draw of 315 mA is available on
each pin. This current is sufficient to power
various intercom user devices such as
small user stations and beltpacks. A com-
mon broadcast application would be to use
RTS BP325 beltpacks. Select the con-
nected devices so that their total current
doesn’t exceed 315 mA. That’s not always
the easiest 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 mA of
current. According to this figure up to three
of these units can be connected to each
Model 47A 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 mA. Four of these “modern” BP325
units can easily be supported.
The Model 47A’s intercom power supply
circuits operate under software control.
This allows detection of fault conditions
and protection of both the Model 47A’s
circuitry and connected intercom user
devices. Upon initial Model 47A power-up
no monitoring of the intercom power sourc-
es takes place for a period of three
seconds. This allows the Model 47A’s cir-
cuitry and the connected intercom user de-
vices to stabilize. The LEDs associated with
pins 2 and 3 will light to indicate that the
outputs are active. After this initial 3-second
period monitoring becomes active. A
fault condition is detected if the power
on a pin falls below 24 volts for a continu-
ous 1-second interval. The hardware and