Manual

Model 47A User Guide Issue 2, January 2020
Studio Technologies, Inc. Page 23
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 volts DC on pins
2 and 3 of the output connector. A maxi-
mum 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
common broadcast application would be
to use RTS BP325 beltpacks. Select the
connected devices so that their total cur-
rent doesn’t exceed 315 mA. That’s not
always the easiest figure to calculate but
a web search will generally find specifica-
tions for all commonly used devices. For
example, a search finds that the original
version of the BP325 consumes a maxi-
mum 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
sources takes place for a period of three
seconds. This allows the Model 47A’s cir-
cuitry and the connected intercom user
devices 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 continuous 1-second interval. The
hardware and software responds to this
condition by turning off the power source
on that specific pin and flashing its associ-
ated LED as a warning. After a 6-second
“cool-down” interval the output returns to
the same condition as upon initial power-
up: power is again applied to the pin, the
pin status LED will light, and monitoring
won’t begin for another three seconds. A
full short-circuit condition applied to the