Studio Technologies 46A - Manual (Page 22)

Issue 1, September 2014 Model 46A User Guide
Page 22 Studio Technologies, Inc.
DC on pin 2 will cause the status LED la-
beled pin 2 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 de-
vices almost always draw power from pin 2
rather than pin 3. However, in many broad-
cast applications, power is provided on
all intercom paths so that flexible channel
assignments can be made. In the external
2-wire power mode the Model 46A is sim-
ply 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 46A’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 is lit. To clarify, if either, or both,
the LEDs are lit then both terminations 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 46A’s termination is “lifted.”
Internal Power Source
When an interface’s front-panel DIP switch
is set for internal 2-wire power the Model
46A’s 2-wire party-line interface supplies 30
volt DC on pin 2 of the output connector. A
maximum current draw of 315 mA is avail-
able. This current is sufficient to power vari-
ous 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 current doesn’t
exceed 315 mA. That’s not always the easi-
est figure to calculate but a web search will
generally find specifications for all com-
monly 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 46A
interface. Newer versions of the BP325 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 46A’s intercom power supply
circuits operate under software control.
This allows detection of fault conditions
and protection of both the Model 46A’s
circuitry and connected intercom user de-
vices. Upon initial Model 46A power up no
monitoring of the intercom power output
takes place for a period of three seconds.
This allows the Model 46A’s circuitry and
the connected intercom user devices to
stabilize. The LED associated with pin 2 will
light to indicate that the output is active.
After this initial 3-second period monitoring
becomes active. A fault condition is de-
tected if the power on pin 2 falls below 24
volts for a continuous one second inter-
val. The hardware and software responds
to this condition by turning off the power
source to pin 2 and flashing the pin 2 LED
as a warning. After a 6-second “cool-down”
interval the output returns to the same con-
dition as upon initial power up: power is
again applied to pin 2, the pin 2 status LED
will light, and monitoring won’t begin for
another three seconds. A full short-circuit
condition applied to the Model 46A’s 2-wire
connectors will result in a continuous cycle
of 4-seconds on (three seconds for start
up and one second for detection) and
6-seconds off.