Studio Technologies 45A - Manual (Page 24)

Issue 2, November 2011 Model 45A User Guide
Page 24 Studio Technologies, Inc.
pins 2 and 3 of the 2-wire (PL) intercom
connector are monitored for the presence
of DC voltage. If a level of greater than
approximately 18 volts DC is not detected
on either pin, 200 ohm terminating net-
works are applied to those same pins. This
ensures that the Model 45A’s hybrid circuit-
ry remains stable, preventing objectionable
audio signals from being sent to the 4-wire
output connectors. One caveat does ap-
ply: there may be a few seconds of severe
noise whenever an interface moves from a
DC present condition to a DC not present
condition. But other than that period, the
audio paths will remain “clean.”
As a visual aid, LEDs on the front panel will
display the DC power status of pins 2 and
3. But when auto terminate disable mode is
active (configuration DIP switch 4 set to on)
the LEDs will no longer indicate the inter-
com circuit’s termination status.
For special applications the auto terminate
function can be disabled. This will primarily
come into play when “splitting” the Model
45A’s resources into two separate single-
channel interfaces. Moving configuration
DIP switch 4 to its on (up) position disables
the auto terminate function. To someone
observing only the Model 45A’s front panel
this change would not be readily apparent.
The front-panel LEDs will continue to dis-
play the DC power status of pins 2 and 3.
But whether or not either or both LEDs are
lit, the Model 45A will never apply 200 ohm
terminations to pins 2 or 3. For the hybrid
circuits to remain stable termination imped-
ances must be provided by the connected
circuits. If these are not present one might
find the hybrid circuits generating very
unpleasant audible noise. This condition,
caused by the 2-wire output circuitry be-
ing in an “unloaded” state, will not damage
the Model 45A’s circuitry. However, without
the auto terminate function a disconnected
cable or other real-world wiring problem
could present users with a rude surprise!
In conclusion, it’s important that technical
personnel working with the Model 45A be
informed when the auto termination func-
tion has been disabled. They will then be
aware of the potential noise issues and be
ready to make corrections should a prob-
lem arise.
4-Wire Mute During Auto Null
Normally, the 4-wire outputs are muted
whenever the auto null process is taking
place. This ensures that the test tones gen-
erated as part of the nulling process will not
be heard by users of the signals associated
with the 4-wire outputs. For special appli-
cations configuration DIP switch 5 allows
the automatic muting mode to be disabled.
This mode is provided primarily for use by
factory personnel or during demonstra-
tion use. During normal operation switch
5 should remain in its off (down) position.
When switch 5 is in its on (up) position
4-wire muting during auto null will not take
place. While not appropriate during actual
Model 45A use, it is interesting to “hear”
the nulling process take place. But unless
there’s a really good reason, configuration
DIP switch 5 should always remain in its off
(down) position.
Technical Notes
Software Version Display
A special Model 45A power-up sequence
allows the unit’s software version number
to be displayed. This is useful when work-
ing with factory personnel on application