Studio Technologies 45A - Manual (Page 18)

Issue 2, November 2011 Model 45A User Guide
Page 18 Studio Technologies, Inc.
Auto Null Button Operation
A pushbutton switch, located on the Mod-
el 45A’s front panel, is used to activate
the auto nulling functions. A configuration
parameter allows selection of the button’s
operation. The choices are dual auto null
mode and independent auto null mode.
When dual auto null mode is selected a
single “tap” (press and release) on the
pushbutton switch initiates a routine that
begins with channel 1’s auto null se-
quence, followed immediately by channel
2’s auto null sequence.
The independent auto null mode allows
the auto null function to be initiated for
each channel as desired. A single “tap”
will start the auto null routine for channel
1; two “taps” will start the routine for
channel 2.
When configuration DIP switch 3 is in
its off (down) position the dual auto null
mode is selected. This is provided spe-
cifically for cases where the Model 45A’s
2-wire party-line interface will be used with
dual-channel intercom user devices. As an
example: the RTS TW-series provides two
independent audio channels, as well as
power, over a single 3-conductor cable.
In a case such as this it’s useful for both
of the hybrid circuits associated with an
interface to be auto nulled at approxi-
mately the same time. The dual auto null
mode allows a single “tap” of the button to
initiate nulling of both hybrid circuits.
When configuration DIP switch 3 is in its
on (up) position the independent auto
null mode is selected. This is appropriate
for applications in which the two audio
paths associated with the Model 45A’s
intercom interface are used with inde-
pendent party-line intercom circuits. This
situation might arise when two Clear-Com
single-channel intercom circuits are con-
nected to the Model 45A. The hybrid cir-
cuit associated with each channel can be
auto nulled as desired. Another example
would be in an RTS TW-series application
that uses source assignment panels. The
“SAP” panels would be used to route mul-
tiple intercom channels to various sets of
user devices as desired. In this case, the
two channels associated with the Model
45A’s interface will often end up not rout-
ed to the same user device. Independent
auto nulling is certainly desired.
Auto Terminate Disable
The auto terminate function is designed
to ensure that each 2-wire-to-4-wire in-
terface circuit remains stable under most
operating conditions. It can only be active
when the Model 45A is not serving as the
intercom power source. The auto termi-
nate function applies 200 ohm imped-
ances to both pins 2 and 3 of the 2-wire
party-line interface when no source of
DC power is detected on either of those
pins. This function is directly compatible
with RTS TW-series intercom applications
in which the three conductors of a cable
support both DC power and two channels
of audio. It’s also compatible in situations
where all three conductors of a Clear-Com
single-channel intercom circuit are con-
nected to the Model 45A. In this latter case
only one of the Model 45A’s audio chan-
nels will be used.
There may be situations where it’s neces-
sary for the two channels associated with
the Model 45A’s interface to be used with
separate 2-wire party-line circuits. An ex-
ample of this might be where two “loops”
of single-channel belt packs, such as units
from Clear-Com, need to be connected. In
this case the intercom circuit’s DC power