Studio Technologies 47 - Manual (Page 18)

Issue 1, August 2010 Model 47 User Guide
Page 18 Studio Technologies, Inc.
Auto Terminate
The auto terminate function is designed to
ensure that each 2-wire-to-4-wire interface
circuit remains stable under most oper-
ating conditions. Specifically, 200 ohm
impedances are automatically applied to
both pins 2 and 3 of a Model 47 2-wire
party-line interface when that interface is
configured for external power and no ex-
ternal source of intercom power is detect-
ed on pins 2 or 3. 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 connected to the Model 47. In this lat-
ter case only one of the Model 47’s audio
channels will be used.
There may be situations where it’s neces-
sary for the two channels associated with
each Model 47 interface 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.
Adapter cables as shown in Figure 2
would be used. In this case the intercom
circuit’s DC power will not be connected
to the Model 47, but only to the user
devices. In this scenario, a DC voltage
won’t be present on pins 2 or 3. Another
example of this might be where adapter
boxes with DC blocking are used to inter-
connect intercom circuits with the Model
47. In both these cases a DC voltage
won’t be present on either pins 2 or 3 and
the Model 47 will automatically apply 200
ohm terminations. In both these cases the
termination will be in error and will result in
incorrect audio levels due a double-termi-
nation condition.
To prevent this problem the auto termi-
nate function can be disabled. Referring
to Figure 7, when DIP switch 1 is in its off
(down) position the auto terminate func-
tion is active. When DIP switch 1 is in its
on (up) position the auto terminate func-
tion is disabled. Repeating for clarity, the
auto terminate function applies to both
interfaces 1 and 2. With the auto terminate
function disabled the two interfaces, when
set for external power, will operate nor-
mally with the exception that pins 2 and 3
will never be terminated by the Model 47’s
circuitry.
Figure 6. Detail of back panel showing the
Advanced Mode DIP switches
Figure 7. DIP Switch 1—Auto Terminate