Manual

Issue 2, January 2020 Model 47A User Guide
Page 18 Studio Technologies, Inc.
Auto Terminate
The auto terminate function is designed
to ensure that each 2-wire-to-4-wire in-
terface circuit remains stable under most
operating conditions. Specifically, 200 ohm
impedances are automatically applied to
both pins 2 and 3 of a Model 47A 2-wire
party-line interface when that interface is
configured for external power and no exter-
nal source of intercom power is detected
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 47A. In this lat-
ter case only one of the Model 47A’s audio
channels will be used.
There may be situations where it’s neces-
sary for the two channels associated with
each Model 47A interface be used with
separate 2-wire party-line circuits. An ex-
ample of this might be where two “loops”
of single-channel beltpacks, 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 47A, but only to the user devices. In
this scenario, a DC voltage won’t be pres-
ent on pins 2 or 3. Another example of this
might be where adapter boxes with DC
blocking are used to interconnect intercom
circuits with the Model 47A. In both these
cases a DC voltage won’t be present on
either pins 2 or 3 and the Model 47A will
automatically apply 200 ohm terminations.
In both these cases the termination will be
in error and will result in incorrect audio lev-
els due a “double-termination” 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 normal-
ly with the exception that pins 2 and 3 will
never be terminated by the Model 47A’s
circuitry.
The auto terminate function should be
disabled only when absolutely necessary;
it’s possible that a significant downside
could be experienced. With auto terminate
disabled it’s important that properly ter-
minated 2-wire party-line intercom circuits
be connected to pins 2 or 3 on the Model
47A’s 2-wire PL connectors. If they are not
connected, it’s likely that audio oscilla-
tions, noise, and distortion will be gener-
ated in the Model 47A’s 2-wire-to-4-wire
converter circuitry. These audio artifacts
will not cause any damage, but will be
sent out the 4-wire line output connectors.
Users of the 4-wire equipment may be less
than pleased with what they hear!
Figure 7. DIP Switch 1—Auto Terminate