Clear-Com TW-12C - Manual (Page 9)

S y s t e m I n s t a l l a t i o n M a n u a l
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CALL SIGNALLING
The TW-12C interface receives Call signalling in the appropriate form (DC level
for Clear-Com, 20k Hz tone for RTS), decodes it, and passes it as logic to the
other side of the interface, via an optical coupler. The receiving side of the
interface sends the signal appropriate for the intercom type that was selected via
the front panel. DC isolation of the Call signal between the two sides is obtained
through the use of the optical coupler.
SYSTEM POWER
Each side of the interface obtains its DC power through Pin 2 of the intercom
connector for that side, through an active impedance buffer that lets the line be
used as an intercom line (in the 2-wire format). Each side of the interface is
powered from the intercom system to which it is connected. If either side is
accidentally disconnected, that side “powers down”, providing no audio feed to
the other side and therefore not disturbing the impedance of the system that
remains connected. No termination sensing or switching is necessary to prevent
the remaining portion of the interface from oscillating. The TW-12C front panel
provides an LED POWER indicator for each side of the interface, which shows
that a powered intercom is connected to either or both sides.