Tandy 1000
Telephone Interface Section
Technical Reference Manual
The telephone interface section consists of a coupling transformer, two sets
of relay contacts, transient suppressors, and a ring detector.
The coupling transformer isolates modem ground and passes tone signals
through
in
both directions.
It
also protects the telephone line from high voltage
due to
its
breakdown characteristics.
As
mentioned before,
K2
is
the hook relay, and
K1
is
the pulse dial relay. When
the modem
is
ON-LINE,
K2
is
on, shorting out Pins 1 and
8.
(Also notice that
it
shorts out 9 and 15.) Assuming that you are not pulse dialing at this time,
K1
is
not active, and a connection occurs between the contacts of K2 and
the primary of
T1
.
When the modem pulse dials,
K1
switches
on
and
off,
breaking and making
the
T1
connection to
TIP
and RING. The Central Office detects these makes
and breaks, and the number
is
pulse-dialed.
R9
and C36 form a stubbing net-
work for protection of
K1
's contacts. CR2 prevents harmful voltage transients
from being passed to the telephone line.
Bridge rectifier CR8 accomplishes ring detection. When ring voltage
is
applied
to
TIP
and RING, CR8 full-wave rectifies the sine wave and applies
DC
to filter
C12. This voltage
is
allowed to turn
on
transistor 01 which,
in
turn, lights the
LED within opto-isolator
US.
The
transistor inside
US
follows the pulsating ring
vo!tage and outputs a 5.0 volt square wave to
Pin
19 of the microprocessor.
The
microprocessor makes the decision whether or not to activate K2 and
answer the call.
89
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