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The
cassette-loading circuitry,
internal
to
the
SALT chip,
is
composed
of
a
zero-crossing detector.
Figure
5-13 shows the input from the
cassette being loaded
by
a
220-ohm
resistor,
R14,
then coupled into the
SALT chip through
a
510-ohm resistor,
R18.
R14 serves
to
load the
capacitively-coupled output
characteristic
of
most portable
cassette recorders, and R18 limits
the
current
of the
incoming signal
to
prevent damage
to the
SALT chip
if an
excessively
large
peak-to-peak
voltage
is
fed
into the cassette
input.
Although Tandy's computer
cassette recorders
do not
produce
more
than
6
volts
p-p, the
circuitry
is
protected from voltages
as
high
as
18
volts
p-p.
The
zero-crossing
detector internal
to the
SALT changes
state each time
the
incoming signal
passes through zero volt. There
is a
small amount of
hysteresis built
in
which provides noise immunity and
prevents false triggering of
the
zero-crossing detector.
The output
of the
zero-crossing
detector
is
an LS
TTL-compatible
level and
is
coupled into PIA
IC4.
The final function
of the
SALT chip
is
to
drive
the
cassette relay. A TTL
signal from
PIA
IC4 enters pin
10
of
the
SALT chip where
it is
connected
to the
base of
an
internal Darlington
transistor,
the
emitter
of
which
is
grounded. The collector exits
the
SALT chip
at
pin
9
and
is
connected
to
one end of the
cassette
relay. The
other
end of the relay
connects
to
+5 VDC,
When
the
incoming signal goes
high, the transistor becomes
saturated and connects
its end
of
the
cassette relay
to
ground,
causing
the
relay
to
energize. When
the
incoming
signal
is
low, the
transistor
is
cut
off.
There
is
no ground return
for
the +5
volts
at the
other end of
the
relay, so
it
is
de-energized. Diode
D5
protects
the
transistor
in
the
SALT from the surge current caused
by
the coil of the relay (when the relay
is
de— energized)
.
TO
PIA
IC4-2
TO
PIA
IC4-10
TO
PIA
IC44
FROM
PIA
IC4-3
CASSETTE
DATA
(ZERO CROSSING)
INPUT
RS-232C
J
R14
DATA
INPUT
RS-232C
CD
INPUT
R15
RS-232C
'DATA
OUT
Figure
5-13. I/O
Circuitry
-46-
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