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Texas Instruments 99/4A - Page 77

Texas Instruments 99/4A
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T
h
U
S
C
1
C
LI
:T:
V
S
Ht
1
H
Cl
t
h
E?
16
t
O
H
bit
i
C:
t
c?
C
6
3,
c
1
F
“
;
q
r~
j
1
'
i
L
DYNAMIC
RAM
INTERFACING
Dynamic
RAM
memory
is
given
in
finure
53,
â–¡
.
133.
uses
pins
1
through
11
and
25
through
32
for
i
dynamic
RAM.
Rins
3
through
10
are
the
address/dat
TMS
4116
(line
10
only
-For
data)
.
RAS
on
sin
1
an
are
the
row
and
address
strobe
lines
for
loading
ths
4116
s
(
a
n
d
r
s
f
r
e
s
h
operation)
,
a.n
d
are
connect
Ths
TMS
99
IS
nt
er-Facing
the
a
lines
-For
the
d
CAS
or*
pin
2
the
a
d
d
r
e
s
s
in
ed
to
ths
corr-
esponding
pins
4
and
14
of
the
RAM's.
R/W
on
pm
11
of
the
TMS
9915
distinguishes
between
a
read
or
write
to
the
-RAM.
This
signal
is
delayed
by
the
-Four
inverting
buffers
U101
For
timing
purposes
in
units
manufactured
not
using
the
TMS
991SA.
During
a
read
all
S
separate
output
bits
of
the
RAM
?
s
form
an
eight
bit
data
bus
which
is
connected
to
RDO
throuqh
RD?
(pin
25
through
32)
of
the
TMS
9913.
VIDEO
OUTPUT
The
composite
NTSC
video
signal
is
available
on
RIN
36
of
the
TMS
9900.
This
signal
is
fed
to
the
video
amplifier
in
figure
53,
p.
133.
This
amplifier
has
two
purposes:
1.
To
amplify
the
video
signal.
2.
To
provide
the
desired
output
impedance
of
75
ohms.
The
first
stage
of
the
amplifier
consisting
of
0200
is
a
common
base
amplifier
which
has
enough
bandwidth
to
amplify
the
video
signal.
R201
is
used
to
adjust
the
gain
of
the
vidio
amplifier.
The
second
stage
with
0201
has
no
voltage
amplification
since
this
stage
is
used
as
an
impedance
tranformer
to
provide
the
75
ohm
output
inpedance.
R203
and
C205
is
a
filter
which
decouples
the
video
noise
from
the
+12
volt
power
line.
L20O
and
C207
have
the
same
purpose
and
the
voltage
from
this
filter
can
be
used
to
supplv
power
for
an
RF
modulator.
In
the
99/4A,
the
first
stage
of
the
video
amplifier
(0200)
is
a
FNF
transistor
connected
in
the
common
collector
configuration
.
Input
to
0200
is
through
a
filter
which
is
designed
to
rol
1
off
signal
frequencies
above
the
6
MHz
video
bandwidth.
Diodes
CR200
v
CR201
clamp
the
DC
level
after
C201.
0201
is
an
emitter-Follower
and
is
used
as
an
impedence
transformer
to
achieve
the
required
75
ohm
video
output.

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