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5.8 DAC
Circuitry
(IC7)
Two special
analog integrated
circuits
are
used
in the
Color
Computer
3
to
implement
a
multitude
of analog functions,
including power
supply regulation, cassette
operation,
the
RS-232C serial
interface, the joystick interface,
and sound production/selection. The
DAC chip (IC7)
is
one
of the
custom
linear integrated circuits used
in
the
Color Computer
3.
As its
name
implies,
it
contains
a
Digital
to
Analog Converter. This chip also
contains
a
sound multiplexer and
the
circuitry necessary
to
interface
the
joystick controllers
to
the
microprocessor. Figure 5-10
shows
a
block diagram
of the DAC chip.
The DAC performs most of
the
functions
of this
chip. Six bits of
control are used
by the DAC to
specify
a
discrete internal analog
level. This level
is
one of
the
sound
inputs
to the
sound multiplexer.
It
is
also used
as
a
reference
for
a
comparator,
the
other input of which
is
one of the four
joystick
inputs.
Finally,
the
DAC signal
is
attenuated
and used
as the
cassette recording
signal
for
data storage.
There are
two
select inputs
to
the
DAC chip: Sel
A
and Sel
B.
These
determine which
of the
joystick
inputs
is
to be
compared against
the
DAC,
as
well
as
which sound
source
is
coupled
to the
sound output pin
according
to
the table on the next
page.
ANALOG
INPUTS
ANALOG
SELECT
'
in
LEI
Vcc
M.
WEIGHTED CURRENT SOURCES
nmi
CURRENT
SWITCH
AND
l/V
CONVERTER
<>V\
SELECT
LOGIC
JTI
BUFFER
D>
O
COMPARATOR
ANALOG
SWITCH
.,
n
W
D
Mm
SND
SND
SND
ENABLE
I
II
It DO
T] 02
T\ D4
DATA
INPUT
T]\
CASS
OUT
]j]C0MP.
OUT
T]
SOUND-OUT
Figure
5-10. DAC
Block Diagram
(IC7)
-42-
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