Atari CX 5200 Field Service Manual page 130

THEORY OF OPERATION
For
the following
discussion,
reference the lower
PC
board assembly schematic. Since
both
X
and
Y
circuit
operation
is
identical,
refer to the
X
circuit for this description.
The
CX53
Trakball
is
a 2-channel
D
to
A
converter which
translates optically
coupled
clocks
into
analog
currents.
The
current sources or sinks into
the
two
paddle
line
inputs
used
for
X-Y
control
in
the 5200. Figure 7-3
is
a
function block
diagram
to
illustrate
the following information.
When
the cue
ball
is
spun,
the
two
roller
shafts rotate.
Each
shaft has a slotted
encoding wheel
at
one end which
interrupts the
two
light
sources
in
the optocoupler
for
that channel. This produces a
pair of
alternating voltage
wave forms
from
the
photo-transistors
in
the optocoupler
(U2).
Comparator A1
straightens the signals
from U1
into
5V
square
waves
and feeds
them
to
A2
and A5. These two clock
waveforms
are
approximately 90°
out of phase.
This
allows correct direction sensing,
depending upon which clock
leads the other.
A2
senses
which
leading
edge
of
the
two
clocks
is
coming
first,
thus
determining
direction.
A5
combines
the
two
clocks
into
one
with doubled frequency, thus
increasing resolution.
This clock
is
in
turn
fed
into
A3
which
sets
the clock's pulse
width
at a
constant duration while
still
allowing the frequency
to
vary with
speed.
The
Q
and
Q
outputs
from A2
are used
to
gate
the fixed-width pulses from A3 with the
directional
clockgates
(A4). This produces alternating
outputs depending
upon
direction (Left or Right).
The
current
integrator
networks on the
outputs
of
A4A
&C
and A5B<3cC then
link
these gated clocks
to
the analog
input
of the 5200
main
console.
TP9 &
TP10 each
should
have
a
ramp waveform
approximately
3 volts in
amplitude.
This
ramp waveform
is
generated
in
the
main
circuitry of the
5200 and
is
controlled
by
increases
or
decreases
in
current
caused by the Trakball
circuit.
When
the
ball
is
stationary, the
ramp
will
remain
at
approximately
3V.
When
the ball
is
spun
in
the
"right" or
"down"
direction
an inverted clock
siphons current off the corresponding
ramp
with a
net reduction
in
the slope of the
ramp (-600mV max.
@ C7 and
C8).
When
the ball
is
spun
in
the
"left" or
"up" direction a positive
clock causes
positive current
integration, with a net increase
in
the slope
of
the
ramp (+900mV
max. (9C7 and
C8).
The
clear
line
(used
for calibration)
is
held
HIGH
for
normal
operation.
During
initialization
the console
pulls
it
LOW.
This prevents any ball-generated clocks
coming from
A3, and allows the
static
output level to
be read and used
as a
reference
reading
in
determining
velocity
change.
CX5200
Field Service
Manual
7-5