Atari CX 5200 Field Service Manual page 130

For the following discussion, reference the lower PC board assem bly 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.
inputs used for X-Y control in the 5200.
illustra te the following information.
When the cue ball is spun, the two roller shafts rotate.
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 for m s from the
photo-transistors in the optocoupler (U2).
Comparator A l straightens the signals from U I into 5V square waves and feeds them
to A2 and A5. These two clock wavefor m s are approximately 90 0 out of phase. This
allows correct direction sensing, depending upon which clock leads the
senses which leading edge of the two clocks is coming first, thus deter mining
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
Q
The Q and outputs fro m A2 are used to gate the fixed-width pulses from A3 with the
directional clock gates ( A4).
direction (Left or Right). The current integrator networks on t he outputs of A 4 A & C
and A5B&C then link these gated clocks to the analog input o f the 5200 m ain console.
TP9 & TP I O each should have a ramp wavefor m approximately 3 volts in amplitude.
This ramp waveform is generated in the m ain circuitry of the 5200 and is controlled by
increases or decreases in current caused by the Trakball circuit.
stationary, the ramp w ill remain at approximately 3V.
"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 (+9 0 0 m V max. @ C 7 and C8).
The clear line (used for calibration) is held
initialization the console pulls it LOW.
coming from A3, and allows the static output level to be read and used as a reference
reading in determ ining velocity change.
CX5 200 Field Service Manual
THEORY OF OPERA nON
The current sources or sinks into the two paddle line
Figure 7 -3 is a function block diagram to
This produces alternating o utputs depending upon
HIGH
T his prevents any ball-generated clocks
7-5
Each shaft has a slotted
other.
speed.
When the ball is
When the ball is spun in the
for normal operation.
A2
During