Functional Description
Address Decode
The select on the Game Control Adapter
is
generated by two
74LS138's as an address decoder.
AEN
must be inactive while the
address
is
201 in order to generate the select. The select allows a write
r-.,
to fire the one-shots or a read to
give
the values
of
the trigger buttons and
one-shot outputs.
Data
Bus Buffer/Driver
The data bus is buffered by a 74LS244 buffer/driver.
For
an
IN
from
address
X'
20 1', the Game Control Adapter will drive the data bus;
at
all other times the buffer is left in the high impedance state.
Trigger Buttons
The trigger button inputs are read via an
IN
from address
X'
20
I'.
A
trigger button
is
on each joystick/paddle. These values are seen on
data bits 7 through 4 (see Software Interface sub-section). These
buttons default to an open state and are read
as
"1".
When a button is
depressed, it is read as
"0".
Software should be aware that these
buttons are
NOT
debounced in hardware.
Joystick Positions
The joystick position is indicated by a potentiometer for each
coordinate.
Each
potentiometer has a range from 0 to 100 K ohms that
varies the time constant for each
of
the four one-shots. As this time
constant
is
set at different values, the output
of
the one-shot will be of
varying durations.
All four one-shots are fired at once by an
OUT
to address
X'20
1'. All
four one-shot outputs will go true after the fire pulse and will remain
high for varying times depending on where each potentiometer
is
set.
These four one-shot outputs are read via an
IN
from address
X'20
1 '
and are seen on data bits
3 through
O.
2-118
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