Functional Description; Address Decode; Data Buss Buffer/Driver; Trigger Buttons - IBM 5150 Hardware Reference Manual

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 ofthe 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'201 '
and are seen on data bits 3 through O.
2-118