Atari Joystick Interface
for the
IBM Game Control Adapter

By Donald R. Blake
0408DRB1 at OWGVM1
December 18, 1984
Owego, NY

The schematic for the Atari joystick interface is shown in
the figures.
[figure 1 and figure 2]

Since CMOS devices are used, all unused inputs should be
tied to ground. Normal handling precautions should be
observed to protect from damage due to static voltages.

Integrated circuit connections not shown in the schematics
are:

IC1 (4049 hex inverter) and IC2 (4050 hex non-inverting buffer):
	
        pins 13, 16 - no connection
        pin 1 - +5V (Vcc)
        pin 8 - ground (Vss)
	
IC3 and IC4 (4066 quad analog switch):
	
        pin 14 - +5V (Vdd)
        pin 7 - ground (Vss)
	

Since the joystick switch contacts are electrically poor,
all joystick inputs are buffered via IC1 and IC2.

Pull-up resistor (R01, R02, R05, R06, R09, R10, R11, R14,
R15 and R18) values are not critical. I used 50K ohms for
the pull-up resistors.

The IBM Technical Reference specifies the game control
adapter input range as 0 to 100K ohm. I used a value of
50K ohm for the game control adapter input resistors (R03,
R04, R07, R08, R12, R13, R16 and R17) which provides inputs
of 0, 50K and 100K ohms. This value doesn't work well with
some game software (e.g., Burgertime). You'll need to do
some experimenting with the particular software
applications you're using. A value of 75K to 100K ohm may
be a better choice.

Construction is not critical. I mounted the components on
a perforated board and used wire wrap. I used a small
plastic case to hold the board and two 9-pin male
connectors for the joysticks. An 8-conductor cable runs to
a 15-pin male connector which plugs into the game control
adapter.

A friend of mine built a one joystick version on the
prototype area of the game control adapter. He added a
9-pin male connector to the game control adapter and a
switch to select between the interfaces.



