Re: joystick with III plus

[dempson@atlantis.actrix.gen.nz (David Empson)]

In article <4ccei8$1f1@opal.southwind.net>,
Randy Shackelford <shack@onyx.southwind.net> wrote:
> I thought I'd plug one of my joysticks into the 9 pin port with the
> joystick symbol on the back of my III plus and play a game or two
> with the II emulator.  The machine weirded out until I unplugged the
> stick though. What's the deal? 

The Apple III joystick port has very little in common with the
Apple II one.  Do not attempt to connect an Apple II joystick to an
Apple III, or vice versa.  At the very least, it will not work.  At
worst, you could damage the computer.

For starters, if you push either button, you will short out the power
supply (+5V will be connected to ground).

Here are the pinouts, for comparison:

   IIe   III

1  SW1   GND
2  +5V   +5V
3  GND   GND
4  PDL2  Joy-X
5  PDL0  SW0
6  SW2   +12V
7  SW0   GND
8  PDL1  Joy-Y
9  PLD3  SW1

Luckily, no single joysticks use pin 7 (SW2).  If you pressed that, you
would short out +12V to +5V, which could do some serious damage to the
motherboard.

The Apple III also uses a different circuit arrangement within the
joystick.

The switch inputs are supposed to have pull-down resistors (270 ohms),
and have a momentary-closed contact to +5V.  (The Apple II doesn't
need a pull-down resistor, but it wouldn't matter if one was there.)

The joystick analog inputs are supposed to be connected so that one
end of each potentiometer is grounded, the other end connected to
+12V, and the wiper goes to the joystick pin.  The potentioment value
can range from 1K ohm to 700K ohm (the manual says that with higher
values, a limited range will be available, so I expect that 100K ohm,
as used in a PC joystick, should be OK).

(The Apple II uses a 150K ohm potentiometer to +5V.  There is no
connection to ground - one end of the pot is left unconnected.)
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand