Re: Game port

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

Frank Carney <frankcNOSPAM@cyberhighway.net> wrote:

> Thanks for the information.  I got everything I need now.

I have a few important bits of information that Rubywand omitted.

On the IIe and later machines, the Open Apple (or Command) and Solid
Apple (or Option) keys are connected in parallel with buttons 0 and 1.
If these keys are pressed, it will appear that your external input has
gone active (high).

The second point is that in any Apple II except the IIgs there is an
optional modification that allows the shift key to be read via the
button 2 input.  The logic for this is opposite to the apple keys:
holding down the shift key grounds the SW2 signal, so the software reads
low while the shift key is being pressed.

Depending on how this has been wired up, it is possible to short out an
external input.  The seriously bad combination is a standard pushbutton
on input 2, which is connected to +5V when pressed, and the shift key is
connected to ground when pressed.  Press them both at the same time and
you short out the power supply.

The easiest way to stop this from happening is to put a low value
resistor between the shift key and the SW2 pin, rather than a direct
wire connection.  However in your case, you can't be sure that the
computer in question has this modification done "safely".

A side note: the "platinum" version of the IIe has this modification
preinstalled on the motherboard (using a resistor).


A final note: the IIc does not have a button 2 input, and its button 0
and 1 inputs are not quite the same as other machines, since they double
up as mouse detection and button signals (which use the opposite logic -
connecting to ground when the mouse is plugged in or the button is
pressed).

You cannot reliably put a TTL level into a IIc.  The only officially
supported input combinations are a switched +5V with pulldown (as on
earlier machines), or a similar transistor configuration (fully
saturated PNP, with Vce <= 0.3V, and current into the IIc >= 30 mA,
according to the techincal reference).

The IIc manual specifically excludes a schottky gate output, switched
ground through a transistor (open collector), and emitter follower
transistor configuration.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand