Re: Shift-key mod

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

In article <014351HJNWLKPANMTTUE@ods.sol.net>,  <dreamer@ods.sol.net> wrote:
> 
> A month or two back there was some discussion about how to do the
> shift-key mod (presumably on the //+ and //?).  One fellow posted how to
> do it, but another guy responded that the wrong annunciator (#3?) was
> listed, but couldn't remember the correct one.

I don't remember this exact discussion.  Annunciators are outputs, and
you need an input for the shift-key mod.

The one-wire shift-key mod requires a wire running from the piggyback
board on the back of the keyboard to switch input 2, which can either
be accessed as a pin on the game socket (which may block the socket),
or to the appropriate pin on the chip which detects the switch.

In this form, the mod only applies to the ][ and ][+.  The IIe has a
motherboard option that allows the shift-key mod to be enabled with a
solder blob.  The "platinum" IIe has the mod pre-installed (and
provides a motherboard option to disable it again).

I am not aware of a standard mod for the IIc to detect the shift key
in this fashion (unless it is already installed), or for the IIgs
(there is another way of reading the shift key on the IIgs anyway).


For the ][/][+: run a wire from pin 24 of the piggyback keyboard
connector (on the pins between the keyboard circuit board and the
piggyback board: pin 24 is one in from the edge opposite the power
supply) to pin 1 of the 74LS251 at board location H14.  I don't have a
II+ motherboard handy, so I'm not sure exactly where this chip is - I
think it very close to the game connector, and is just under it
judging from the diagrams in the Red Book.


For the original IIe motherboard: solder across option X6.


In both cases, this modification has one nasty side effect: if you
connect a button to switch input 2 on the game connector (e.g. a
three-button joystick, or two joysticks, or four paddles), and the
button is pressed at the same time as the shift key, the power supply
will be shorted out, causing the computer to turn off until either the
button or shift key is released.

You can avoid the short by using a low value resistor (e.g. 100 ohms)
in series with the one-wire mod (or by soldering a resistor across X6
in the IIe).


With the shift-key mod installed, software can detect the shift key
state by examining bit 7 of memory location $C063.  If bit 7 is set,
the shift key is up.  If bit 7 is clear, the shift key is down.  Note
that this is the opposite sense to the pushbutton inputs.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand