Re: Apple II 256K Memory Expansion Kit

["David Wilson" <mcs6502@gmail.com>]

On Feb 25, 7:06 pm, "mdj" <mdj....@gmail.com> wrote:
> I've never looked up the exact 'technical' need for this jumper, but
> it is necessary due to the hardware change that introduced double hi-
> res graphics - I believe installing the jumper will inhibit 14Mhz
> video when a graphics mode is active, which is necessary to prevent
> hardware lockups on the revision A Apple IIe (which doesn't support
> double resolution graphics modes)
>
> Someone else here probably knows the exact details of why this change
> is necessary.

The jumper joins pin Aux-55 to pin Aux-50.

Rev A: Aux-55 = ENFIRM (when low disables ROM1 & ROM2 on motherboard)
[*]
Rev B: Aux-55 = FRCTXT' (when low allows 14MHz dot clock even when GR
is high)

Both: Aux-50 = AN3 (Annunciator #3 - low after hardware RESET, set
high by monitor firmware)

If the jumper is in place on a Rev A motherboard the system will not
boot due to lack of ROM RESET routine. On a Rev B motherboard it will
allow the use of Double HiRes and Double LoRes graphics.

[*] see page 178 of 030-0357-A "Apple II Reference Manual For IIe
Only". Interestingly, the circuit diagram on pages 179-182 is of the
Rev B motherboard.