Re: overriding motherboard ROM with a peripheral card in a platinum iie

[Silicon Sam <SiliconSam@gmail.com>]

On Mar 20, 4:57 pm, Mike Willegal <m...@willegal.net> wrote:
> One other thought.
>
> Inhibit only controls enable of the onboard ROM D000 to DFFF.
>
> The 4K memory range from C000 to Cfff is intended to be handled somewhat
> differently than the D000 to FFFF PROM range. There should be a
> flip-flop on each expansion card that controls a PROM up to 2K bytes in
> size that can share the address space C800 to Cfff with all other
> peripherals.
>
> For slots 1 through 7, I/O select is driven low when location CNXX is
> addressed and the PROM on that slot should be enabled (the flip flop is
> set).  When location CFFF is addressed the flip flops on all peripheral
> cards are reset, disabling all the peripheral PROMs.
>
> Rather than checking all 16 address bits for $CFFF, usually the inverted
> version of I/O strobe is anded with (A8 & A9 & A10).  This results in a
> reset of the flip-flop with any address CFxx and the loss of 256 bytes
> of PROM space, but fewer chips are required to implement this.
>
> Regards,
> Mike Willegal
>
> Mike Willegal wrote:
> > Although I have no first hand experience with a IIe, I took a look at
> > the apple tech note for the IIe inhibit line.  It can be found here.
> >http://www.1000bit.net/support/manuali/apple/technotes/aiie/tn.aiie.0...
>
> > It appears that an Apple firmware card will do exactly what you want,
> > even though it was originally designed for a II and II plus.  It has an
> > enable/disable switch and also supports multiple copies of the board in
> > the system through a daisy chaining system.  Priority of multiple boards
> > is controlled using the Apple bus DMA control lines.  It also has the
> > capability to be switched on or off via software.
>
> > Find a copy of  "Understanding the Apple II" by Jim Sather, which has
> > schematics.  There are copies available for download on the internet.
>
> > I recently built a slightly more modern version (uses a single 27c128 or
> > 27c257 PROM).  Details are available on my website  Let me know if you
> > need more information on my version and I'll help you if I can.
>
> >http://www.willegal.net/appleii/appleii-first_page.htm
>
> > Regards,
> > Mike Willegal
>
> > jonnosan wrote:
>
> >> Howdy all.
>
> >> I'd like to understand how to have a peripheral card override the ROM
> >> on the motherboard, to allow access to alternative ROM images (e.g.
> >> Senior PROM / EDM).
>
> >> What I want to do is have a peripheral card with an EPROM in it, and
> >> an 'on/off' switch, and when the switch is 'on', any attempt to access
> >> to either the Applesoft or Monitor ROM code goes to the code in the
> >> EPROM instead.
>

  Back in the old days, I used to sell a kit that would toggle between
the Unenhanced ROMS and the Enhanced //e ROMs.  When the enhanced //e
came out, not everything was compatible right away. All I did was take
a couple of 27128 EPROMs and toggle a pin (can't recall which one
right off the top of my head) and it would toggle between the upper
64K and the lower 64K half of the 128K ROM.

  Just toggle the switch, and reboot into either enhanced or
unenhanced.  Substitute the upper or lower 64K with the code you want
and swap between the two.