overriding motherboard ROM with a peripheral card in a platinum iie

[jonnosan <jonnosan@gmail.com>]

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.

I'd initally thought I could  hook up d0..d7 and a0..a13 on a 27128 to
the corresponding bus signals, and then connect a14,a15 and the 'on/
off' switch via AND gates, and then NOT the result to the EPROM OE' so
OE' only goes active when both A14 & A15 are high (i.e. the address
requested is in the range $C000..$FFFF). I'd then also connect OE' to
INHIBIT' (so that when the EPROM gets activated, the motherboard ROM
is disabled)

But I think this would cause problems during access to any of the
other things that can also reside in the memory range $C000.$FFFFF,
i.e. any of the softswitchs or RAM banks.

So how would a peripheral card detect when access is attempted to
firmware ROM, so it can override it?