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.05.htmlIt 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. 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?