I don't have time to properly go over it before work, but I'd recommend using two 62256 32K x 8 SRAMs. They're readily available (if you don't have any let me know...I bought several hundred for a good price years ago, and I'll never, ever use them all) and if you do it right you can swap an SRAM for a 27256 EPROM or one of its EEPROM equivalents.
You should also provide an inhibit line for your "board enable" decoder (the 74LS30). Drive it by ORing together /PHANTOM and a signal from a mask-off decoder. I'd use a 74LS682 comparator, which has a "greater than" output -- that allows you to set the high byte of your address that you want memory to be blank above.
On Mon, 15 Oct 2012 23:26:04 -0500
Kyle Owen <kylevowen@...
> So I've been working on designing my own 64k SRAM card for my N* Horizon. I
> don't ever expect to need more than 64k of RAM (hah!), so I'm not
> implementing any banking features on this. I am allowing the top 8k to be
> deselectable in 2k blocks, as I don't want to mask over my floppy
> controller at $E800.
> Anyways, I would certainly appreciate someone else looking over my
> schematic before I start on my layout. If you see any way I can save on
> ICs, or have a better easy-to-find alternative to my L2 cache chip, please
> let me know. I'm all about this board being user-friendly and very
> buildable for others needing a very cheap, possibly easy-to-prototype
> board, much like Michael Holley's 32k RAM card for the SWTPC:
> Here's the link: