Prototype new Apple /// memory board

Eric Smith spacewar at
Wed Aug 25 15:28:15 CDT 2021

I've been working on a new memory board for the Apple ///, using (somewhat)
modern and still-in-production components, especially CMOS static RAM
rather than DRAM. Last night I soldered the connectors, sockets, and
passives of my first prototype:

I need to do some testing for shorts, etc. before I attempt to actually use
it in an Apple ///. I expect that some debugging of the design will be

The Apple /// design is _much_ more complex that the Apple II and IIe. I
intended this board to provide 512KiB of RAM, but I've already determined
that some design changes will be required for that, so this prototype will
only support 256KiB.

The early Apple /// design, as documented in US patents, would have
supported up to 512KiB of RAM, but the actual shipped design reduced that
to 256KiB. There was a third-party 512Kib emory board from "On Three",
which required pulling various chips from the motherboard and running
cables from those to the memory board.

The SOS operating system, as shipped, only supported 256KiB. On Three
modified the SOS bootloader to detect and use 512KiB. Some Apple ///
application software also had trouble with 512KiB, and On Three patched
some of those.

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