With respect to BDD's concern, the /OE of the EEPROM is tied to CPU pin 40. So shouldn't that mitigate the bus contention?

@Jeff, what should I look for to see if you concern has been handled?

@All, while trying to find parts on Mouser I noticed that the CY7C199CN is out of stock and obsolete. It looks like it will be hard. Not sure what to do about that, although I suppose I could use a regular width static RAM in an adapter.

Mousers results for the DS1813 contain a number of parts in the through hole format that differ slightly in part number. See
https://www.mouser.com/Search/Refine?Keyword=DS1813

Usually when this happens there's something I don't know about part ordering.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

I read some of the preceding posts a little too hastily and I overlooked the fact the manufacturer says an '816 will work in this board. And that claim seems plausible, based on the schematic (from your link upthread). BDD's concern is satisfied because both of the memories' /OE are driven by an inverted copy of Phi2, and my concern is satisfied by the manufacturer's claim and by a superficial inspection of the glue logic.

-- Jeff

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

Something like the IDT 71256SA seems to be available from Mouser still , and 0.3" wide DIP

@Dr Jefyll, thanks.

@BDD, I misread the schematic because it crossed across pin 40, but didn't have the dot. As Dr Jefyll, it continues to an inverter to Phi2.

@Martin A, thanks for the suggestion.

Hmmm...this is the first time I've seen a schematic.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

No worries. It was in the Google Drive folder link in the first post. I didn't upload it to the forum as given this was a commercial product, rather than forum member designed.