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From: "Mike McLaren, K8LH" <k8lh@...>
Sent: Thursday, December 5, 2019 12:40:21 AM
Subject: Re: [cosmacelf] Todd's ELF-ish to get an FDC #Homebrew #microboards
On Sun, Dec 1, 2019 at 07:00 PM, cmdrcosmac wrote:
Ok, here's an untested design for inserting both ROM and I/O into the address space... The I/O selector has priority so you could even map I/O into the middle of ROM address space. You can also deselect I/O or ROM by driving the /OE pin high on the respective '688 chips. So, yeah, you could swap ROM or I/O into and out of address space as needed. While the design is relatively simple when using a 64K or 128K RAM chip, I suspect it wouldn't be difficult to use the other half of the 74HC139 decoder to provide the chip select signals for a pair of 32K RAM chips.
Cheerful regards. Happy Holidays. Mike, K8LH