Re: Todd's ELF-ish to get an FDC #Homebrew #microboards


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:

It's such a simple circuit. Use two of 'em!. One is set to the size and location
of the ROM. The other is set to reserve 256 bytes and addressed anywhere else
as needed. When placing the hardware, watch out for routines that scan for RAM
by writing and reading back. (As IDIOT does) Perhaps a control bit fed to the
688's enable pin, so the hardware cannot be accessed until software enables it.
 Then you run the /PQ outputs of the 688's thru a gate so either one will deselect
the RAM, and the one for the hardware can enable the chip selects of the hardware
devices, and the lowest address bits operate the register address bits of the devices.

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

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