[sebhc] 64K memory

Lee Hart leeahart at earthlink.net
Sun May 16 23:47:38 CDT 2004

Carroll Waddell wrote:
> I have been designing a 64K memory card for the H8... If there is
> enough interest, we could get a PCB fab house like PCBExpress to
> make the PC boards... This is a half size (6 x 6) card.

Carroll, rather than make it a half-size board, I would rather see it a
full-size board, and make good use of the space.

First, H8 boards have large spacings, so they are easy to build. Clumsy
fingers, tired old eyes, and poor soldering skills aren't a problem. Now
that *I* am clumsy, old, and poor, these attributes would be most

Second, the speed of a computer is largely set by the CPU-to-memory
link. Putting the CPU on one board and the memory on another forces all
transfers to be limited by the motherboard. The H8 has a very simple,
but slow motherboard.

So, I would put the Z80 and memory on the same board. Then the Z80 can
run *very* fast (up to 20 MHz with modern chips) when accessing on-board
memory. When accessing OFF-board memory or I/O, slow the Z80 back down
to its standard 2 MHz. This avoids any problems with motherboard or
other board speeds.

I did this on my H-1000 design (a replacement CPU board for the H89, and
it was very successful. The Z80 could run at 4 MHz or 8 MHz for on-board
RAM, but slowed to 2 MHz for ROMs, I/O boards, or accessory memory
boards. Every stock accessory board we tried worked perfectly with no
cuts or patches. No BIOS patches were needed, unlike all the 4 MHz mods
for the H89.
"Never doubt that a small group of committed people can change the
world. Indeed, it's the only thing that ever has!" -- Margaret Meade
Lee A. Hart  814 8th Ave N  Sartell MN 56377  leeahart_at_earthlink.net

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