[sebhc] Flash memory interfaces

Lee Hart leeahart at earthlink.net
Mon Sep 20 15:58:51 CDT 2004

Mark Garlanger wrote:
> I was just looking at the prices of the little USB Flash memory
> devices and was wondering if anyone has investigated the
> feasibility of adding a USB interface to the H89? Or maybe one
> of the ther Flash memory interfaces (compact flash, SD, memory
> stick, etc.)?

> It seems like it would be possible to model it like a harddrive,
> and have much more than the H67 provided for less than $50 +
> cost of he interface card.

It's a great idea, but the practical difficulties abound. Most of these
modern cheap-mass-memory devices are tailored strongly toward high-end
PCs. They use complex protocols and specialized parts that make them
very difficult to be used by hobbyists. So, while it is possible, no one
is willing to spend the time and effort to figure it all out and get it
to work.

I think a "low tech" solution is far more likely to succeed. You can
readily buy very large conventional EPROM, EEPROM, flash ROM, and RAM
chips in normal DIP packages. The pinouts are highly standardized. The
protocols for talking to them are simple and well-documented.

For example, years ago when the 32k EPROMs became cheap, I plugged one
into the ROM socket of my H89, and wired the extra address lines to the
spare output bits of the 8250 UART. I wrote software to bank-switch the
EPROM. The extra ROM space was used by my "Superset" software to
drastically enhance the H19 and H89's terminal capabilities.

Now that 512k EPROMs are $6-7, you could expand the concept. Plug it
into one of the CPU board's 2k ROM sockets, and add an 8-bit latch to
control the EPROM's extra address lines. This gives you 256 x 2k = 512k
bytes of non-volatile read-only memory. Add a RAM disk drive to the CP/M
BIOS to access this memory as a read-only disk drive. (I already have
such software written for the H-1000, which would work on the H8 and H89
with only minor modifications). Half a meg is enough to hold virtually
all the programs that one could ever want on an H89. You could even boot
from this ROM disk!

If this isn't enough memory, the concept could be expanded. If it was a
RAM chip and a little battery, it would be non-volatile and work like a
real disk (but drastically faster). You could put multiple ROM/RAM
sockets on a little board, which plugs into the bus or ROM socket as an

The advantage of this approach is that it is so straightforward and
simple that it has a good chance of actually getting built. All the
approaches with interfacing USB or CompactFlash sound great, but will
never get done.
"Never doubt that the work of a small group of thoughtful, committed
citizens can change the world. Indeed, it's the only thing that ever
has!" -- Margaret Mead
Lee A. Hart  814 8th Ave N  Sartell MN 56377  leeahart_at_earthlink.net

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