Hi folks -
The topic of reading or writing older "odd" disk formats
(hard-sectored, GCR, whatever) comes up here pretty often. And often
the discussion leads towards the Catweasel floppy interface (currently
available commercially, as I understand) or the Compaticard interfaces
(not currently being manufactured, as I understand).
I've recently "modernized" my foreign format disk reading
hardware. The idea of both the "old" version and the "new" versions
are very similar to each other, and to what other tools like the
Catweasel and the Compaticard do:
Data from a floppy disk drive comes as a series of pulses on
the read data line. All these circuits (using different means) buffer
up to an entire track of data in RAM, and then allow a user program to
look at the buffer and analyze the data on its own terms. They also
allow control over rudimentary floppy functions such as step in/out,
load head/start motor, etc.
*I* buffer the pulses on the read data line by simply recording
a "1" bit if there was a pulse in a window, and a "0" if there wasn't.
I sample at 4MHz, meaning that a complete revolution of a floppy
requires most a megabit of RAM. I also buffer information such as
index pulse data (essential for decoding many hard-sectored floppy formats.)
The bit rate from most standard floppy formats is roughly 500 kHz, meaning
my 4 MHz sample rate oversamples by a factor of 8 or so. Yes, it's overkill
for many applications, but I want to be sure that all information on the
floppy is faithfully represented in the buffer. Besides, many GCR and
some MFM variants use half- or even third-fundamental-frequency components
(sliding the pulse in the window to encode more data without increasing
the clock rate) and oversampling is necessary for these formats.
Other buffers (I believe the Catweasel
falls in this category) record the times between successive pulses in
RAM, instead of the "raw bits".
My "previous generation" buffer had a megabit of SRAM and
interfaced via a 16-bit-in and 16-bit-out parallel interface (usually
to a DR11-style interface on a PDP-11). The megabit of SRAM was made
up of a whole bunch of 2k*8 SRAMS, meaning that just the buffer memory
required a couple of sizable PC boards. (If you're familiar with the
technology, you'll get the idea that "previous generation" buffer was
based on chips readily available around 15 years ago.)
My "new generation" buffer has the same megabit of SRAM - but
now in a single SRAM chip - and interfaces via a PC-clone bidirectional
parallel port. Other than the big SRAM chip, the rest of the buffer is
just eleven SSI and MSI HCTTL IC's. The eleven chips form the PC-side
interface, the floppy-side interface, the timing clocks, the buffer
sequencer, and the buffer address (17 bits) counters.
So how is my buffer different than, for example, the Catweasel?
1. The Catweasel uses some custom LSI parts, as far as I can figure out.
My circuit is much more "hackable", anyone with a TTL databook can figure
out what it does and improve on it. Or you can build one yourself from
scratch. (Other than the 128K*8 SRAM, all the other parts were literally
purchased from the local electronics shop. Heck, most of the chips can
be bought at Radio Shack!) Total cost for the chips in my buffer is
about $30.00, about half of that in the SRAM chip.
2. The Catweasel uses a proprietary, largely undocumented programming
interface. My circuitry is entirely open, and I think it's pretty easy
to program. (My first hack at acquiring data with the new buffer was
dashed off in about half an hour under QBASIC.)
3. The Catweasel requires a bus slot inside a PC-clone. My new buffer uses
a much more general purpose parallel interface. So you can hook it up
to a laptop, or even to something that isn't a PC-clone at all.
Those are what I see as advantages over the Catweasel. There are also
some disadvantages:
4. You can't just go out and buy my buffer, but you can buy Catweasels
off the shelf.
5. My buffer is strictly "read-only" as I use it. I think the Catweasel
(and Compaticard) both allow writing.
Would it be worth writing up my new floppy disk data buffer so that
others could improve on it? Would anyone be interested in unstuffed and/or
stuffed PCB's? Should I give it a name? (The "Timweasel", anyone? I
gotta think of a better name!)
--
Tim Shoppa Email: shoppa_at_trailing-edge.com
Trailing Edge Technology WWW: http://www.trailing-edge.com/
7328 Bradley Blvd Voice: 301-767-5917
Bethesda, MD, USA 20817 Fax: 301-767-5927
Received on Tue Jul 04 2000 - 08:03:25 BST