Tim's own version of the Catweasel/Compaticard/whatever

From: CLASSICCMP_at_trailing-edge.com <(CLASSICCMP_at_trailing-edge.com)>
Date: Wed Jul 5 07:36:17 2000

>> 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.

>I sympathize with that, but for those of us who are much better at software
>than hardware, something off-the-shelf is a big plus.

OTOH, something I can put together on a Sunday afternoon with parts I bought
at Radio Shack is an even bigger plus for me :-).

It's not like my buffer uses any complex electronics. It's all perfectly-
standard TTL parts and a SRAM chip, and it's currently residing on a pretty
randomly wired solderless breadboard so you don't need any fancy construction
techniques. Except for the capacity of the SRAM chip, this is all quarter-
century-old technology.

As Chuck pointed out, maybe the fact that this is quarter-century-old
technology put together with quarter-century-old construction and design
techniques makes it less accessible to some of the younger members
of this list. Maybe the way to make it more accessible to them is to put
the circuitry on a CPLD, I dunno, I think it's fine as it is.

I suppose there *are* folks who might be interested in using such a device
who don't know which end of a soldering iron to pick up, but a very valid
point is that I built this without even touching a soldering iron!

And the fact that I built it without even drawing a schematic first would
tend to implicate the design as being on the naively simplistic side, too :-).
(I still gotta draw that schematic up for you guys...)

>Another point to note is that the Catweasel samples at 7 or 14 MHz (software
>selectable). In reading some old 8" MFM disks, I found that there had
>been a lot of bit-shifting over the years (or maybe there was not enough
>write precomp applied to begin with)

I will admit, MFM *does* require at least twice the timing resolution of FM.
I once got in a small argument with some other members of this list about
AC circuit design of MFM vs FM data recovery circuits. IIRC, they were
insisting that MFM did come "for free" if you had the frequency response
necessary for FM at half the data rate. My point was that it
wasn't the max pulse frequency which made life difficult, it was the phase
response (finding where the pulse occured in the window) that was the tough
point.

>, and I had to use an extra heuristic
>to make them readable at all. I'm not sure that 4 MHz would have been
>a high enough sample rate for these.

Yeah, well, with my circuit if you don't like the sample rate, you buy
a different off-the-shelf oscillator in a can and plug in. If you
now need more buffer RAM, you plug in a second RAM chip and wire it up.
That's IMHO the beauty, but maybe a software-only hacker doesn't see that.

Tim.
Received on Wed Jul 05 2000 - 07:36:17 BST