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

From: Richard Erlacher <richard_at_idcomm.com>
Date: Wed Jul 5 10:36:40 2000

Quite so, but please see my embedded comments below.

Dick

----- Original Message -----
From: <CLASSICCMP_at_trailing-edge.com>
To: <classiccmp_at_classiccmp.org>
Sent: Wednesday, July 05, 2000 6:36 AM
Subject: Re: Tim's own version of the Catweasel/Compaticard/whatever


> >> 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.
>
The choice appears to be between something that may work and something that
will, though I believe the sample rate ought to be increased.
>
> OTOH, something I can put together on a Sunday afternoon with parts I
bought
> at Radio Shack is an even bigger plus for me :-).
>
Buying parts at Radio Shack is risky. I once bought a significant number of
parts from them only to find that not one of them was in spec. I've not
bought any components from them since, and that was in '77. They do have
some interesting tools, but they immediately discontinue them if they're any
good.
>
> 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 believe that the CPLD makes it accessible to those who otherwise might be
disinclined to build any hardware. The advantage is that the file
associated with the definition of the hardware is part of the package. The
programming cable is, likewise. The CPLD makes using components known to be
working and already in the user's possession, on an old motherboard or video
card, say, workable.
>
> 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.
>
I think you'll need to go faster than that, else missing an edge will create
more phase shift than you can trivially resolve.
>
> >, 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.
>
If you want to use a single oscillator and still write with typical precomp,
you'll have to use 24 MHz and divide it down into 6 and 8 MHz.
>
> 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.
>
Agreed, though the software will have to be adjusted accordingly. There
will have to be a fair amount of work to deal with the write side of the
problem. (Is that a right-brain activity?)
>
> Tim.
>
>
Received on Wed Jul 05 2000 - 10:36:40 BST