Is there a Catweasel like device for hard disks?

[Jules Richardson]

Chuck Guzis wrote:
>> Date: Wed, 02 Jan 2008 14:28:23 -0600
>> From: Jules Richardson <jules.richardson99 at gmail.com>
> 
>> Andrew Lynch wrote:
>>> Hi,
>>> Just out of curiosity, is there a technique or device which can do raw
>>> reads of tracks on ST506/ST412 style hard disk drives similar to how a
>>> Catweasel can with a floppy disk drive?
>> To the best of my knowledge, no - and I've been keeping an ear to the
>> ground for such things for a while (plus it's a discussion which crops up
>> here every once in a while, but I don't believe anyone's produced any
>> working hardware yet).
>>
>> The speeds involved (particularly if over-sampling the data) are
>> reasonably high - enough that it'd be tricky[1] to throw something
>> together out of OTS TTL parts. That puts such a project more within the
>> realm of people who know all about interfacing to high speed
>> microcontrollers, and the pool of available carbon units with the time,
>> skills and inclination to make such a device is pretty darn small.
> 
> I'll assume that we're talking about (surviving--and that's a real 
> gotcha) ST506/ST412 interface drives here.

Indeed - I sorted through my pile of 'reserves' recently and got around a 2/3 
mortality rate. On the flipside, it's been a while since I've had a working 
drive suddenly fail on me - as though long periods of disuse are real killers 
(perhaps things like seized spindles and heads sticking to platters, rather 
than logic or head faults).

 >   Why would a Catweasel-
> type interface (i.e. pulse time sampling) be even desirable?  All the 
> drives used to store digital data that I'm aware of were recorded as 
> MFM, M2FM or some flavor of RLL (2,7 probably being the most common). 

Hmm, I thought I read once that you could throw pretty much anything you 
wanted at a ST506 drive so long as it was within the various tolerances - is 
that not true of ST412-type drives?

Although I suspect things fall into the same category as for a floppy reader - 
the choices boil down to either oversampling the data in order to capture bit 
transitions, or to recording the time between bit transitions (as I believe 
the catweasel does).

> The rated speeds of old ST412-type drives were pretty modest; about 
> 5MHz for MFM and 7.5MHz for RLL, IIRC.  

Yep... I was thinking an 8x sampling rate would probably suffice, but with a 
7.5MHz raw data rate that's probably a bit much for standard TLL ICs. 
Recording the elapsed time between bit transitions might be better, though 
(providing it's still possible to record the worst-case scenarios)

cheers

Jules