Replacements for ST-406/ST-512 drives

From: Eric Smith <eric_at_brouhaha.com>
Date: Mon Feb 7 19:03:01 2005

Dwight wrote about ST506 emulation:
> As I stated before, if you place a simple toggle flip-flop that
> is clocked by the pulse from the drive, the rate of sampling
> is reduced by a large factor ( at least about 10 ).

I don't see how that helps. If the drive emulator is not going to
decode the MFM (or RLL, etc.) to store the bits in a decoded form,
then it has to preserve the timing of the pulses with very good
resolution in order to make sure that on playback they are where
the host's disk controller expects them. It's nowhere near good
enough to sample at 2x or even 4x the data rate (10 Mbps or 20 Mbps,
based on a nominal bit rate of 5 Mbps). For MFM, the bit cells occur
at a 10 Mbps rate, and I doubt that good results would be had from
sampling at less than 5x that rate (50 Mbps). For RLL, the timing
is even more critical.

And that's not even considering the way the controller applies
precompensation to account for peak shift in the recording channel.
A really good drive emulator would model the peak shift.

Since the highest-capacity drive ever made with the ST506/ST412
interface, the Maxtor XT2190, had a formatted capacity of only 150 MB
(225 MB w/ RLL), there seems to be little reason when building an
emulator using a modern drive as the storage to try to sample at less
than 50 Mbps. I'd probably be inclined to sample at 100 Mbps to make
sure that RLL works reliably. That results in a total storage
requirement of 3.825 GB, which is small enough to fit in even a
Hitachi Microdrive.

Eric
Received on Mon Feb 07 2005 - 19:03:01 GMT