[sebhc] hard sector substitute

Dwight K. Elvey dwight.elvey at amd.com
Tue Jun 29 18:19:17 CDT 2004

Hi Lee
 I believe it can be done if one also includes a slight
shift, based on the write gate. If it sees the write gate
turned on, delay the hole slightly longer with a one shot.
This way, the read has a little more space between the
hole and the sync byte. One just needs to be careful
to not overlap with the previous data for the read but
there is a reasonable gap for the previous data to the hole.
I'd have to look it up to see how long it was in uS.
 One would also allow a little for speed error. The other
thing is that this delay would be greater for holes in around
sector # 5 because the average rotational rate is usually
quite constant ( at least when seen from my strobe wheel ).
 These disk wouldn't be easily moved from machine to machine
but they should work reasonably well on the same machine.

>From: "Lee Hart" <leeahart at earthlink.net>
>I'm speaking anecdotally and from memory, so please allow for errors.
>Steve Thatcher wrote:
>> a HD drive is able to read DD disks, so there should be a way
>> to reduce the speed to 300rpm
>Older 5.25" and 3.5" HD drives could operate at two different speeds;
>300 rpm for low-density, 360 rpm for high. But to save money, this
>feature was left out of most PC drives; instead, they have an extra
>crystal in the disk controller, and read/write even low-density data at
>the higher speed.
>> I have been thinking about a PIC that would be able to not only
>> supply the missing hard sector pulses, but also be able to remove
>> them from hard sector disks.
>I built a circuit to do this years ago. A CMOS phase locked loop chip
>used the single index hole of a soft-sector disk to create a 10x clock.
>This clock was ANDed in with the real index pulse to produce the 11
>pulses per revolution to fool an H17 controller and software into
>thinking it was a hard-sector disk.
>> I don't think rotational stability will be an issue.
>It worked, but not very reliably. The disk drive needed good speed
>stability, and the disks had to rotate easily in their sleeves. I had
>too many disk drives and disks for which this wasn't true.
>> If that were the case then DD SS disks would be problematic.
>Except that soft-sector disk controllers extract the clock from the data
>stream itself. There can be substantial variations in disk rotation
>speed and the data can still be recovered. It's the same as tape
>recorder that runs at the wrong speed; the pitch may be wrong or even
>vary moment to moment, but everything is perfectly intelligible.
>In the old days, they tried to make the disk drive rotation speed
>precise, and so simplify the hardware in the controller. We went from
>synchronous motors in 8" drives (perfect speed control) to
>servo-controlled motors in full-height 5.25" drives (1% accuracy), to
>open-loop direct-drive motors in half-height 5.25" and 3.5" drives (2%),
>and in some extremes (Commodore and Macs) no speed control at all (5% or
>> The only latency issue with regards to hard sector disks would
>> be the computer response time to "seeing" the index hole and when
>> it really starts writing the sector data.
>Yes. My PLL circuit took a couple rotations of the disk to "sync up".
>This is inherently longer than a real disk takes to get up to speed.
>> The PIC would work for both 5 1/4" and 8" drives and be smart
>> enough to adjust itself based on the index hole. A few jumpers
>> on it would set it for how many sector pulses to generate
>> (1, 10, 16)  and what drive size.
>I'm sure a PIC could do it, but it is a *challenging* software problem
>in real-time control!
>"Never doubt that the work of a small group of thoughtful, committed
>citizens can change the world. Indeed, it's the only thing that ever
>has!" -- Margaret Mead
>Lee A. Hart  814 8th Ave N  Sartell MN 56377  leeahart_at_earthlink.net
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