[sebhc] hard sector substitute

Lee Hart leeahart at earthlink.net
Tue Jun 29 13:43:17 CDT 2004

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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