[sebhc] hard sector substitute

Lee Hart leeahart at earthlink.net
Wed Jun 30 13:09:25 CDT 2004

Dave Dunfield wrote:
> Does the H17 switch each drive motor off when selecting drives
> for a disk to disk copy?

Like your Northstar, the Heath H8 and H89 have a single motor select, so
all disk drives connected to a controller start/stop their motors at the
same time.

The extra delay when switching disks in my hardware hard-sector circuit
occurred because the PLL circuit had a "lock" detector output that
inhibited the extra sector pulses if it wasn't phase-locked. If you
switched disks, it lost lock until it had seen a couple index holes
(more than one complete revolution). With a real disk drive, the new
drive selected was already spinning up to speed, so the software started
reading as soon as it saw the first index hole (typically half a

However, you can have two disk controllers, each with its own motor
select line. For example, when copying files between an H17 and an H37
disk. In this case, when a drive on the second controller is selected,
its motor select line may have already timed out and turned off; so the
time to start the motor and wait for it to get up to speed was an
additional delay. My circuit's delay was equivalent to this happening
every time a new drive was selected.

> I've thought about building such a device...

You know, the funny thing is that we have source for the Heath software,
and all the tools for writing 8080/Z80 code. We could just write a new
H17 driver that doesn't depend on the sector holes. No new hardware at
all is needed.

The interesting thing about this approach is that we know so many more
"tricks" today for high performance disk systems. In the "old days", the
designers were delighted just to get it to work at all. They didn't have
the luxury of extra time to perfect the software to get greater
"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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