[sebhc] hard sector substitute

[Dave Dunfield]

>Suppose you are copying files from one H17 to another H17 disk. The
>device that is creating the fake sector holes has to wait for a pair of
>index holes (one full revolution) each time it switches disks. So, while
>my hardware circuit worked, it was slower than real disks because of the
>extra time the H89 spent waiting for the disk to get "up to speed"
>(really, for the PLL circuit to lock in and start producing valid sector
>pulses).

Does the H17 switch each drive motor off when selecting drives for a disk
to disk copy?

The N* does not - when copying one disk to another, both drives stay on,
and the drives simply get selected back and forth.

In fact, there is only one motor-on signal, so all drives come on for any
operation. Most drives can also be modified for "motor on select", however
every drive I've seen that does this also implements a timer so that the
drive remains on for a period of time between selects.

I've thought about building such a device, and I decided that it would be
best to attach it to the drive (ie: one per drive - not one per controller).

Basically, an itty-bitty micro (PIC, AVR etc.) would have to be able to see:
  - Motor on signal (to the motor - not necessarily the drive interface)
  - Index hole sensor (even when drive deselected

And be able to drive the index hole output back to the drive (so it would be
gated by select) - alternatively - gate it externally.

The MCU would watch the index hole, and could time how long from one edge
to the next - after 1-2 revolutions (depending on position if index hole) it
would be able to insert phony pulses to simulate the other 10/16 holes.

By watching how long after motor-on before the first index pulse, you can
handle cases where the index hole is near the sensor and the drive is not
up to speed for the first pulse.

As long as the motors are not turned off, you can continue to "sync" and
no further time is wasted. If the system turns the motors off when switching
from one drive to the other (I can't imagine a system that would do this),
you could add the Motor control to the output of the device, and implement
your own timer to delay shutting off the motor.

An alternate solution:

- Take a dead drive And modify it with a homemade punch that can punch
  through the index hole.
  - Put a 10 sector disk in it - carefully line up the 11th hole and put it
    in the drive - mark this as the "master" position on the drive wheel
    (Assuming a direct drive)
  - Move the wheel for each of the other sectors, and mark the position on
    the wheel

- Now you have a jig for making 10-sector disks:
  - Align soft-sector diskette to put index hole in position
  - Aligh wheel to master postion and insert disk.
  - Move wheel to other marked position and activate punch at each position.

Tough part would be making the punch - any takers?

Regards,
Dave
-- 
dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Vintage computing equipment collector.
                http://www.parse.com/~ddunfield/museum/index.html

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