speaking of 3.5" floppies, is 720K R.I.P.?
cclist at sydex.com
Thu Feb 5 12:11:40 CST 2009
On 4 Feb 2009 at 18:14, Chris Elmquist wrote:
> I'm finding it much more difficult than I expected. It seems that the
> speed of these drives is not very constant. So, when I trigger off the
> real index pulse from the drive and then generate very accurately timed
> pulses simulating the 10 other holes, it appears that they are not lining
> up with the data some number of rotations later.
I can't say much about any experience with 3.5" drives in this; I've
been using 5.25" units and measuring the speed with every rotation.
Given the low mass of 3.5" direct-drive motors and media, I can well
imagine that speed variations are all over the place. I wonder if
one can tap into the PLL used to control the motor--there should
certainly be enough information there to determine short-term speed.
However, after 5.25', I'm going to an all-electronic solution;
basically a sampling plug-in replacement for a drive. I figure that
rusty goo smeared on plastic cookies is not only going to be harder
to find, but show a pronounced decline in quality as remaining
production gets shoved out to the margins of the commercial world.
I have customers who would probably kill to replace the floppy drives
in their equipment.
> The result is a high number of soft errors during read back.
> My simple plan was to just watch for the real index hole from the selected
> drive and then generate the synthetic ones back to the controller until
> the drive was deselected. I could see I had a timing problem which I
> initially blamed on the microcontroller's RC clock (I am using an ATMEL
> AVR instead of a PIC). So, I improved this in various increments,
> ending up with a +/- 50ppm osc can and at each increment of improving
> the stability of the clock, my soft errors INCREASED!
> I'm now reworking the whole design and I'm going to keep the soft sectored
> drive always selected, always motor on and continually watch the real
> index pulses and compute the instantaneous RPM. From that, I'll generate
> the pseudo index pulses which will hopefully better match the current
> speed of the drive. The pseudo pulses and all the other drive signals
> will be externally gated onto the floppy bus since I need to keep the
> emulating drive always active-- and I want to emulate two drives at the
> same time. Kind of a pain but I don't see how else to get there now.
> Any suggestions on what I am missing?
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