19 Apr
2009
19 Apr
'09
5:57 p.m.
My finding (with the help of Andrew Lynch) was that
the rev-to-rev
speed variation on old 5.25" 100 tpi drives was nowhere near stable
enough; if the speed error from rev to rev varied by more than a few
msec., you were dead. With the old open-loop DC tach circuit found
on old 5.25" drives, this was more than likely.
Random though (I've not tried anything like this..) Most of those speed
control circuits used an AC tachogenerator built into the drive motor.
Provided the beld doesn't slip, or you have a direct-drive motor (common
in half-height drives) you should get a constant number of cycles from
that generator per revolution. Could you divide that down to produce hard
sector pulses?
Another random thought.... Most of these hard-sector drives were
single-sided. Most modern drives are double-sideed. How about recording
timing pulses on the normally unused side of the disk and adding extra
electronics to the drive (basiclaly just a second read amplifier chain)
to pick them up and produce sector pulses? Some hard drives used
divided-down signals from the sevo surface for this sort of thing.
-tony