What happens
with one of these drives is you slow the steppign rate
(at the cotnroller) right down, say to 20ms steps? Does it still
sometimes kil the index pulse?
This is actuall older than one might think. I've got a FH 5.25"
Micropolis 96 tpi drive that uses buffered seek--it blocks index
until the head is on-cylinder. Probably 1979 or so; I could check
the date codes. It uses a MOS Technology microcontroller (house
numbered). It has a HC51/U-sized 2MHz crystal on the PCB.
Problaby one of those 6500-series microcontrollers, which were used in
some disk drives...
And it's old enoguh that the Beeb should have taken account of that
design .Perhaps it was just so obscure that nobody over here had come
across it (I certainly hadn't...)
However, can you step it slowly enough (sensibly) that this is not a
problem? If you snd, say, 12ms or 20ms-spaced step pulses, will it remain
ready all the time?
For pure bad deisgn, nothing beats the Shugart SA4000 hard disk, IMHO. On
that thing, you can eitehr send the step pulses one at a time, waiting
for hte 'on cylinder; signal to go active after each one. Or you can send
a burdt sufficiently fast that the drive will buffer them, then move the
head and assert 'on cylinder' when it's done. But if you send the burst
of sstep pulses too _slowly_, the drive wil lget confusedand end up o
nthe worng cyulinder. Ther reason (and I have the schematics...) is that
there's on coiunter that is controleld both by the incoming puses and by
the stepper motor derive clock. If the heads are moving when a step pulse
comes in, there's a good change the control logic will be in the wrong
state and will consider the incoming pulse to be a stepper motor clock
pulse or vice versa, so the counter changes in the wrong direction...
-tony
Cheers,
Chuck
