Re: Apple II Disk Drive Question

Don Bruder <dakidd@xxxxxxxxx> wrote:

In article <1hc5pp1.ked6xj1br0avnN%dempson@xxxxxxxxxxxxx>,
dempson@xxxxxxxxxxxxx (David Empson) wrote:

There is a bit on page 9-45 (with reference to the ProDOS driver) which
says that the maximum variation due to rotation speed and disk flutter
must be less than plus or minus 4.7%. This translates to about 2.3%

There's no nicer way to put this, and I've seen you making the exact
same *JUST PLAIN WRONG* statement multiple times now, so I'll just be
blunt:

I beg your pardon? That is the first time I've even READ that section of
the book in question, let alone referenced it, so I don't know how I
could have made the statment "multiple times" (apart from in a single
post, which was derived from the same value).

It doesn't "translate" to anything, period. "+/-4.7%" explicitly states
that 4.7% below OR above 300 RPM is acceptable - no translation required
- There's an explicit 9.4% window, with the target speed of 300RPM
centered in it.

Quote from the book:

.... the 30-cycle period between reads means that there can be no more
than 3 parts in 32 variation in speed between the reading and writing
drives. In other words, drive speed variation plus speed variation due
to disk flutter must be less than plus or minus 4.7% or occasional
reading errors will ocucur at the bottom/middle border in a data field.

[end quote]

3/32 = 0.09375. Therefore you are correct, it is a +/-4.7% variation
from 300 rpm, so my figures were incorrect, and the actual limit on the
range of rotation is speeds is about twice what I said, about 286 to 313
rpm should be OK.

It does not say anything about 2.3% - If that were the intent, there
would be no "+/-" involved - it would be a straight "4.7% variation".

The paragraph in question was talking about variation in speed between
reading and writing drives, and I interpreted the quoted 4.7% figure as
being the allowed variation in speeds between read and write drives,
rather than the allowed variation from the midpoint. I didn't notice
that the 3/32 ratio was twice that.

4.7% of 300 is 14.7. Therefore, 300-14.7 = 285.3, and 300+14.7 = 314.7,
meaning any speed from 285.3 to 314.7RPM is within specs.

Your incorrect interpretation of what's spelled out explicitly in black
and white narrows that range considerably - 2.35% of 300 = 7.05, making
minimum RPM under your interpretation 292.95, and maximum 307.05.
Yes, it's a tighter tolerance. But it's not the tolerance specified.

I know that's nit-picking, but if you're going to throw math around, at
least get it right, huh?

If you are going to post to Usenet, at least have a modicum of courtesy.
I made a simple mistake, there is no need to be rude about it.

--
David Empson
dempson@xxxxxxxxxxxxx
.

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