Since I have several of the drives, I do, indeed have the service documents
for the model 277 (single-sided) drive.
The difference between FM and MFM is not the rate at which thd data and
clocks are written to the drive, but rather, the modulation technique
itself. Instead of haveing a discretely presented clock 180-degrees out of
phase with the data window, it has an imbedded clock, in which, like
manchester code, the data is shifted in phase from the clock. If there is a
long string of ones, they are written in phase, one pulse per clock, while
if there's a long string of zeroes, the zeroes are represented by a string
of pulses 180 degrees out of phase , omitting the first and last pulses in
order to avoid crowding them to such extent that it violates the maximal
flux reversal density. As a result, you get a modulated waveform which
contains both the data and the clock in it in such a way that you can write
and read double the amount you can write and read using FM, since FM
automatically consumes half the channel bandwidth for discrete clock pulses.
It should come as no surprise that this works much like Manchester Code.
Dick
-----Original Message-----
From: Tony Duell <ard_at_p850ug1.demon.co.uk>
To: Discussion re-collecting of classic computers
<classiccmp_at_u.washington.edu>
Date: Friday, April 09, 1999 4:44 PM
Subject: Re: stepping machanism of Apple Disk ][ drive (was Re: Heatkit 5
>> Actually, Persci floppy drives in 1976 or so were voice-coil (and quite
>> a pain to maintain, even then - these days the glue that holds the
>> optical graticules in place is often failing, and gluing and realigning
>> from scratch is even harder, even with all the special Persci
>> realignment jigs and electronic panels.) And a common modification
>
>Does anyone have the service manuals for these drives? I have one on my
>old CASU S100 system, and although it's not failed yet, I'm sure I'll
>have to fix it sometime.
>
>Are they any worse to maintain than RK05s, say? The overall design of the
>positioner seems to be very similar.
>
>> >One interesting thing about the Apple GCR modulation format is that it
>> >essentially was a "double-density" technique.
>>
>> Eric said the same thing, and I disagree with you both. To me (and all
>> the tech pubs I've read) the density is how many flux transitions you can
>> do per second (or revolution). GCR is a way of getting more real data
with
>
>Hmm... Are you implying that conventional MFM double density is just FM
>with a faster clock rate? Surely not.
>
>I thought the whole point of MFM was to reduce the number of flux
>transitions per (user) data bit. An FM bit cell _always_ has a clock
>transition, and may have a data transition as well. MFM removes some of
>the 'wasted' clock transitions.
>
>> the same number of flux transitions. Apple GCR drives use single-density
>
>As, IMHO is MFM.
>
>-tony
>
Received on Sat Apr 10 1999 - 02:52:32 BST