Lisa "twiggy" specs (Was: selling a blank floppy disk

From: Fred Cisin <cisin_at_xenosoft.com>
Date: Fri Jan 1 19:31:09 1999

Who here knows the specs on the Lisa "twiggy" disks?


some simple arithmetic combined with info from Eric's post:
"360K" diskettes have 40 tracks, 48 Tracks per inch, double sided, with
about 4.5 formatted K per track. (about 6300 unformatted bytes per
track, with a LOT used up in sector headers, gaps, and other "overhead")
250K bits per second data transfer rate, with 300 RPM. MFM
The diskettes are 300 Oerstedt (magnetic coercivity).
"High" density runs 7.5 to 9 formatted K per track. 1.2M diskettes are
600 Oerstedt. 500K bits per second data transfer rate with 360 RPM. MFM
(360K disks in a 1.2M drive can be handled by changing the drive speed,
or by keeping the 360RPM and upping the data transfer rate to 300K)

If we assume that the Lisa did not go any nearer the center nor edges,
then 62.5 TPI would lead to about 52 tracks. Assuming double sided, that
would mean about 8.375 (average) formatted K per track. That is
consistent with "high" density. GCR would tend to provide a flux
transition density of about 1.5 times that of FM, and slightly less than
that of MFM. Flux transition density would seem to be the most important
factor to determine the correct coercivity.

Did they have the same number of sectors on each track ("constant
rotational speed"), or did they have different numbers of sectors on
different tracks ("constant linear velocity")?

Therefore, it would seem likely that 1.2M would indeed be the correct
blanks to use for homemade "twiggy" disks.
I had a chance to play briefly with a Lisa before they were released to
the public - the homemade disks that I made out of "360K" did NOT work.

--
Fred Cisin                      cisin_at_xenosoft.com
XenoSoft                        http://www.xenosoft.com
2210 Sixth St.                  (510) 644-9366
Berkeley, CA 94710-2219
On 1 Jan 1999, Eric Smith wrote:
> Tony wrote about Twiggy (Lisa 1) diskettes:
> > Oh right.... (Just looked). Is there a more sensible way of making that 
> > sort of media should I ever manage to obtain a machine that uses it?
> 
> Before I acquired any "real" Twiggy diskettes, I hand-made a few by
> cutting up the jackets of standard 1.2M high-density floppies (which weren't
> readily available when the Lisa 1 was current in 1983; they became common
> sometime after the introduction of the IBM PC-AT in 1984).
> 
> Recently I realized that I don't know whether the 1.2M media is actually
> appropriate (although it does seem to work).  In order to fit 871K on a
> double-sided diskette, the Twiggy uses a 62.5 TPI track pitch, rather than
> the standard 40 tracks at 48 TPI or 80 tracks at 96 TPI.  Since a common
> format for a DSDD (not high-density) floppy gets around 720K, and a DSHD
> gets 1.2M, the bit density may be closer to that of the DD drive.
> 
> But it's hard to say, since the Twiggy uses GCR at a variable rotation
> rate.  I need to get the specs on a standard 5.25 inch diskette, such as
> the radius of the innermost and outermost tracks, in order to work it out.
> 
> I'm guessing that the reason for 62.5 TPI may have been that they may have
> been able to do that with the standard heads that were intended for 48 TPI
> drives.
> 
> More information on the Twiggy drives is on my web page:
> 
> 	http://www.brouhaha.com/~eric/retrocomputing/lisa/twiggy.html
> 
> > And if I hear one more person who claims the Millennium is the 31/12/1999 
> > - 01/01/2000 (as opposed to the correct date of 1 year later), I am 
> > liable to get out a very large LART..
> 
> I saw a letter to the editor of PC magazine (US edition) sometime within
> the past year.  The writer claimed that the idea that the millenium would
> end at the end of 2000 was stupid; I don't recall exactly what he said about
> it, but it was something to the effect that it was OBVIOUS that the millenium
> ended at the end of 1999, and that 2000 began a new millenium.
> 
> That's not the interesting part.  We hear about bozos like that all the time.
> 
> He went on to "prove" that he was right.
> 
> He explained that if you are counting coins, that your 100th penny was
> part of your first dollar.
> 
> Obviously he didn't actually think about the relationship very hard.
> 
> Your 1st penny is part of your first dollar, as is the 2nd, 3rd, 4th, [...],
> 98th, 99th, and 100th.  Your 101st penny is the first penny of your second
> dollar.  [Just as the 101st year was the start of the second century, etc.]
> 
> I was just about rolling on the floor when I read this "proof" of his point.
> If it had been UseNet, I would have assumed that it was a troll.
> 
> A lot of people gripe about the lack of a year zero.
> 
> I try to explain to people that the year numbers are ordinal numbers, not
> cardinal numbers, which usually gets blank stares.  So I then explain that
> A.D. years are numbered as the first year after the [whatever you call the
> divider between B.C. and A.D. dates], the second year after, etc., and that
> the B.C. years are numbered as the first year before, second year before,
> etc.:
> 
>                                baseline
>                                  event
>                                    |
>                                    |
> <------|------|------|------|------|------|------|------|------|------|----->
>  fifth |fourth|third |second|first |first |second|third |fourth|fifth |sixth
>  year  | year | year | year | year | year | year | year | year | year | year
>  B.C.  | B.C. | B.C. | B.C. | B.C. | A.D. | A.D. | A.D. | A.D. | A.D. | A.D.
>                                    |
> 
> After seeing it presented this way (usually wider so I can mark the first
> and second decade), people that aren't completely brain damaged will usually
> concede that this makes some sense, even if they won't concede that it is
> "correct".  The rest of the people aren't even worth arguing with.
> 
> Cheers,
> Eric

Received on Fri Jan 01 1999 - 19:31:09 GMT