Date   : Tue, 23 Mar 2010 21:35:29 +0000
From   : pete@... (Pete Turnbull)
Subject: Re-using floppies

On 23/03/2010 19:34, Dr. David Alan Gilbert wrote:

> Woah! That's the 2nd time I've seen reference to 100 tpi discs - can
> you give some more information?
> 
> The Commodore drives to my knowledge were VERY odd; they used
> special mechanisms and had a hell of a lot of processing in them (and HPIB
> or similar out the back - one I picked up 2nd hand had a 6502 and 6510
> (from memory) and a fairly useful haul of 6532s and other 65xx chips).

Not special mechanism, just special electronics.  That was not unusual 
at the time.  Those drives have a lot of intelligence and buffering in 
the drive system, rather like modern SCSI/SASI/whatever hard drives. 
The Commodore drives changed the data rates on different groups of 
tracks; later the same designer (Chuck Peddle) used variable speed 
drives to achieve a similar result -- ie approximately the same /linear/ 
bit rate and hence bit density on inner, middle, and outer tracks.

> However, what really interests me is that I got a pair of 100tpi Teak
> drives many years ago (full height, 5.25") by accident in a junk
> sale (we were lucking for a pair of 80 track drives for our Model B!).
> 
> Those Teak drives have standard Shugart interfaces (and were in a nice
> external box) - but if Commodore drives always had special interfaces,
> I'd love to know what else used 100tpi ?

A few CP/M machines did, but 96tpi (or 48tpi) was always more common.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York