Early 3.5" Floppy Drives

[Fred Cisin]

On Thu, 15 Dec 2005, Allison wrote:
> 765A write clock rate by drive and density, bit rate is clock/2.
> Size   density  format   writeclock
> -----------------------------------
> 8"       DD      MFM     1000khz
> 8"       SD       FM      500khz  (8"SSSD 241k CP/M standard)
>
> 5.25     DD      MFM     500khz (40track is 360k, 80track 720k)
> 5.25     SD       FM     250khz
>
> 3.5"     HD      MFM    1000khz (1.44mb) (looks like 8" different CHS)
> 3.5"     DD      MFM     500khz (720k)   (same rate as 5.25 DD and 8" SD)
> 3.5"     ??       FM     250khz (not used obsolete)

one more entry to add to that table:
  5.25    "HD"     MFM    1000KHz (1.2M) (looks like 8", could be SAME CHS)

> None of this has anything to do with rotation rate of the media.
Very true.  8" and "1.2M" is 360RPM, all others are 300 RPM,
with a few exceptions (early Sony 3.5 at 600 RPM, NEC, Weltec, etc.)

> Actual data storage capability is format dependent.
and the format choices open an even bigger can of worms







> One example that was known the to CP/M world was 5.25" 80track (FD55F)
> two sided at either 720k or ~780k  I was sometimes called QD as it
> was really the same as the 360k but twice the tracks (48 tpi vs 96).
> So happens that the 3.5" drive can be plugged in and used exactly
> as if it were a FD55F for the same 720k as I do it all the time
> from a CP/M system to DOS and the CPM80 side has a utility that
> read/writes DOS FAT files. I'd have used 1.44 but the WD1770 literally
> cannot run at the required rate (not rated to either!).
>
> I'll let you all in on a dirty trick.  The 765(A) outputs a signal on
> pin26 called FM, that is used to select data rates /2 ALWAYS.  If you lift
> the pin the data rates for FM mode are now twice as fast and suitable
> for many other uses like 8" media.  For the integrated flavors of 765
> the same effect can be had by twiddling bits in the drive control register.
> If all else fails, you can double the the 8 or 16 mhz clock source
> used to 16/32 as needed.  I have taked the 9.6mhz out and used higher
> on one board 16mhz so that switching to AT 5.25HD got me 8"DD instead
> without futzing with drivespeed (rotation rate) that means nothing to
> most 3.5, 5.25 (including FD55GFR with the jumper pulled) and 8" drive.
>
> >That was the beauty of the NEC APC line--from 8" right down to 3.5", the
> >data format didn't vary one iota.  The NEC 9801 floppies still record 1.3MB
> >on a 3.5" drive spinning at 360 RPM.
> >
> >But the  PC-XT 8" drive controllers were a special beast, honest.
>
> Not really.  I can take the stock IBM XT long board and with one change
> make it do DSDD 8" (other than cable adaptor).  Common parts cost 'bout
> $1, acutally cheaper now than 20 years ago then it would have been 1.89.
> Replace the 8mhz clock source with 16mhz.
>
> Thats how it's done.
>
> Allison
>
>

--
Fred Cisin                      cisin at xenosoft.com
XenoSoft                        http://www.xenosoft.com
PO Box 1236                     (510) 558-9366
Berkeley, CA 94701-1236