Early 3.5" Floppy Drives

Allison ajp166 at bellatlantic.net
Thu Dec 15 12:18:04 CST 2005

>Subject: RE: Early 3.5" Floppy Drives
>   From: "Chuck Guzis" <cclist at sydex.com>
>   Date: Thu, 15 Dec 2005 09:32:54 -0800
>     To: cctalk at classiccmp.org
>On 12/15/2005 at 7:42 AM Allison wrote:
>>Any PC controller that can do 720k 3.5" format can do 
>>8" as it's the same data rate.  it's not what chips was 
>>used it's how it was used.
>Whoa.    Sorry, I couldn't let this one go by unanswered.
>8" drives use a 500K data clock rate, not 250K like the DS2D 720K 3.5".  A
>controller that supports 1.44M DSHD 3.5" should do just fine on 8". While
>there were early 8" drives that used a lower clock rate, they were pretty
>much gone by the time of the dawn of the PC.  FM support with a modern
>controller is a somewhat different kettle of fish.  The nearest AT medium
>to the 8" drive would be the 5.25" high-density diskette, which also spins
>at the same rate--360 RPM.

;) your assumption is double density.  8" SSSD is not that fast.
I never said formats were the same or even dive interface only that
the data rates fly.

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)  

None of this has anything to do with rotation rate of the media.
Actual data storage capability is format dependent.

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.


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