3.5" floppy drive question(s)

Chuck Guzis cclist at sydex.com
Sat Oct 11 12:29:13 CDT 2008


On 11 Oct 2008 at 11:34, Curtis H. Wilbar Jr. wrote:

> Was wondering if 3.5" HD floppy drives can be jumpered/used in
> a system that only supports 720K, using them as 720K only
> drives ?

Without running down my documentation on the drives, here's an 
approximate answer.

Any 1.44MB drive will read and write 720K diskettes just fine (as you 
probably already know).  However, using one as a 720K-only drive or 
to replace a 720K drive has a few issues, some of which can be 
resolved by reconfiguring the drive, sometimes not.  For example, 
there are *many* variations on the Teac FD235HF; newer drives tend to 
be less configurable (or more difficult to configure) than older 
ones.

The first issue is to get the drive to ignore the difference between 
1.44MB and 720KB media (i.e. the media type aperture in the floppy 
jacket).  If a drive could be configured to operate in "host media 
select" (also sometimes called "PS/2 mode") where the media type is 
signalled by driving pin 2 on the floppy interface, it's possible to 
get the drives to ignore the aperture.  Otherwise, you have to use 
care to make sure that if you're using 1.44MB media as 720KB, the 
aperture is covered over.

The second is the function of pin 34.  The normal (PC) function of 
this pin is as a "disk changed" indicator; but some drives allowed 
for redefining this pin as "ready", which is the 720K convention.  
I've done that with Sony 1.44MB drives and know that it was a jumper 
option for some Teac models.

If you've got one, also check out the Teac FD235J (2.88MB drive).  
Some models of this drive had jumpers for just about anything you 
could imagine, including media select and ready-vs-disk change.

> Also, anyone know if there if it is possible to slow the spindle speed
> so that 1.44M can be done on machines that only have DD data rate ?
> (and if so, any of the above drives capable of being slowed ?)  Would
> that even work ?

While it's possible to change the spindle speed on many 1.44MB 
drives, the speed change is to 360 RPM from 300 RPM.  As far as 
slowing the spindle speed to 150 RPM, while it might be possible with 
some hacking, you wouldn't like the result.  Read signal strength on 
a disk drive is proportional to the square of the speed of the medium 
passing by the head; halving the drive speed quarters the read signal 
level (one of the reasons, but not the only one, that inner tracks on 
a diskette tend to be less reliable than outer ones).

In the early days of the PC AT, there were some offerings of 5.25" 
drives with 180 RPM spindle speeds to allow them to be used with a PC 
XT diskette controller for 1.2MB media.  They barely worked at all.

Cheers,
Chuck




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