3.5" floppy drive question(s)

[Mr Ian Primus]

--- On Sat, 10/11/08, Sridhar Ayengar <ploopster at gmail.com> wrote:

> >> The first issue is to get the drive to ignore the
> difference between 
> >> 1.44MB and 720KB media 
> > 
> > I think drives nearly always do the detection via a
> microswitch, don't 
> > they? (It wouldn't surprise me if at least one
> wasn't using some kind of 
> > optical arrangement). In the event of no jumper,
> cutting traces or 
> > bridging switch pins with solder should do the job I
> would have thought.
> 
> IBM PS/2s did it optically.

Hehe. Or not at all... I remember using one PS/2 that just assumed everything was HD unless told otherwise (or the disk was already readable). Didn't even need to drill holes in the disks to 'upgrade' them. Of course, these days, I find myself taping over the holes in disks, since DD media is hard to find.

I've made high density 5 1/4" drives into 80 track double density (720K) drives by cutting the trace on the board that leads to pin 2, and tying it to ground. This forces the drive into DD recording. 

Never tried it with a 3 1/2" drive.

Now, what I wonder, is how does the _controller_ know what kind of disk is inserted in the drive? According to the documentation I have, the density select line is defined as a unidirectional signal TO the drive, not from it. Having not worked as much with 3 1/2" drives, I don't know, but I would assume that the drive senses the disk's hole, and operates in that data rate and density recording, and the computer simply picks up on the data rate and works accordingly. Or, perhaps, in 3 1/2" drives, pin 2 is bidirectional, allowing the drive to tell the computer the type of disk inserted. I don't know.

But, 1.44mb floppy drives are cheap and easy to get. Grab a soldering iron and try it :)

-Ian