View Full Version : Floppy density detection (PS/2 formatted diskettes)

July 28th, 2009, 05:30 PM
Does anyone have experience overriding the density detector in modern 3.5" floppy drives? I don't mean formatting HD diskettes as DD; I mean reading DD diskettes that have been written in HD format!

This weekend while helping my dad clean out his attic, I discovered a box of 3.5" DSDD floppies, carefully labeled "Full Hard Disk Backup - 4/16/91" in my own handwriting, that I hadn't seen in many, many years. I'm almost 100% certain these came from an IBM PS/2 Model 55 that my mom had at her workplace in 1991.

None of the disks are readable, but I suspect it's because they've been formatted as high density in the PS/2, which notoriously lacked density detection hardware. I'm a bit embarrassed, but 18 years have gone by, so I'll cut myself some slack!

I'd like to read the disks back, but to do so I'll have to disable or short the density hole sensor somehow, so the drive believes that the diskette is HD. I'm sure I can figure out how to do this through trial and error, but I hope that someone here may have done the same thing, and will have some pointers about how to make a shorter job out of it. A reversible modification would be best, but I have enough floppy drives that if I have to sacrifice one I don't really care.

July 28th, 2009, 05:43 PM
Hi! That usually means the floppy disk has a hole in it to indicate HD capable media. If you have a floppy disk drive you can spare, there is probably a LED to photo-transistor pair you could short out to simulate the effect of the missin hole. Since the photo-cell is connected to the base of the phototransistor it should be a just a matter of shorting the collector to the emitter and you are good to go (assuming a BJT).

You'd have to take apart a floppy disk drive so pick one you are not too fond of and experiment with that. Possibly you could fake out the controller by setting the density pin on the floppy drive interface. I think its pin 34 and grounding it would indicate high density. Better double check that though as I am not sure it would work.

I hope this helps.

Thanks and have a nice day!

Andrew Lynch

July 28th, 2009, 06:20 PM
Depends on the drive. Most drives over the past 15 years have teeny microswitches in them to determine density, disk insertion, and write protect.

July 28th, 2009, 08:13 PM
The early PS/2 drives were more like their 5.25" cousins--that is, the drives used "host media select" (pin 2 on the interface connector) rather than "drive media select" (media type aperture governs things). I've even seen a DSDD diskette formatted to 2.88M.

While it's possible to find a drive (some models of the Teac FD-235J can do it) that can be jumpered to do "host media select", they're not awfully common. Failing finding a suitable drive to read them, you can always just drill a 1/8" hole in the disk jacket in the appropriate place.

July 28th, 2009, 09:37 PM
Or you can desolder the tiny switch and jumper any pads on it that may need to be closed.

July 29th, 2009, 05:14 AM
Drill a hole in the diskette? :eek:

July 29th, 2009, 05:43 AM
Find a spare drive with microswitch sensors, and break off the plastic peg that pokes up through the HD hole. This way, the disk jacket won't press the switch down, so the drive always thinks it has a HD disk in it.