IBM PS/2 Formatted Diskettes - Non-Standard
I have 3.5 diskettes that are IBM PC formatted and I am trying to read them with the Supercard Pro. They are low-density diskettes formatted as high-density. Unfortunately, they are standard formatted but non-standard bit rates and possibly rotation.

The IBM PS/2 series allowed a 720 K floppy disk, without holes punched, to be formatted at 1.44 MB. The diskettes work flawlessly in the PS/2 series (50z, 55sx, etc.) and they weren't readable on IBM clones. When the PS/2 was readily available, I never thought to copy these diskettes to the standard, dual punched high-density diskettes.

I ran across several old diskettes formatted this way. The good news is that I have a physical IBM PS/2 Model 50z (Micro Channel) on my shelf. The bad news is the diskette drive motor no longer spins. 

The attached image shows the results of the Supercard Pro reading the diskettes. My other IBM diskettes, both 5.25 and 3.5, work flawlessly. The splice method has recovered bad sectors in nearly every instance. These diskettes are simply a problem at this time.

Is there a way to get the Supercard Pro to read these diskettes short of buying a PS/2?

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The SuperCard Pro is just a flux level reader/writer. It doesn't actually "read" anything, it just collects the flux that is read by the disk drive. If the drive can't read the flux (due to the disk being thought by the disk drive that it is low density when in fact it is high density) there might not be anything that you can do to force the drive to read the disk differently than it normally does.

No, having said that, there is one possibility though - you can try changing the DENSITY in the pull-down menu. That will invert the density line. This option was made because some disk drives have a jumper that sets this and sometimes it is backwards. If that disk drive pays attention to that signal (some ignore it), then that might force the drive into reading the opposite density that it normally does. It's worth a try.
Hi there,

I realise that this is a relatively old thread, but I was wondering what would happen if you did punch those disks so that they would be read as high-density disks by normal drives?

Kind regards,

Dreamland Fantasy Studios
(05-08-2022, 04:05 AM)dfstudios Wrote: I realise that this is a relatively old thread, but I was wondering what would happen if you did punch those disks so that they would be read as high-density disks by normal drives?

I never tried notching the disks with SCP. Back in late 80's/90's, though, I did run through the various scenarios when I found these low density/high density formatted disks were uniquely formatted and didn't work elsewhere. A fellow student had a clone PC and the clone couldn't read the disks I formatted for a group project.

Notching a disk formatted this way didn't fix the problem with non-IBM PS/2 machines. The student and I tried this. Interestingly enough, if you formatted a disk in this way and later notched the disk, the IBM PS/2 that created the disk would no longer read it.

I hooked up the SCP tonight and found one of these disks to try. I changed all of the options available, including using splice mode, with no success. I found a couple disks formatted this way and they all produce the same results.

SCP has done a great job with my disks. Reading this type of flux would be a bonus. The drive is a TEAC FD325HF 3291-U has block of jumpers and I haven't tried changing them yet. The drive is set to automatic density, A3-B3.
Just adding a density notch to a 720k disk that has been formatted as 1.44mb should make it readable using a SuperCard Pro or similar device. It is easy enough to cover the new notch back up if needed.

In normal 1.44mb drives, the density notch controls the analog filter the drive uses to create the digital data out signal. In most 1.44mb drives a high density (500kbps) signal filtered as low density (250kbps) will get partially filtered out as "noise" and therefore will be unreadable.

Almost all 1.44mb floppy drives IGNORE the density line signal sent by the SupoerCard Pro, similar flux device, or floppy disk controller. These drives exclusively rely on the density notch to determine which density filter to use.

I have only one 1.44mb drive - a Sony MPF520-D P/N 04H7405 FRU 93F2361 - that does indeed respect the density line, thereby allowing a SCP or floppy controller to override a density notch. (Note: some drives use the density line to engage the 360RPM Japanese "Mode 3") I think that drive was used in IBM Valuepoint systems in order to maintain compatibility with these disks. It is otherwise a perfectly standard 1.44mb drive.

It is likely that some 720k disks may not have been capable of holding a high density signal, and have degraded over time. These will not be readable in any drive, no matter what. The magnetic formula used to make the disk cookies differed between manufacturers. That said, I have many 720k disks that were intentionally notched, successfully used as high density, and are still readable today.

I don't have any sample disks like this off hand, so I can't check - but It is also conceivable that some PS/2 systems may have written something differently on the boot sector or file system that may have prevented *DOS* from reading them properly. But in that case, a flux level reader, once the drive's density filter is set properly, should still be able to read the content. Make a dump, feed it to HxC and the sectors should light up green.
I have never seen a drive not respect the state of the density line (which is why that option was even there in the first place). I have a dozen different brands of drives. The density line also typically changes the drive speed along with the analog filter. HIGH DENSITY typically runs at 360 RPMs, and LOW DENSITY typically runs at 300 RPMs. This is not a rule, but it is very common with the drives.

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