Kryofluxing PC Floppies

By Michal Necasek

Last year I finally bought a Kryoflux, unfortunately in the middle of moving house. Now I’m finally able to use it beyond verifying that it’s not completely broken. After imaging a few dozens of floppies, I can say one thing–Kryoflux is surprisingly difficult to use with PC 5¼″disks. There is a distinct impression that Kryoflux was designed to deal primarily with Amiga and C64 floppies, and although PC floppy formats present absolutely no difficulty for the Kryoflux hardware as such, using the software for archiving standard PC 5¼″ media is very far from simple.

Let’s start with the easy part. Imaging 3½″ media is relatively simple because PC 3½″drives are straightforward (well, let’s omit the special Japanese 1.6M media). 3½″ drives always rotate at 300 RPM and usually automatically handle media density based on the floppy itself. But if everything were easy, life wouldn’t be very interesting.

Density

With 5¼″ media it gets a lot more complicated. Drives often have no media density detection and the density signal must be driven appropriately by the controller. The Kryoflux GUI has an option for that, with the caveat that “density line low/high” refers to the logic level density signal, not how a drive interprets it. And it just so happens that on typical PC drives, the DENSEL (DENsity SELect) signal is active low, which means that “density line low” in the Kryoflux GUI must be used for high-density disks. To be clear, that is not Kryoflux’s fault, that’s the reality of PC floppy drives.

My experience with the density selection is that for HD floppies, it is critical and when it’s wrong, Kryoflux won’t be able to read anything (DTC reports tracks as unformatted). For low density 360K disks, it makes almost no difference in my experience. For good 360K disks, there’s no discernible difference between using the low and high density setting, Kryoflux decodes the MFM data just fine either way. But for marginal disks, it does make a difference–if the density setting is incorrect, there will be considerably more bad sectors.

RPM

Another troublemaker for 5¼″ PC floppies is the rotational speed. Old (i.e. 5¼″) PC drives rotated at 300 RPM and worked with 250 Kbit/s data rate. When IBM introduced high-density 1.2M drives in the PC/AT, the rotational speed went up to 360 RPM (same as high-density 8″ drives) and a 500 Kbit/s data rate was used.

For working with low-density media in HD drives, a special 300 Kbit/s data rate is used by a PC floppy controller designed for 1.2M drives. The rotational speed is 20% faster (360 vs. 300 RPM), and that is compensated for by a 20% faster data rate (300 vs. 250 kbps).

The Kryoflux default for rotational speed is format specific, and for MFM images it’s 300 RPM. That prevents 5¼″ HD floppies from being successfully decoded by DTC, and the speed has to be explicitly set to 360 RPM.

Double Stepping

Low-density 5¼″ media present another difficulty. 360K floppies only have 40 tracks (48 tpi), while 1.2M disks have 80 tracks (96 tpi). On high-density drives, “double stepping” must be used when working with low-density media, i.e. stepping two tracks at once when performing any seek operation.

Kryoflux, again, can deal with it, but the method is far from obvious. The “track distance” has to be set to 40 tracks, which might sound like Kryoflux would cover 40 tracks of an 80-track floppy… but no, it actually means double stepping.

Weird Stuff

I have come across one floppy (5¼″ low density, mass duplicated, circa 1990) where DTC reported a single track as unformatted and couldn’t read any data (there was just a bunch of zeros in the resulting image). That is, there were no errors reported at all, DTC simply decided the track wasn’t formatted. There certainly was supposed to be data on that track.

Reducing the default RPM for decoding from 300 to 295 allowed DTC to decode all tracks of the floppy without errors, including the one previously seen as unformatted. I don’t know what that’s about.

Alternatives

There are things about Kryoflux (specifically DTC, the software) that are just plain weird, annoying, or broken. For example, when a bad sector is found, there’s no good way to see which one it is. For each track, the number of bad sectors is listed, but without identifying them. In a raw image, the corrupted data is simply written out; that makes sense (the beginning of the sector is often OK), but that makes it quite difficult to precisely identify the bad sector(s).

The DTC GUI allows setting the output directory, which is very useful, but when that directory does not exist, reading floppies fails (simply does nothing) without any indication why. Not ideal.

Again, these are problems with the software, not hardware. There are some alternatives which can work with Kryoflux dumps. I tried the tools that come with PCem but found them significantly harder to use than Kryoflux DTC (in itself an achievement) and simply could not make them work for me. I also tried tools from the HxC emulator which impressed me much more than DTC, were easy to use and more capable, although decoding Kryoflux images was interminably slow.

This seems to be an area undergoing development and things may very well look different a year from now.

Notes

After archiving a few hundred floppies, with or without Kryoflux, I have formed several distinct impressions:

  • Mass-duplicated floppies are very reliable, typically reading with no errors after 25-30 years.
  • User-written floppies on the other hand are very much a mixed bag, with a significantly higher error rate.
  • High density 5¼″ floppies are surprisingly reliable; that is almost certainly a function of their larger physical size and higher rotational speed, which translates to a lower bit density and therefore lower error rate.
  • On the other hand, 5¼″ floppies are susceptible to physical damage caused by poor storage. Floppies can get bent or dented and become partially or completely unreadable. I have floppies bent so far out of shape that they won’t even rotate.
  • 3½″ floppies are significantly better protected with their hard shell. A pack of 3½″ floppies is quite stable and can withstand significant force, surviving poor storage unscathed.

Conclusion

The Kryoflux hardware is perfectly capable of working with any PC floppy format, and the Kryoflux software can handle common formats just fine. The only deficiency is missing presets for common PC media, notably 5¼″ floppies.

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