Give my "ImageDisk" a try.

I designed it to be able to read anything the PC FDC could read.
It is specifically designed to handle very unusual and non-standard formats,
and is pretty much the "tool of choice" to backup and recreate media for very
old/unsual formats, and is used by many museums around the world to preserve
such material.

You can get it from "Daves Old Computers" -> "Download disks/software images" (near the bottom)

Note: To be able to completely control the PC FDC (Nec 765 compatible) it accesses the
FDC directly and in somewhat non-standard ways ... in other words it needs DOS (as DOS doesn't
restrict your ability to access hardware, and doesn't have "other stuff" running in background which
could affect it's ability to timer accesses) - I do offer an image of a DOS boot disk with ImageDisk
on it which you can use if you want to use it on a non-DOS system)

Dave ::: https://dunfield.themindfactory.com ::: "Daves Old Computers"->Personal

Grzyb wrote on 2024-12-12, 01:33:

I know that some CP/M formats are readable with a standard PC FDC, so maybe there's a chance?
Or do I need to aquire stuff like KryoFlux or Greaseweazle?

I vote Greaseweazle because I own one, and it's great. You can do raw dumps of the floppies, with that you can re-do any conversion later on without having to use the real media again.
The only problem - but that is true to all the methods - is you need a floppy drive with the same step as the original. I'm not sure what TPI the Elwro used but I would guess it's the more-or-less "standard" 48 or 96 for the 5.25"?

UPDATE: So apparently it's 720k format, 80 tracks on 2 sides but with only 9 sectors per track. This would be readable by PC if the markers and gaps are standard, the rotation speed (300 vs 360) is worked around by the FDC using 300kbps instead of 250. Just keep that in mind if you ever need to re-create a working floppy. Still, a raw dump is always superior.

The big thing something like GreaseWeasel will do for you is it can record the
raw bit stream from the drive ... so it can read ANY format at a very low level
it then requires lots of software to convert that raw bit dump into meaningful
sectors.

Floppy disks are essentially circular magnetic tapes - so digital data can be
encoded "any way the vendor feels like".. but almost all use an off-the-shelf
FDC "chip". The FDC used in the PC implements a very well known and standard
way of encoding data "sectors" - and *almost* all disks are encoded/written
in a compatible method.

There *ARE* disks encoded well outside of that standard (eg: Apple II)
which the PC FDC can't read.

So there is usually a reasonable chance that any random diskette can be
read on a PC - but not always.

This standard way of encoding disks has a LOT of variables, some major, some
minor - the native PC software only knows about and can deal with a small list
of the *many* encoding possibilities of the FDC hardware.

ImageDisk will perform an analysts if the disk encoding and determine what
parameters are used to read/write the disk. This information is stored into the
.IMD image file - so it knows how to recreate it.

IMDU (IMageDisk Utility) is a tool which can perform various operations on a
.IMD file - including the ability to extract the disk sectors content as raw
binary data - this allows those "in the know" about that disk format of the
original OS to access/manipulate the data as needed.

Do be aware that not all PC FDCs are equal, and some of the non-PC formats
may be readable on some, and not readable on others. (eg: single-density and
many of the possible sector sizes are not used on a PC and not supported by
FDC build into some PC chipsets). If you have having trouble it's advisable
to try it on a few systems. (and if you find a "good one" that can read most
of the non-PC formats - hand on to it!)

And a major advantage of ImageDisk over the likes of GreaseWeasel - you can try it for free!

Dave ::: https://dunfield.themindfactory.com ::: "Daves Old Computers"->Personal

Deunan wrote on 2024-12-12, 14:25:

The only problem - but that is true to all the methods - is you need a floppy drive with the same step as the original. I'm not sure what TPI the Elwro used but I would guess it's the more-or-less "standard" 48 or 96 for the 5.25"?

UPDATE: So apparently it's 720k format, 80 tracks on 2 sides but with only 9 sectors per track. This would be readable by PC if the markers and gaps are standard, the rotation speed (300 vs 360) is worked around by the FDC using 300kbps instead of 250. Just keep that in mind if you ever need to re-create a working floppy. Still, a raw dump is always superior.

FWIW, single/double step, sides and kbps are among the many parameters that ImageDisk determines during it's analysis.
- it can read/recreate most anything the FDC can access.

Dave ::: https://dunfield.themindfactory.com ::: "Daves Old Computers"->Personal

IMD can handle single/double stepping just fine, the issue I had in mind is those more uncommon drives thare are not 48/96 TPI. For example there are some quad density 77-track drives with 100 TPI. Floppies written with different TPI need a specific drive to read them back. The early Elwro 800 Junior seems to have used another format, 150k only. That is possibly single-sided, and FM instead of MFM. I've yet to find a PC FDC controller that can do FM...