Preserving a Floppy Disk With a Logic Analyzer and a Serial Cable
owl writes:
Preserving a floppy disk with a logic analyzer and a serial cable:
Being involved with retro computers, I have a few floppy disks (of the 3.5-inch variety) that I would like to preserve as faithfully as possible. Of course, I know there are dedicated devices for doing that, such as the Kryoflux or the SuperCard Pro. But it occurred to me that I already own the required hardware to capture the low-level data from a floppy disk: my Saleae Logic 8 logic analyzer.
Side note: While I can only highly recommend the Saleae analyzers for their features and easy-to-use software, the things described here can also be done with other logic analyzers - including those available for less than 10 from your favorite Chinese online store - and using, for example, the free Sigrok software.
Contrary to more modern mass storage devices such as ATA hard drives or USB sticks, the interface to a floppy drive is much more low-level. E.g., you can ask a modern hard drive to read sector 1337 and it will return you the bytes stored in that sector. In contrast, as soon as it is selected for reading and the disk is rotating, a floppy drive will simply give you a pulse each time the magnetic flux changes, i.e. whenever the magnetic field changes orientation. It is important to know that the magnetic field orientation does not directly represent the individual bits that are stored on the disk. Instead, an encoding scheme is always used. The details of the encoding differ between systems - which is why you cannot read an Amiga disk in an Atari ST, for example. Regardless of the implementation, the encoding always needs to take care of several things: 1. Encode the data bits, obviously. 2. Clock recovery. This is essential because different drives may rotate at slightly different speeds and the floppy disk controller thus needs to determine the actual data rate. 3. Marking the start of a sector. This is often achieved by flux patterns that do not occur in regular data.
Raise your hand if you remember storing data on a floppy disk. Or was that on a floppy disc?
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