![]() If you zoom in on the read data signal, you will see that – for MFM encoding and DD (double density) disks – the flux transitions are 4 µs, 6 µs or 8 µs apart. The side 1 select signal tells the drive which side of the disk to read. During that time – nominally 200 ms for a 300-rpm, 3.5-inch drive – an entire track on the disk is read. Only the falling edge is relevant.įurthermore, so that you know the duration of one revolution of the disk, the index signal provides one pulse for every revolution. Then the disk drive will return flux transitions as pulses as described above on the read data line. The write select signal, write gate, is forced high, which means deasserted, since floppy disks use active low logic signals. This image shows the signals involved in reading data from a floppy disk. It is important to note, however, that the floppy disk imaging is performed at raw flux level and as such will work with every encoding. The following examples will consider MFM (modified frequency modulation) encoding as used in IBM PCs and Atari STs. This is often achieved by flux patterns that do not occur in regular data. 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. Regardless of the implementation, the encoding always needs to take care of several things: 1. The details of the encoding differ between systems – which is why you cannot read an Amiga disk in an Atari ST, for example. Instead, an encoding scheme is always used. It is important to know that the magnetic field orientation does not directly represent the individual bits that are stored on the disk. whenever the magnetic field changes orientation. 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. E.g., you can ask a modern hard drive to read sector 1337 and it will return you the bytes stored in that sector. A primer on the floppy drive interfaceĬontrary 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. 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. ![]() 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. Of course, I know there are dedicated devices for doing that, such as the Kryoflux or the SuperCard Pro. Preserving a floppy disk with a logic analyzer and a serial cableīeing 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. ![]()
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