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When the 3.5" FDD was introduced for the IBM PC and compatibles (or possibly earlier), someone made a decision to make the data cable between the controller and drive unkeyed. This, of course, meant that you could plug it in the wrong way if you weren't paying close attention to which side pin 1 was on.

If you did plug the cable in the wrong way, two obvious user-observable things happened:

  • Neither the floppy disk drive nor the controller broke; if you realized the error of your ways in time, all you needed to do was flip the cable over and give it another try, and no harm came of it
  • When the system was powered on, the disk drive's activity light would be continuously illuminated

Also, as I recall (though this has been argued against in response to this question), if you inserted any floppy disk, whether the read-only tab was in the read-only or read/write position, however briefly, while the drive was in this state, then that disk was trashed and basically only good as a coaster afterwards (due to their almost square form factor, floppy disks never made good frisbees).

What was actually going on inside that floppy disk drive, and why?

  • "someone made a decision to make the data cable between the controller and drive unkeyed" I like how modern connectors like Apple lightning en.wikipedia.org/wiki/Lightning_(connector) don't care which way you plug them in. – Caltor May 22 at 10:58
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One side of the cable is normally all ground. The other is all signals. This was done so a ribbon cable would have a ground wire between each signal wire, which helps prevent crosstalk.

Flipping the cable swaps signals for grounds, grounding all signals. Since all the signals on a floppy disk are considered to be asserted when a zero (a logic 1 = ground) all the signals are activated.

The cable has many signals, but these are the key ones that erase the disk.

  • /MOTEA ( Motor enable) is asserted, which starts the motor spinning.
  • /DRVSA (drive select) is enabled. It engages the head load solenoid which pushes the head against the floppy diskette.
  • /WDATE (write data) is asserted low, so a logic 1 will be written to the floppy
  • /WGATE (write gate) is asserted so the current to the head is turned on.

The floppy head is a small electromagnet, so the data on the track under the head is erased.

I learned to always power up and check the led before putting on a floppy. Many years ago I erased a very valuable Unix partition on a hard disk by powering up the disk and then the controller. The exact same thing happened, because I had two power cords instead of one.

  • Could you please elaborate on some things? (I'm not sure what; it's a good answer, but seems short.) – wizzwizz4 Jul 19 '16 at 6:30
  • I added cable details on the exact signals involved. It's actually making logical 1's on the cable, but I don't have enough rep t comment above. This asserts all the signals. /DRVSA is asserted when low. This requires a logical 1, indicated by the slash. – Ferd Sep 24 '16 at 1:41
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    Now I understand why this happens! It's such a shame I can't upvote a second time. I've making a small edit to improve the formatting; if you don't like it you can revert the changes. – wizzwizz4 Sep 24 '16 at 8:00
  • I found a pinout at pinouts.ru/HD/InternalDisk_pinout.shtml which has all of the signals. – a CVn Feb 27 '17 at 16:17
  • I believe this is still the canonical method to format a floppy disk in *nix operating systems. ;) – Caltor May 22 at 10:52
17

According to the standard pinout only even pins are used for actual signals, and all odd pins are used for ground. (some floppy drives are actually missing every nth pin or more from the odd row - because there is no need for that many ground pins, this makes very easy to identify which row is ground).

Reversing the cable simply grounds everything on the motherboard floppy interface - but since the interface is of "Open Collector" type - that causes no harm to signals coming out of the controller to the drive.

Conversely, digital signals that the floppy controller tries to read would all be seen as binary "0" - this is effectively what grounding them does.

Even though signals are swapped with ground the drive still does get "correct" ground from the power connector - this is how it grounds all other signals.

Activity light would be illuminated because the drive still does get the power from the 4-pin cable - I'm not sure whether this was on purpose to indicate a problem, it might depend on a drive.

I've never heard of data corruption caused as a result of the cable - I'm sure the heads don't write anything. Physical corruption can't happen either - the heads make contact as designed when the disk is inserted (the head's arm are lowered only then). - Corrected as per feedback.

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    Interesting side note: If I recall correctly you the Atari ST uses a standard drive but with the cable connected the wrong way around! I guess they hooked up the odd pins instead. – Matt Lacey Jun 21 '16 at 0:43
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    "I've never heard of data corruption caused as a result of the cable" Well, now you have: It happened to me. I definitely recall breaking one or two boot disks when building PCs and inserting the boot disk before turning on power for the first time (or maybe it was just thinking "hey that light being on is odd, okay let's proceed anyway"). Another tech at the company at the time said afterwards "throw it away, it's ruined now" about that floppy. – a CVn Jun 21 '16 at 7:53
  • I second that. I trashed several floppies that way. – chris-l Jun 21 '16 at 7:55
  • Ok, so be it. Thanks, correcting my answer then. – Kuba Tyszko Jun 21 '16 at 8:00
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    I'd never heard of data corruption either, and I worked for many years back in the day as a PC repair tech. I was about to comment the same thing on the question. To be fair, I always checked to see that the floppy drive light went off during POST whenever I added a drive, but I'm surprised at least one time I hadn't left a floppy in the drive. – Cody Gray Jun 21 '16 at 15:59

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