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Atari floppy drives for the 8-bit series, the 810 and 1050 specifically, ran at 288 RPM rather than 300. I can't imagine why this might be, and my google-fu fails to turn up a reason.

Worth noting that the XF551, Atari's last 5.25, did indeed run at 300, which I believe was because they used the drive's built-in controller. This did cause minor problems with some disks, but nothing serious. So it seems one could have used 300 at any point.

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  • 4
    Surely, it's related to the Flux Capacitor ;)
    – Brian H
    Nov 23 at 16:57
  • What a blast from the past. I remember (vaguely) disassembling the code of the XF551 for whatever reason... good old times!
    – AnoE
    Nov 24 at 15:57
  • @AnoE - you wouldn't happen to have it anywhere would you? I'm looking for some images of it for a wiki article. Nov 24 at 19:52
  • I held on to that old stuff for a few years back then, but (blasphemy) after a few decades I threw all my old hardware, software, magazines etc. from the 8-32bit era away, and am now only keeping the "previous gen" hardware around for spares... Sorry, @MauryMarkowitz.
    – AnoE
    Nov 25 at 7:50
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the 810 and 1050 specifically, ran at 288 RPM rather than 300. I can't imagine why this might be,

I do believe it was needed to securely store 18 sectors per track while using the WDC1771 at 1 MHz feed by a 6507 at 500 kHz. It might have been a sweet spot for components, with slowing down the drive (zero part cost) enabling some savings.

Worth noting that the XF551, Atari's last 5.25, did indeed run at 300, which I believe was because they used the drive's built-in controller.

No. It was done by simply upping the master clock from 8 MHz to 8.333 MHz, which is almost exactly the 4.1% speed increase from 288 to 300 rpm.

This did cause minor problems with some disks, but nothing serious.

Not really. With the changed master clock differences were way within the original specs. Genuine Atari floppies suffer only with early 810 drives which were not really stable in their analogue parts.

Then again, rotational speed is an issue when disks formatted on a 810/1050 are written on non XF551 300 rpm drives (like on a CP/M machine, or PC), as they may write over the header of a following sector.

So it seems one could have used 300 at any point.

If they had increased the clock speed of the CPU/controller system.

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  • No, the issue with the problem disks is ones that did copy protection by timing reads. The slight difference was enough. Nov 24 at 19:50
  • @MauryMarkowitz Sounds strange as the 'slight' difference (between 810/1050 and 551 if this is what you refer to) was more than 10 times smaller (0.08%) than the allowed variation between drives (1%)
    – Raffzahn
    Nov 24 at 20:07

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