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While look at some 3.5" floppy disk drives I have, I noticed they have a little resistor missing next to RDY and, looking it up online, I find some instructions for adding a resistor there to enable it. What is it and why would I want to enable it (or why is it not already enabled)?

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Ready was traditionally the signal that went active after the motor had been turned on and two index holes had passed. So it primarily indicates that a disk is present, and also implies that the drive has had time fully to accelerate.

IBM redefined that pin to indicate disk change — it goes inactive when a disk is ejected and active again when the head steps a track.

I would dare imagine the resistor allows you either to pick a behaviour, or to intermingle them; a combination of both tests works on some machines.

  • Oh well, far too slow. But I'll leave this here as I think it adds detail. – Tommy Dec 29 '17 at 22:42
  • Thanks, that is indeed useful! I was wondering whether a drive with the ready signal enabled would still work in a PC... – Stephen Kitt Dec 29 '17 at 22:57
  • If memory serves, the drive's 'ready' signal isn't used directly by an 8272/765, other than to expose it in response to the sense drive status command. So it'd be up to the operating system. So I have no idea. – Tommy Dec 29 '17 at 23:20
  • Why was it necessary to have a disk-change switch? It's generally not possible to swap disks without the write-protect indicator going from "beam is blocked" to "beam is not blocked" (which is how the Commodore drives like the 1541 detect disk swaps). Why would that not be sufficient? – supercat Dec 30 '17 at 0:47
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PCs don’t use the ready signal, and since most floppy drives ended up used in PCs, manufacturers could save a little by ignoring it.

You’ll need it if you want to use the drive in some other systems (notably the Amiga, the Amstrad CPC and PCW). It must be active low when a disk is in the drive, and importantly, it must switch to high and back when the disk is replaced.

Some drives have jumpers which can be used to enable the ready signal; others like yours require modifications.

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