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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3 Answers
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.
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Oh well, far too slow. But I'll leave this here as I think it adds detail.– TommyCommented Dec 29, 2017 at 22:42
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Thanks, that is indeed useful! I was wondering whether a drive with the ready signal enabled would still work in a PC... Commented Dec 29, 2017 at 22:57
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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.– TommyCommented Dec 29, 2017 at 23:20
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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?– supercatCommented Dec 30, 2017 at 0:47
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.
answered Dec 29, 2017 at 22:29
The Ready signal is part of the original Shugart floppy bus specification, but the IBM PC and compatibles used a different pinout. (One key benefit was to eliminate the need to configure floppy drive numbers with jumpers, by configuring them all the same at the factory, and using a twist in the cable to distinguish between two floppy drives.)
PCs expect to receive a different signal on pin 34, to indicate whether the disk is still in the drive or if it's been ejected and swapped (Disk Change). Shugart-compatible systems such as Amiga, Acorn, or Amstrad machines still expected to receive a Ready signal on this pin.
Rather than design different floppy drive circuit boards for each market, it would make economic sense to have one circuit board which could be configured to operate to either standard. The addition (or omission) of this resistor is how the configuration would be selected when the board was assembled, and how you can modify the board yourself later if you wish.
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1The additional information on this history of this is good, but the answer could be improved by explaining exactly what the "READY" signal is in that spec, linking to the spec (there's a manual on archive.org), and perhaps explaining why and how IBM's changes changed the pinout, and where the pinout changed (on the drives themselves?).– cjsCommented Nov 4, 2019 at 5:50