aureal wrote on 2022-05-14, 08:02:

Alistar1776 wrote on 2022-05-14, 06:42:

Question... if a 3.5 1.44mb FDD can read and write 720k 3.5 disks, can those disks still function just fine in a normal 720k FDD?

Yes they should read fine unless the 720k disk was formatted into a 1.44mb disk. On the ibm ps/2 it was not necessary to have the extra hole to format to 1.44mb either.

and a 1.44mb FDD cannot format 720k disks as 720k, only 1.44m?

It can format it to 720k or 1.44mb if you mod it with a extra hole. (unless you have a ps/2 drive and you dont need to mod it)

Alistar1776 wrote on 2022-05-14, 08:14:

aureal wrote on 2022-05-14, 08:02:

Alistar1776 wrote on 2022-05-14, 06:42:

Question... if a 3.5 1.44mb FDD can read and write 720k 3.5 disks, can those disks still function just fine in a normal 720k FDD?

Yes they should read fine unless the 720k disk was formatted into a 1.44mb disk. On the ibm ps/2 it was not necessary to have the extra hole to format to 1.44mb either.

and a 1.44mb FDD cannot format 720k disks as 720k, only 1.44m?

???
it's no problem to format a 720k disk in a 1.44MB drive. just specify it, ie FORMAT /F:720

weedeewee wrote on 2022-05-14, 10:38:

???
it's no problem to format a 720k disk in a 1.44MB drive. just specify it, ie FORMAT /F:720

oh ok. I didnt know that, thanks

aureal wrote on 2022-05-14, 08:02:

Yes they should read fine unless the 720k disk was formatted into a 1.44mb disk. On the ibm ps/2 it was not necessary to have t […]
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Alistar1776 wrote on 2022-05-14, 06:42:

Question... if a 3.5 1.44mb FDD can read and write 720k 3.5 disks, can those disks still function just fine in a normal 720k FDD?

Yes they should read fine unless the 720k disk was formatted into a 1.44mb disk. On the ibm ps/2 it was not necessary to have the extra hole to format to 1.44mb either.

Tetrium wrote on 2022-04-28, 09:50:

Virtually all the other 3.5in FDDs I found were HD drives except for a few which were 2.88 ED drives.

How could you tell when you had a 2.88 ED drive without a 2.88mb disk. Is it always on the label?

I have a weird 1.44mb fdd which on the back has less ground pins than normal. I thought they were ripped out and the drive was damaged but when I popped the model into google it seems to be like that. It is a Mitsumi D359m3d https://www.amazon.com/D359M3D-Mitsumi-Floppy … e/dp/B000EISIQ6 . Any ideas know why this is so? Its the only fdd I've seen like this.

One way (and imo the best way) is to look up the part number. This doesn't have to be 100% absolutely reliable though as there can still be some mis-information online about these drives. This was before the internet so when internet was finally a thing the 2.88MB drives were already archaic so little exists online from when these drives were actually current.

Most 2.88MB drives actually have "2.88" somewhere on the front bezel (usually either the eject button or it's just somewhere on the bezel). In some cases the stickers have a different color.

I found mine mostly from memorizing part numbers and going through stacks of 3.5in drives. I'd pick out any with "2.88" on the bezel right away so nobody else could snatch it up 😜

Some drives have fewer ground pins, I presume because it was cheaper to make them that way. Iirc Mitsumi D359 is a more recent drive so probably more cheaply made. The really old ones usually are build somewhat more robustly, kinda similar to how old ODDs are often a bit longer and often a lot heavier.

Sorry to revive an old thread!

waterbeesje wrote on 2022-04-27, 22:07:

Indeed. ED: 3,5" 2,44MB HD: 3,5" 1,44MB or 5,25" 1,2MB DD: 3,5" 720kB or 5,25" 360kB SD: ??? […]
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Indeed.
ED: 3,5" 2,44MB
HD: 3,5" 1,44MB or 5,25" 1,2MB
DD: 3,5" 720kB or 5,25" 360kB
SD: ???

SD and DD disks are the same thing. (What?!)

SD means that the disk is formatted with FM (Frequency Modulation) encoding. DD means that the disk is formatted with MFM (Modified Frequency Modulation), which is a more efficient encoding that can fit two bits into the same space that FM used for one bit by encoding both the clock signal and the data in the same clock window instead of separate clock windows. All you had to do was switch from FM to MFM to double the capacity of a disk. SD and DD disks both had 77 tracks on 8" disks and 40 tracks on 5.25" disks; they're physically identical. Any 48 tpi 5.25" disk can be formatted as SD or DD by encoding with FM or MFM, respectively. MFM was introduced to floppy disks in 1976, which is why no PC user ever had a SSSD or DSSD disk.

Alistar1776 wrote on 2022-04-27, 22:18:

Isnt there 720k 5.25 also?

Indeed there is! DD 5.25" disks are 48 tpi (40 tracks). On a PC, the drives were 300 RPM and had a 250 kbps data rate. 300 RPM = 5 RPS. To get the unformatted capacity of a disk, divide the data rate by the RPS, then multiply by the number of tracks to get the single-sided capacity, and double it to get the double-sided capacity.

250,000 / 5 = 50,000 bits per track * 40 = 2,000,000 bits per side = 250 KB single-sided and 500 KB double-sided.

IBM originally formatted them with 8 sectors per track (160 KB SSDD / 320 KB DSDD) until Tim Paterson pointed out that there was room for a 9th sector, so disks were 180 KB SSDD and 360 KB DSDD starting with DOS 2.0.

In 1982, 96 tpi 5.25" floppy drives were introduced. Doubling the number of tracks t0 80 doubled the unformatted capacity to 1000 KB double-sided. If you used the IBM standard of 9 sectors per track, your formatted capacity was 720 KB. These drives were called DSQD (quadruple density). This format was never used by IBM nor supported by DOS.

By adding cobalt to the magnetic coating, coercivity doubled from 300 to 600 Oe, which made it safe to increase the number of sectors per track. DSHD disks doubled the number of tracks to 80, and DSHD drives doubled the data rate to 500 kbps. That, by itself, would have allowed for disks with a 2000 KB unformatted capacity and 1440 KB formatted capacity; a 1.44 MB 5.25" floppy disk.

For reasons that are unknown to me, DSHD drives also increased the rotation speed to 360 RPM, or 6 RPS. That means that you have to divide the data rate by 6 instead of 5. 500,000 / 6 = 83,333 kb per track * 80 tracks * 2 sides = 1⅔️ MB unformatted capacity. Maintaining the IBM-standard 28% overhead, there was only room for 15 sectors per track instead of 18, which is why the capacity was only 1200 KB. That's why we got 1.2 MB floppy disks instead of a quadrupling of capacity to 1.44 MB.

waterbeesje wrote on 2022-04-27, 19:26:

There was this issue with 5,25 drives. The DD (360kB) drives spin at a different speed then a HD (1,2MB) drive. Some HD drives do have the capability to spin at the speed, but not all.

Nope! Floppy drives have an electric motor that spins at a fixed speed and drives a belt. The gears on your bicycle change the ratio of rotations of the pedals to rotations of the wheel. Floppy drives would have needed a gearing system to switch the ratio between the motor and the disk to support disk speeds of 300 and 360 RPM, which would have added cost and complexity, and it would still have needed to support two different data rates: 250 and 500 kbps. Instead of supporting two rotation speeds and two data rates, the same thing could be accomplished by just supporting two data rates: 300 and 500 kbps. In order to read and write a DD disk correctly, you just need to set the data rate so that there are 50,000 bits on each track. Instead of multiplying 50 kb by 5 RPS, you multiply it by 6. 300 / 6 = 250 / 5. An HD drive spins all disks at 360 RPM, using a 500 kbps data rate for HD disks and a 300 kbps data rate for DD disks.

DOSGuy wrote on Yesterday, 14:22:

Nope! Floppy drives have an electric motor that spins at a fixed speed and drives a belt.

I'm going to be "that guy". Floppy drives with belts are old and rare. And a major PITA. There are some newer exceptions - mostly pre-1990 Japanese 3.5" drives and some later laptop ones, but that's about it.
And with direct drive you can switch speeds, in fact DOS will try to switch speeds before it tries a different bitrate. It'll pick the first method that works.