IBM used 720k floppies pretty extensively. They were available in the AT/5170, and were the default drive in the PS/2s powered by the 8086, including the Model 25 and Model 30.
They were also present in the IBM 5140, and in many other early portable computers, including the Toshiba T1000 and it's descendants. Tandy used 720k drives in the 1000sl2 and the 1000TL line.

So, they were used quite a bit. The later 286 machines did tend to use 1.44mb drives, as did almost all 386es.

- Alex

Complete guess, but the 3.5" format really came to the PC with the IBM PS/2. Wikipedia says only 8086-based PS/2s had DD 720K drives, while 286s and above had HD 1.44. I would be surprised if 8086-based PS/2s were particularly popular in 1987 and later...

When did clones really start to adopt 3.5"? I think it was much closer to the end of the 1980s, if not the early 1990s. By that point, HD was widely established.

Also, some historical context - Macs (which were always centered around 3.5") had DS 3.5" in 1984, DD 3.5" in 1986, HD 3.5" in 1988.

In 1984, IBM AT introduced 5.25" HD drives, ie. 1.2 MB.
3.5" DD = 720 KB, it would be a step back, that's why it got mostly skipped.

The only exception was a bunch of XT-class machines...
- the original HD floppy controller was designed for the AT (16-bit ISA), XT-style controllers were typically DD
- BIOSes in XT machines were also limited to DD
In effect, the only cheap way to use a 3.5" drive in an XT was 720 KB.

Note that the only XTs with 3.5" drives were portables and other compact stuff - full sized XT machines used 5.25" DD 360 KB drives until the very end.

Edit: it should also be noted that 5.25" 720 KB drives got TOTALLY skipped in the PC world...

Yeah the PC's with 720k 3.5" that quickly come to mind are all XT's of IBM and certain Laser XT's and Epsons.

They seemed popular with some European OEMs and Japanese. Apricot I think used them quite early in preference to 5.25. My Sharp PC-4640 has one as original equipment. Olivetti liked them too I think. Come to think of it, I think those companies that also did electric typewriter/word processors with a save to disk facility on 3.5 DD floppy also liked to put them in their PCs more than anyone else. I remember them as quite a bit cheaper than HD drives for a bit, but might have entered the market as it were at the tail end of popularity, so might have been closeout pricing more so than having been half the price of HD in late 80s. At the mid 90s though they were getting harder to find and more pricy again as Amiga users began buying them up for replacements when modded or 5.25 media began getting less easy to find and exchange with other machines, and ppl who were happy DOSing around on XT class still with WP and 123 tried belatedly to upgrade to 3.5 to give them a better change of exchanging data with newer systems. I know I found it more handy to stick DD 3.5s in XTs in early 90s than keep trying to use 5.25 on the daily.

edit: Media quality was a factor for 5.25s as well, unless you were "happy" paying $30+ for a box of 10 disks still, each of which only got 360kb on, the economy options tended to get unreliable quick, like about 3 rewrites and they were a third bad sectors. The cheap 720s at this time were half the price for twice as much space and seemed to do 10+ on the cheap ones before problems, and didn't seem to go bad "just sitting there" like the cheap 360s seemed to do.

Tandy loved 720k 3.5 floppy drives keeping them as options even in 386+ machines which made no sense since 720k/1.44 were fully compatible with DD Media

I remember that a lot of late 80s/early 90s software for DOS/Windows was available on 3,5" 720 KB diskettes.

Things like PC-DOS 3.30, MS-DOS 5.0, Quick Basic 4.5, Visual Basic 1.0 etc.

If it was a big box, then often both 3,5" and 5,25" floppies were included.

The 5,25" types originally were formatted 360 KB capacity, but 1,2 MB got popular soon.

_
In the 90s, I often saw 3,5" 720 KB floppies, 3,5" 1,44 MB floppies and 5,25" 1,2 MB floppies.

Edit:

wbahnassi wrote on 2023-11-04, 00:18:

It seems despite there are lots of software shipped for PC on DD 3.5" disks, PCs always had HD 3.5" drives.

Laptops.. I've seen quite a few late 80s laptops having 3,5" DD drives.

The Schneider Euro AT I once had was equipped with a DD drive, as well.
That's a 286 desktop PC from circa 1988.

XT class PCs may have had 3,5" DD floppy drives, too.
720 KB format is upper limit for the bandwidth of the XT era floppy controller.
For HD floppy drives, a faster controller was needed (available for XTs, but rare).

Edit: The 720KB or DD format was also very popular thanks to Amiga, Atari ST and Macintosh. And last but not least, MSX machines.
That's why there plenty of 3,5" DD diskettes available on market, I believe.
The drive manufacturers thus had been prepared to provide DD drives to the market.

Edit: I forgot to mention, I once dismantled and cleaned a 720KB PC drive myself.
So I know these are real. 😁

VivienM wrote on 2023-11-04, 00:58:

the 3.5" format really came to the PC with the IBM PS/2

The 5140, which is the "obvious beta" of PS/2s 😀
Which in turn is probably the main reason there was an official 3.5 drive for the AT...

My Dad had his Olivetti M24 upgraded with a 3.5" DD 720KB disk drive. The second of the two 5.25" floppy drives (the B: drive, which was the upper one) was swapped out for it. It was a black 3.5" drive in a black 5.25" adapter bracket, which matched the color of the drive it replaced.

Indeed, that was the highest capacity disk drive that PC/XT floppy controllers would support.

In autoexec.bat, we would load the 800 II Diskette BIOS Enhancer TSR by Alberto Pasquale, so we could format and use 3.5" DD diskettes with 82 tracks and 10 sectors per track, which squeezed out 800KB of storage capacity on such disks. 🙂

There was a company that released a floppy controller that would support high-density drives even on PC/XT machines. It was called the CompatiCard. Probably unobtanium these days.

I also heard of some (later?) PC/XT clones that had motherboards that somehow supported high-density diskette drives.

I'm not sure if that's interesting, but there are two "fun facts" that I remember.

- The MSX standard ('82 onwards) specifically used FAT12 filesystem to be compatible with MS-DOS PCs (though no sub directories; perhaps because of MS-DOS 1.0).

- The operating system of the Atari ST ('85) supported FAT12 for interchangeability with MS-DOS PCs. TOS 1.04 (aka Rainbow TOS) improved compatibility even further.

That's interesting, because both platforms used 3,5" drives as their primary storage.
By contrast, 5,25" drives were more popular on PC-98, C64 and IBM PC platforms.

Though they also existed for Amiga and Atari ST platform as an option. Third-party drives were also available.

Are 720K drives completely redundant compared to 1.44MB? In other words:

* Can hook a 1.44MB drive to an XT floppy controller and it can operate as 720KB without issues.

* A DD disk written in a 1.44MB drive can be read on a true 720KB-only drive.

I know the second one indeed works, as I often write DD disks using my PC's HD drive to read on my MSX, which has a 720KB drive.

I don't have an XT so I can't tell about the first point though, but if it does work, then indeed there is no point in a 720KB drive at all, at least in the PC world.. can't tell if this also applies to Amiga and Atari ST. Probably some drive modding can make it work there too.

wbahnassi wrote on 2023-11-04, 13:38:

* A DD disk written in a 1.44MB drive can be read on a true 720KB-only drive.

Hi, I think that's about the only compatibility/reliability issue.

A DD drive uses "wider" tracks than an HD drive.

Thus, reading problems can occur if an used DD floppy gets being written to by an HD drive.

Since the HD drives uses smaller tracks, it can't overwrite the old ones properly.
There will be left over data, essentially.

That situation may confuse a true blue DD drive that tries to read such a floppy.

Its read head will pass over both the small track of fresh data, as well as the old garbage data on the outskirts of the track.

Otherwise, there are no bigger issues.
The formatting itself is hardware-independent.
A HD floppy+HD floppy drive can use 360KB or 720KB formats just fine.

The only thing that should be taken care of is that overformatting a DD floppy to a HD floppy might not be reliable.
Back when 720 KB floppy disks were plentiful and cheap, many users formatted them to 1,44 MB.
It worked, yes. But it's not clear how reliable this was on the long run.

wbahnassi wrote on 2023-11-04, 13:38:

can't tell if this also applies to Amiga and Atari ST. Probably some drive modding can make it work there too.

There is a file on Aminet in the hardware directories with instructions for modding a high density JU-144 drive to use in an Amiga... I think there's one for another model too. Possibly a part of that mod would apply to getting one to work on an XT, but the Amiga uses the drive IDs rather than twisted cable and has something different about the diskchange I think so mods applying to that part might not apply to XT.

Jo22 wrote on 2023-11-04, 13:48:

Hi, I think that's about the only compatibility/reliability issue. […]
Show full quote

wbahnassi wrote on 2023-11-04, 13:38:

* A DD disk written in a 1.44MB drive can be read on a true 720KB-only drive.

Hi, I think that's about the only compatibility/reliability issue.

A DD drive uses "wider" tracks than an HD drive.

Thus, reading problems can occur if an used DD floppy gets being written to by an HD drive.

Since the HD drives uses smaller tracks, it can't overwrite the old ones properly.
There will be left over data, essentially.

That situation may confuse a true blue DD drive that tries to read such a floppy.

Its read head will pass over both the small track of fresh data, as well as the old garbage data on the outskirts of the track.

Otherwise, there are no bigger issues.
The formatting itself is hardware-independent.
A HD floppy+HD floppy drive can use 360KB or 720KB formats just fine.

The only thing that should be taken care of is that overformatting a DD floppy to a HD floppy might not be reliable.
Back when 720 KB floppy disks were plentiful and cheap, many users formatted them to 1,44 MB.
It worked, yes. But it's not clear how reliable this was on the long run.

That problem is applicable to 5.25 drives, 3.5 handle it fine.

BitWrangler hinted at it, theoretically most DD and HD 3.5 floppy drives are fully interchangeable, in practice modern "it's there to install HDD drivers on XP and update the bios if we bother releasing one" ones aren't necessarily suitable but it's for reasons at best tangential to capacity (ready vs disk-change vs nothing, being switchable between 4/2/1 drive numbers, dual speed for "Japanese 3 mode" drives, if we also consider ED drives those have an additional control signal, ...)

Ryccardo wrote on 2023-11-04, 14:45:

BitWrangler hinted at it, theoretically most DD and HD 3.5 floppy drives are fully interchangeable, in practice modern "it's there to install HDD drivers on XP and update the bios if we bother releasing one" ones aren't necessarily suitable but it's for reasons at best tangential to capacity (ready vs disk-change vs nothing, being switchable between 4/2/1 drive numbers, dual speed for "Japanese 3 mode" drives, if we also consider ED drives those have an additional control signal, ...)

Yeah the Amiga guys were noticing the rot set in around 1998 or so I think, 1.44 drives becoming trimmed down, cost optimised and not adhering to original standards, you are lucky if you find a later one that can even switch drive IDs between 0 and 1. So definitely a probability that you can't get the one you pulled out of a 2002 Dell working in an XT ... even maybe not with an XT High Density Controller.

wbahnassi wrote on 2023-11-04, 13:38:

* Can hook a 1.44MB drive to an XT floppy controller and it can operate as 720KB without issues.

Not necessarily: A standard PC-type 1.44MB drive has a "density select" input on pin 2. The PC BIOS is supposed to pull this pin down to ground if a 720KB disk is in the drive, and keep the pin high (there should be a pull-up resistor, and an open-collector driver on the interface card) if a 1.44MB disk is in the drive. On 3.5" drives, this makes no sense, because the drive can also use the HD hole in the floppy to determine what kind of floppy is inserted. Instead, it would make sense to have the drive output whether the currently inserted disk has a HD hole or not. AFAIK that's what IBM did on some PS/2 models.

I don't think the XT controller grounds pin 2 to permanently switch the drive into DD mode, so the 1.44MB drive in an XT will be confused by seeing a "HD" density select input and a DD "no-hole" indication. This can "borderline work". First hand experience: I tried connecting a HD drive as "B: drive" to an Amstrad CPC 664. That computer has an DD floppy controller, just like the IBM PC/XT. Without grounding pin 2, this drive could read what had been written to DD media by another drive. Furthermore, that other drive could read what the HD drive connected to the Amstrad wrote. But the drive connected to the Amstrad was unable to read the sectors it wrote itself. These issues disappeared after grounding pin 2.

digger wrote on 2023-11-04, 13:29:

There was a company that released a floppy controller that would support high-density drives even on PC/XT machines. It was called the CompatiCard. Probably unobtanium these days.

I also heard of some (later?) PC/XT clones that had motherboards that somehow supported high-density diskette drives.

You can perfectly plug an AT multi-I/O card into an XT, and everything except IDE will work on the hardware side just as it works on an AT. Only the IDE functionality depends on the extra signals on the 16-bit ISA bus. The problem is just software: An AT floppy controller needs to be told what bitrate it is supposed to use, and the XT BIOS does not contain any code to configure the bitrate of the AT-type floppy controllers. An upgraded mainboard BIOS or an extension ROM can solve this issue. The CompatiCard thus needs to contain a standard AT-type floppy controller with rate-switching support, and a BIOS extension that cares about rate set-up.

Hmmm, what happens if you plug an IDE HDD jumpered to 8 bit XT mode into a 16 bit controller in an 8 bit slot? Nothing because it's using the wrong IRQ??