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Can a 1.44 MB FDD replace a 720 KB one?

Ruud

Ruud

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I ran into this article: Boost your 1571. The article tells you to use a DD drive. But these are becoming rare. So I wondered if one can use a HD one. The fact is that HD drives can format, write and read DD floppies. I myself have only two machines with DD drives: Sergey's XI-8088 that also supports HD drives and the Commodore PC20-III. But putting a HD drive into the last one means I have to reconfigure some jumpers on the drive and at the moment I have no idea which one. Having several spare drives for these machines I never took the time to have a look at that.

Therefore the question: can a 1.44 MB FDD replace a 720 KB one?

Thank you very much for any info!
 
Yes, a HD 3.5" drive can act as a drop in replacement for a 720kB one. Just make sure to only use DD diskettes
 
Yes, the 3.5" HD drive is designed to depend on the extra hole for activating the HD mode, so if you use a DD disk, without the extra hole, then it should always operate in DD mode.

I'm pretty sure that you should always set such a drive as 1.44 in the BIOS, as that is still what the drive actually is, even if you always use it with DD disks in DD mode.

I have 4 older PCs here with 3.5" drives, and all are set in the BIOS as 1.44 Mb, and all work perfectly normally with DD disks if such is inserted.

I don't remember seeing a 3.5" drive with jumbers. Which drive is that?

Geoff
 
If I understand his question correctly, it is not that easy. In a system that does support 720k disk drives only, a 1.44 drive will not "just" work. And I mean not at all, even with dd disks. At least that is my experience from systems like the Amstrad PPC512 and others I tried.
 
Timo hits the nail: would a XT machine accept a 1.44 MB drive as a 720 KB one? It seems I have to dig up a clone XT, find a DD drive in the first place and start to test things my self.

But thank you anyway for all your comments!
 
Ruud said:
Timo hits the nail: would a XT machine accept a 1.44 MB drive as a 720 KB one? It seems I have to dig up a clone XT, find a DD drive in the first place and start to test things my self.

But thank you anyway for all your comments!
Click to expand...
In most cases it will and I'm not theorizing, this has been done. They computer doesn't even know it's HD and the drive just works if you use DD disks.
Now there are reports of "special" machines that are not exactly generic XT clones and use other proprietary components too, I agree that there it's hit and miss
 
And to remember that in the case of DOS (this thread is in 'hardware' not 'IBM PC, Clones and Descendants'), an extra step, per [here], may need to be done in order to format a 720K diskette.
 
konc said:
They computer doesn't even know it's HD and the drive just works if you use DD disks.
Click to expand...
Yes, I fully agree. But remember in the early days, you could set various jumpers on the floppy disk drive to adapt for variations from the Shugart bus. DD drives had those jumpers, most HD drives however did not (that's e.g. the reason why replacing the floppy drive in an Amiga with a HD one running in DD mode is such a pain).

In the end, I guess he just needs to try. I had no luck with that. However, like the already-mentioned Amstrad PPC512, all XT-clones I ever tried were portable systems. They all gave a seek error at POST. But for a Commodore 1571, that is of course of no relevance.
 
GiGaBiTe said:
Or just use a HD disk with the capacity notch taped over to make it a DD disk. Works fine in many cases.
Click to expand...

I'd like to know more about the cases where this would and wouldn't work. I have some raw unformatted disks that I tried taping over the notch to turn them into 720K disks and they seem to work fine in 1.44M drives formatted as 720K, but they wouldn't format on the T1100 plus with 720K drives. It does fine with real 720K media. Could its head be different? Or something else?
 
There is some fairly scientific discussion about this on the web, and the conclusions are not certain - too many ifs/buts. Comes down to what level of reliability you want/expect.

If you look at the magnetic material in the two disk types, you can see easily that they are different. The HDs are clearly thinner, almost transparent. The magnetic properties of the disks are different. The recording parameters for the two disk types are also different, to suit the magnetic properties of the disks, and the squeezing in of the different number of sectors.

If you try to use one disk type as if they were the other, then it may well work, or appear to work, but it may never be as reliable as using the correct combination. Certainly fine for emergency use, or temporary storage, but I'd not trust any recording for more than a few months. The correct combinations should be good for decades! I'd trust DDs for longer than HDs mind you.

ref: https://www.retrotechnology.com/herbs_stuff/guzis.html

Geoff
 
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I have a 1.44 MB aliasing as a 720 KB in my Tandy 1000SX with no issues. As maxtherabbit mentioned, be sure to use on 720 KB floppies.
 
Re: 3.5" drive jumpers. The Teac FD235J has enough jumpers to satisfy just about anyone. Other drives may need removal-insertion of 0-ohm SMT shunts here and there to get the thing to work the way you want. Unfortunately, most drives have very poor documentation on those shunts.
The Teac FD235HF has gone through many revisions, from having a bunch of jumpers to having none.
 
alank2 said:
I'd like to know more about the cases where this would and wouldn't work. I have some raw unformatted disks that I tried taping over the notch to turn them into 720K disks and they seem to work fine in 1.44M drives formatted as 720K, but they wouldn't format on the T1100 plus with 720K drives. It does fine with real 720K media. Could its head be different? Or something else?
Click to expand...

I suspect it has something to do with variances in magnetic head strength in different drives or, I dunno, bad karma. I do the tape thing with 1.44Mb media myself to use as scratch disks in 720k drives, and my experience is it *mostly* works okay once you can get the disks formatted, although getting that initial format on there can be hard. If you have access to a tape/media eraser to blow away the existing 1.44Mb format it might help; I know this will sound like voodoo, but I would swear I've been able to get disks that won't format to work by taking a powerful refrigerator magnet and ritualistically whirling it around the floppy for a minute or two...

But that said, it definitely seems like some low density floppy drives are more tolerant of this than others. For instance, I have a pair of old Macintoshes, a 512k with the original single-sided 400k drive, and a Mac Plus with the 800k drive, plus two external Apple 800k drives (the original Mac styled one, and the later drive that was styled for to match the Apple IIgs), and taped HD disks work *pretty well* in most of them except the original 400K drive, which seems to have intermittent write issues. (Which, to be clear, it doesn't have with real 720k media.) The PC Convertible also seemed a little "iffy" with them, although I can't say I played with it long enough to really be sure.
 
The failures with the Mac 400K drives shouldn't have been too much of a surprise since those were designed to use the original Sony disks which were thicker and had an even lower Oersted value. I have noticed similar failures using high density disks with the early Sony built drives used by HP. Drives designed to write to high density disks should have less difficulty writing 720K on high density disks.

The Sony drives join the 360 RPM drives and the Samsung internal drive that lacks double density as items to watch out for when troubleshooting.
 
Sometimes you have to punt. I transferred a bunch of disks from 5.25" 100 tpi media with a bit clock of about 400 KHz GCR to 3.5". I have lots of drives and the brand didn't seem to matter. Using new DD media, I was getting one good copy for about every 5 attempted--and the one good copy was always in a HD-capable drive (the DD drives that I have (Teac and NEC) didn't work at all.

Okay, so the bit cell frequency is a bit too high for DD. How about HD? Absolutely no luck there with brand-new HD media. How about if I tape over the HD sense hole and try again? Bingo--every single copy wrote and verified correctly. The copies still read fine a year later.

I'll also mention that the original 5.25" drives were altered at the factory to extend their high frequency end.
 
And during this nice discussion I almost forgot that the 1.44 MB FDD has to be connected to a Commodore 1571 FDD. But thanks to you all I have more confidence!
 
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