720kB and 1.44MB drive compatibility riddle with 720kB floppy disks

[ac0mputerguru]

Sometimes you have to wonder.

Consider the Teac FD235HF (most common Teac HD drive that I can think of). It can step 80 tracks at 135 tracks/inch or a track spacing of 0.1875mm.
The stated recording width is here stated as 0.115mm after guard erase (you need this to separate the tracks and avoid inter-track noise). So the total size of the guard band is 0.0725 mm.
Now, let's go to the granddaddy of 720K 3.5" drives, the Sony 0A-32D, a full-height drive with a 26-pin interface. OEM manual here Note that the track recording width is given as 0.125mm with a guard band width of 0.063mm.
In short the recorded track width is slightly wider (0.0095mm or 9.5µM). I suspect that if I run across a spec sheet for the FD-235F (720K), the recorded width will match that of the FD-235HF.

So what's the difference? (I've gone over this multiple times and have documentation from the 3M National Media Laboratory to back me up. Namely, coating thickness, particle size and less importantly, coercivity. The same situation applies to the ED 2.88M recording method, save that a different formulation is used to obtain coercivity in the area of 1200 œ and a particle and coating size that works with perpendicular recording. Track width remains the same.

I can see where the difference between 48 and 96 tpi 5.25" drives gets conflated with the 3.5" drive picture, but they're not the same, aside from the magnetic coercivity of the two media.

If someone can offer hard evidence otherwise, I'd be delighted to review it.

All I have is imperial knowledge from working on the machines at the time. I know what works and what doesn't. Specs are only specs. Real world implementation is what I know.

 

[Chuck(G)]

Against my better instincts, I'll tell you what. Get a couple of degaussed disks and format one to 720K on an appropriate drive and another to 1440K on an different drive. Use some KyRead or other magnetic developer and show me some photomicrographs to bolster your point. I'll not argue with provable facts.
(P.S. I did this about 30 years ago)

 

[ac0mputerguru]

30 years ago, I worked for an IBM VAR, trained and certified by IBM in Chicago with annual recertifications by IBM, Microsoft, Cisco, and Novell. We installed and serviced several hundred IBM PS/1 and PS/2 POS workstations across the nation. We lived and breathed IBM. I've had my own consulting business since 1984, specializing in hardware and database softwares. So, of course I don't know what I'm talking about.

Tell you what: YOU format a 720K floppy in a 720 drive. Copy some files to it. Then go to a 1.44 and change any file on that disk. Take it back to the 720 drive. It will fail. Just like the OP stated happened with his computers!

I don't need a microscope to tell you that, Maynard. It's called "actually working in the field and having to deal with that specific problem, first hand." Not sitting there, spewing specs, and saying that it HAS to work because the specs say so.

 

[Lutiana]

Let's keep it civil. It sounds like it's time for the two of you to agree to disagree.

Tell you what: YOU format a 720K floppy in a 720 drive. Copy some files to it. Then go to a 1.44 and change any file on that disk. Take it back to the 720 drive. It will fail. Just like the OP stated happened with his computers!

I have done this, many times, over the last 30ish years, and have never had any problems with data not being read in the original 720Kb drive. In fact I have done this recently, it's how I get data to and from my IBM5140, which only has 720Kb drives in it, but my tweener machine only has a 1.44Mb drive. Again, never had issues.

I have, however, experienced this with 5.25" drives, going between 360Kb and 1.2Mb drives. So I am wondering if perhaps you have the two confused? Actually, don't answer that, let's just go back to worrying about OP's issue and stay on topic.

 

[jafir]

My guess is that there is some confusion about 360KB and 1.2MB compatibility that is being accidentally applied to 720KB and 1.44MB.

I have also tried this with my Toshiba T1100 Plus and I’ve had no issues. Unfortunately that machine doesn’t POST right now.

 

[Chuck(G)]

have done this, many times, over the last 30ish years, and have never had any problems with data not being read in the original 720Kb drive. In fact I have done this recently, it's how I get data to and from my IBM5140, which only has 720Kb drives in it, but my tweener machine only has a 1.44Mb drive. Again, never had issues.

I do this quite often also, and have no difficulty.

All I have is imperial knowledge

Um, don't you mean "empirical knowledge"?

My name's not Maynard. But if you want to make an issue of it, why not trot over to Herb Johnson's retrotechnology site and put your conclusions out in the open. I'm sure Herb would love to hear from you--and he has a large number of resources, who, like me, "was dere". I'm done with this. Life's too short.

 

[vwestlife]

If there was any minor difference in track width between 720K and 1.44MB 3.5" drives, I'm sure the updated heads got put in 720K drives once 1.44MB drives become widely available, because economies of scale meant it would be cheaper to make one type of heads and put them in all drives, regardless if they were 720K or 1.44MB. Heck, I bet the entire drive was identical between the two densities, except for the controller chip on the circuit board.

And IBM was very cautious about the potential compatibility problems between 360K and 1.2MB 5.25" drives; on the AT they insisted that if you wanted to exchange data with PCs and XTs using 360K drives, you needed to install a dedicated 360K drive. But I've never seen IBM or any other manufacturer give any such warning about potential compatibility problems between 720K and 1.44MB 3.5" drives.

 

[krebizfan]

I think the drives were identical between 720k and 1.44MB except for jumpers. I remember a few people changing a jumper on a 720K floppy to make it into a 1.44MB. That was with drives from the mid-90s when the 720k market was limited to replacements for older systems.

The 720k 3" floppy had problems reading and writing to the 180K flippy disks. When Amstrad was placing both a 180k and a 720k drive in the same machine, the compatibility issue had to be serious.

 

[Chuck(G)]

I must admit those CF2 3" drives always struck me as being cheaply made. I think I still have a stash of replacement belts in my freezer, along with QIC tension bands, etc. Only way I've figured to make them last.
I've never run into a Teac-made CF2 drive. Are they any better than the Matsushita ones?