On Fri, 9 May 2003, Nick Garnett wrote: 20 years ago, IBM brought forth onto the microcomputer world a "new" computer, with "advanced technology", called the "IBM AT". I doubt that they sold more than a few million of them with the 1.2M drive. Counting other manufacturers, there were probably less of them sold than MacDonald's hamburgers. The "AT" used a 80286 microprocessor, which could handle up to 16M of RAM, although for half a dozen years there was little if any software available to make use of any of its special features. But the most interesting part of the "AT" was the floppy drive. A number of years prior to the "AT", several drive manufacturers (Mitsubishi was the first one that I'm aware of) created a 5.25" disk drive that was INTENDED to be usable as a replacement for 8" drives. Well before the AT, Amlyn produced a 5.25" 1.2M drive with a 5 disk changer built in; now THAT is an unusual drive! Unlike the previous (360K DSDD) drives which turned at 300 RPM, 1.2M drives turned at 360RPM (like an 8" drive). And when used with 600 Oerstedt diskettes ("High density"), its electrical interface was almost indistinguishable from that of a "standard" 8" drive, such as a Shugart SA8xx, or a Tandon 848. The power supply for it was 5V and 12V, exactly like a "standard" (SA400) 5.25" drive, as opposed to the distinctly NON-standardized 8" drive power requirements. The interface cable for it was 34 pin, like a "standard" (SA400) 5.25", rather than the 50 pin that was the de-facto standard for 8" drives. It had 80 tracks per side, at 96 tracks per inch, instead of the 77 tracks of a "standard" 8" drive. The format that IBM used was 512 bytes per sector, with 15 sectors per track. That gave a capacity of 1228800 bytes, or 1.121875 "honest" megabytes (where a K is 1024 and a M is 1024 K (1048576)), or 1.228 Million bytes, or 1.2 IBM SLEAZY BOGUS "mega"bytes (where a "megabyte" is creatively redefined to be 1000*1024). But the format was largely a software issue, and the machine could do any format that the floppy disk controller was capable of, (see, for example: http://www.xenosoft.com/fmts.html#8 ) It could mimic almost any 8" MFM format. With different FDCs, it is capable of any format that an 8" drive could do with the same controller. The ORIGINAL intent of the drive, according to a few drive manufacturers, was that by changing the power connector and interface cable, it could be used as a drop-in replacement for "standard" 8" drives. There is even an unsubstantiated rumor that when MICROS~1 was working on the OS mods for the new machines that would use it (without having seen the machine yet), that the programmers at MICROS~1 thought that IBM was planning to release a machine with 8" drives! But IBM used the 1.2M drive as the primary floppy for those machines, and even modified their disk controller design to incorporate a 300K bits per second data transfer rate to permit the "new" drive to be able to read disks from 360K drives. It could also write such disks, although with a few complications. [You prob'ly don't want to hear my rants about THAT] Some other manufacturers built 1.2M drives that had both a 360 RPM AND a 300RPM motor speed capability as an alternate way to incompletely solve the interchange issues. IBM made the 1.2M drive their "standard" 5.25" drive from then on, and even started stamping an asterisk on the front of any further 360K drives that they made in order to label them as being different. NOTE: It was intended as a substitute for "STANDARD" 8" drives. In the words of the late George Morrow: "Standards are a wonderful thing; everybody should have a unique one of their own." As has been pointed out by a few people, there existed numerous NON-STANDARD drives. The 1.2M is NOT very suitable for use in replacement of 8" drives where only one brand and model of 8" drive could be made to work, such as hard sectored with PERIMETER holes, drives with non-standard electrical or mechanical specs, drives where the drive circuitry was implemented elsewhere in the machine, etc. It is quite easy, in 8", 5.25", and even 3", 3.25", and 3.5" drives to find examples where NO other drive can replace the one used. Probably, but anybody who can't replace an 8" drive with a 1.2M 5.25", will NOT be able to replace it with a 1.4M 3.5" (all of the changes needed for a 5.25" 1.2M would also be needed for a 3.5" 1.4M, plus a number of other changes). The BEST way to change over from an 8" drive to a 3.5" 1.4M is to FIRST change over to a 5.25" 1.2M, and get THAT working, THEN make the change to the 3.5" 1.4M. Mostly, with some exceptions, those were documenting the existence of "NON-STANDARD" 8" drives, for which ANY change, even to a different make or model of 8" drive would be a challenge. You need to start by identifying the exact machine, and exactly what drive it currently uses. If it "doesn't matter" what brand and model of 8" drive, then it also won't matter whether it is an 8" drive or a 5.25" 1.2M. There has been some discussion here recently about how to identify whether a 5.25" drive is a 360K v a 1.2M. There have been proposals involving latch design, color, LED, weight, etc. The best way is to identify it by model number, and then confirm experimentally. Some of the many model numbers to look for include: Teac 55G, Shugart/Matsushita 475, and Mitsubishi 4854. Going rate for used ones is about a dime a dozen. -- Grumpy Ol' Fred cisin(a)xenosoft.com

Oh yes, the 1.2M drive (that's 1.2 'IBM Megabytes' -- 1000*1024 bytes) was a common drive on PC/AT (286) and later PCs. The PC I am typing this on has one fitted. It's a double-sided 80 cylinder unit turning at 360 rpm (like an 8" drive, all other 5.25" and most 3.5" [1] drives turn at 300 rpm). It uses the higher data rate (500kbps) like an 8" drive. I have successfully used such a drive in place of an 8" drive. It's not a drop-in replacement -- the cable/connectors are different, and there are minor differences in some of the signals. Btu it can noramlly be got to work. The problems really start if the drive you're replaceing wasn't a standard 8" drive (the cannoical example of this is the DEC RX01 or RX02), or if it uses some of the features on the 8" drive (data separator, hard sector sector/index hole separation) that were never provided on 5.25" drives [1] Early 3.5" drives, often used by HP, rotated at 600rpm. And for completness, there was a 5.25" drive that rotated at 180 rpm, allowing the use of 1.2M disks on a normal (not high density) controller. It's possible, but if you want to go that route, get the machine working with a 1.2M 5.25" drive first. Once that works correctly, then get it working with the 3.5" unit. By doiing the conversion in 2 stages, you will have a clearer idea as to where any problems are coming from. [...] I sometimes forget that not everybody was born holding a soldering iron and screwdriver :-).. Although I do find it hard to understand how anyone can attempt to maintain and/or modify a computer without understanding some hardware (and for that matter some programming). They are not rare. I don't know how easy they are to find for sale -- certainly I am not selling any of mine. But there are plenty of working drives still around (Even if most of them are attached to working vintage computers). I find that very hard to believe. These drives were _very_ solidly made. I supplse it's possible the heads have worn out with all that use, but just about anything else can be fixed quite easily. Heads can be replaced if you can find them, BTW.... -tony