3" disks Was: InfoWorld - May 11, 1992 (3" disk formats)
cisin at xenosoft.com
Mon Dec 2 16:26:54 CST 2019
I haven't heard from Brett Glass in decades, since he moved to Idaho. He
ran the numbers and decided that the differential in market value between
his housing here and similar in Idaho was enough to support him for quite
a while. Since he was working as a writer, he didn't have to be
physically close to his work, and he could get decent internet access
through the university there.
3" drives were readily available in MFM compatible forms. The drive was
designed to be a drop-in replacement for 5.25" SA400 style drives. (OK,
"SA450"?). And the 3" drives even used a 34 pin card edge and a "molex"
power connector (like 5.25"; unlike 3.5")
Depending of format choices, MFM from 180K to 720K.
Amstrad used them, as did early Gavilan and some others. The Gavilans
that I had (both 8 and 16 line models) were later ones, with 3.5" drives.
Gavilan's MS-DOS 2.11 3.5" format was not the same as IBM's PC-DOS 3.20
720K format. But, some development continued, even after Gavilan
collapsed, and the Gavilan MS-DOS 2.ll version K was the same format as
IBM. For those not familiar, MS-DOS 2.11 and 3.31 were versions that were
heavily modified by OEMs, particularly for drive types (including >32M in
3.31). Hence, 2.11 and 3.31 are DIFFERENT from one OEM to another!
But, early on, AMDISK marketed two drive external boxes for Radio Shack
Color Computer and for Apple2. Coco was box standard SA400 compatible
I never had one of the Apple2 3" boxes. So, I have questions about the
Q: Was it a different logic board on the 3" drive for compatability with
the Apple2 "DISK2" interface? (GCR encoding)
Q: Or did their external 2 drive box come with its own MFM FDC for the
Apple2? (In which case, like the SVA FDC, it could adapt an Apple2 to
"standard" drive types)
I assume that the 2 drive external boxes came after the original drive as
5.25" drop-in retrofit, but I could easily be wrong, and it is POSSIBLE
that the Apple2 version could have been the first release of the drives.
The 3" drives were available in 40 and 80 cylinder models.
The 3" drives were available in single and double sided. The single sided
drives would permit "flippy" operation, to use the other side of the disk
as if it were another disk. BUT, the double sided drives (at least
the few that I had) would NOT let you insert a flipped disk; therefore,
the double sided drives could not access the "B" side of a "flippy" disk!
I never got around to looking into modifying the drive for that.
Grumpy Ol' Fred cisin at xenosoft.com
On Sun, 1 Dec 2019, Sellam Abraham via cctalk wrote:
> I thought this was fun; stumbled upon it while looking for what words of
> wisdom Fred had to share about the format of 3" disks:
> Hopefully Fred will see this and tell me whether the 3" disk format was MFM
> or GCR given that the Orwellipedia says the 3" disk format was initially
> designed to work with the Apple ][ floppy drive interface.
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