quad density floppies

Floyd Davidson floyd at ims.alaska.edu
Sun Mar 3 21:29:07 AEST 1991

In article <5238 at umbc3.UMBC.EDU> motteler at umbc4.umbc.edu (Howard E. Motteler) writes:
>A while back, Michael G. Beirne writes:
>> I use a Mitsubishi 5 1/4 inch 1.2 meg floppy jumpered to fix the
>> disk speed and this gives me a 720K 5 1/4 floppy. This way I can
>> just reformat my old 360K backup floppies and use them again.
>Is this the same (or similar to) as the "quad-density" from way
>back in the CP/M days?
>I'd be interested in trying this.  How reliable are the 5 1/4's
>used this way?  Would it be possible (e.g., with an external
>switch) to toggle the drive between regular and half speeds, and
>read both sorts of disks?

Yes to all of the above, and more.

I have both the original 48 tpi drive, and a 96 tpi drive mounted in an
external case (along with an external second hard drive).  The data
cable is daisy chained to both drives, both are set up to be the first
drive, and only the last one has the terminating resistor pack plugged
in.  On the data cable coming from the motherboard cut the drive select
line (I think that is pin 10, but you need to check it out), and put a
single pole double throw switch in.  The pole goes to the line from
the motherboard and the two sides of the switch go to either disk
drive.  The switch then selects which drive is used.

I have not used 3.5" drives, but others have with the same results.

With the 5.25" drives it is also possible to get a little more space
than is specified (I don't know how this works on 3.5" drives, it
may not).

To use the 96 tpi drive you need to change the description file
in /usr/lib/iv to have 80 tracks instead of 40.  That is were a
little diddling can change things.  Use 84 instead of 80.  And
on 48 tpi disks (for the original drive) use 42 instead of 40.

If you try more than 84 on a Teac 55F it will bang the stops on
the head mechanism.  If you try 44 with the original drive (mine
is a Teac 55B) it will format just fine, and you can even write
to it, but...  when you read back a cpio archive it can't read
the disk!  So stick with 84 on a 96 tpi drive and 42 on a 48 tpi
drive.  Or if you have other than Teac drives, try it and see
if they are different.

The capacity for 96 tpi drives is 835k for cpio (819k for a file
system).  For a 48 tpi drive it is 395k and 386k, respectively.
The description file used is /usr/lib/iv/FD10nl, with the
number of cylinders changed as appropriate.

A set of several description files, described as for use with
3.5" drives, was posted a long time ago, and is on osu-cis,
I think.

It has been very reliable for me.  I've been using 96 tpi drives
for years with plain old 1D disks (thats right, single sided
disks.  The cheap ones too!) I've got disks that are 6 years old
that are readable (on a Kaypro, not on a UnixPc...).  And it
hasn't hurt the drives either, as I've been moving the drives
from one computer to another as the old computers byte the dust.
Actually I have 6 Teac 55F's that date back to 84-85 and have
not had any of them fail (unlike the Tandon's I had...).

The only problem is that if you just one time write on a disk
with a 48 tpi drive, it has to be bulk erased to use it on a 96
tpi drive.  The narrower track that a 96 tpi drive writes on the
disk will be right down the middle of the 48 tpi track and even a
small mis-alignment of the head will read part of the 48 tpi
track and cause errors.  I bulk erase with two very large speaker
magnets (4" in diameter each).  Anything less will erase the data
well enough that you can't read it, but will leave enough to
cause problems.  I was actually amazed at what it takes to really
clean off a disk!

Floyd L. Davidson  |  floyd at ims.alaska.edu   |  Alascom, Inc. pays me
Salcha, AK 99714   |    Univ. of Alaska      |  but not for opinions.

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