tim@j.cc.purdue.edu (Timothy Lange) (01/09/88)
How is this for explaining the different floppy disk types? No flames, just
helpful criticism!
I get questions concerning the differences between double sided, double density
(DS/DD) disks versus double sided, high density (DS/HD) disks and double sided,
quad density (DS/QD) disks. To explain these differences I have come up with a
chart or table to explain what is going on and what works with what. To save
space, from now on DD means Double Density, QD means Quad Density, HD means
High Density, SS means Single Sided, and DS means Double Sided.
So far I have the following table for 5.25" diskettes used on IBM PC:
Disk Drive Type: PC Drive Type: AT
Type IO Freq-48 TPI IO Freq-48 TPI / IO Freq-96 TPI / CO Freq-96 TPI
SS/DD 180K 180K*1 / 180K*2 / Note *3
DS/DD 360K 360K*1 / 720K*2 / Note *3
DS/QD 360K 360K*1 / 720K / Note *3
DS/HD Note *4 Note *4 / Note *4 / 1.2MB
The IO and CO Freq means using the frequency for Iron Oxide or Chromium Oxide.
The TPI abbreviation means Tracks per Inch.
*1 Track width and alignment may cause problems when used on other drives.
*2 Disk not be certified for this track density.
*3 Iron Oxide media not compatible with this frequency.
*4 Chromium Oxide media not compatible with this frequency.
So far I have the following table for 3.5" diskettes used on IBM PC:
Disk Drive Type: model 30 Drive Type: model 50
Type IO Freq-135 TPI IO Freq-135 TPI / CO Freq-135 TPI
DS/DD 720K 720K / Note *1
DS/HD Note *2 Note *2 / 1.44MB
*1 Iron Oxide media not compatible with this frequency.
*2 Chromium Oxide media not compatible with this frequency.
I know that HD drives can deal with DD floppies by changing the recording
frequency to match the oxide used on the media and the track to track
distances. At format time, the drives do not know what kind of disk is in
them, you have to tell it with formatting options. Once the disk has been
formatted though, the drive will figure out what it is, if it is capable of
supporting the media being used.
A DD drive can handle DD disks, a QD drive can handle DD and QD disks,
and a HD drive can handle DD, QD, and HD.
You should NOTE try to exceed the certification of the disk, i.e., use a SS
disk as DS, use a DD disk as QD or HD, use a QD disk as HD, or a HD disk as DD
or QD. If the disk is marked as SS that means the manufacturer has certified
only one side for use. The other problems are recording frequency/Oersted
versus media oxide type mismatch.
What is a QD disk by definition? From what I have been told, it is basically a
DD disk certified at the 96 TPI track density.
Here are some other facts: DD and QD disks have an Iron oxide media, HD disks
have a Chromium oxide media. DD and QD are recorded at 300 Oersted, HD disks
are done at 670 Oersted. A DD disk can have two sides, each having 40 tracks,
each track having 9 sectors, each sector having 512 bytes. A QD disk just has
80 tracks per side, (but you can raise the number of sectors per track, example,
10 sectors per track would give you 810K for disk space, non-IBM then, if
ever). A HD disk, has two sides, each having 80 tracks, each track having 15
sectors, each sector having 512 bytes.
IBM compatible machines use soft sectored disks, this means at format time the
software writes it own sector marks on the disk instead of using hard sectors
which are defined by timing holes in the disk itself.
__________
Oersted. The cgs emu of magnetic intensity exists at a point where a force of
1 dyne acts upon a unit magnetic pole at that point, i.e., the
intensity 1 cm from a unit magnetic pole.
--
Timothy Lange / PC Learning Resource Center / Mathematical Sciences Bldg.
Purdue University Computing Center / West Lafayette, IN 47907
317-494-1787 / tim@j.cc.purdue.edu