4 floppy disk drives on a PC?

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Mon Jan 17 17:48:40 2005

> It'd be nice if there was a FAQ about floppy formats in general TBH.
> Covering some of the different geomery and encoding schemes out there
> etc. (not necessarily mentioning any machines by name).

I'll stick my 5.25" MFM (PC, essentially) disk FAQ at the end of this
message. You (or anyone else) is welcome ot make use of it in a more
comprehensive FAQ provided you give me an acknowledgement.


5.25" floppy disk Frequently Asked Questions

2000 A. R. Duell with comments and additions from Fred Cisin. Please feel
free to distribute this document (on web sites, ftp sites, mailing lists,
etc) provided this notice is intact.

This doecument was written as a response to questions on the classic
computer mailing list regarding the use of various types of 5.25" floppy
disks in various types of drives. It attempts to explain what
combinations work and why. For the moment I am only considering
soft-sectored drives...

1) What types of (soft sectored) 5.25" drives are there.

There are 5 common types :
48 tpi single sided, double density. These have 40 cylinders (and 40
tracks). On a PC they'd store 180K bytes, however on other systems, their
capactiy could be anything from 160K to 200K. A few very early drives
(Shugart SA400, for example), and systems designed to use them, only used
35 of the 40 tracks, but kept the same 48tpi track pitch

48tpi double sided, double density. These have 40 cylinders (80 tracks
total, one for each cylinder on each side of the disk). This is the
common PC 360K drive. When used on other types of machine, the capacity
is in general twice that of the single-sided drive, or somewhere between
320K and 400K.

96 tpi, single sided, double density. These have 80 cylinders (and being
single-sided, 80 tracks). These are not common on PCs, and in fact most
versions of PC MS-DOS don't support them, but if used there would store
360K. The DEC RX50 is a (double) drive of this type. The capacity of
these units is the same as that of the 48 tpi double sided drives (not
too suprising, as they have the same number of tracks).

96 tpi, double sided, double density. Again, they have 80 cylinders (and
thus 160 tracks). On a PC, they'd store 720K, although they're not
commonly found on PCs, although the IBM PX=JX used them. Theyre are
common on non-PC compatible MS-DOS machines, CP/M machines, Acorn BBC
micros, and early-ish unix workstations, where they store between 640K
and 800K depending on the formatting.

96 tpi double density drives are sometimes called 'quad density' units.

96 tpi, double sided, high density. This is the PC 1.2Mbyte drive, and is
not commonly used elsewhere. These drives have several differences
compared to the double density version, although the PC controller hides
some of these. This drive is, in fact, somewhat similar to the 8" drive
-- it uses the same data rate and even the same spindle speed (360 rpm,
as against 300rpm for all the other types of drive). It thus can often be
used as a substitute for an 8" drive in older machines, if connected up
with a special cable.

2) What are the real differenced between the various types?

The difference between single and double sided drives is obvious -- the
double sided drive has an extra head (mounted on top of the disk) and a
switching circuit to select between the 2 heads. In passing at this
point I'll mention that single-sided drives record on the bottom
(non-label side] of the disk and that this is 'side 0' on double sided units

The differences between 48 tpi and 96 tpi double density drives are again
fairly obvious. The head positioner (mechanism that moves the read/write
head) is designed to move the head only 1/96" per step rather than 1/48".
The actual head in a 96tpi is narrower (radially) than the one in a 48tpi
drive so that it writes a narrower track on the disk (so that adjacent
tracks don't overlap at the closer spacing).

The high density drive has several differences wrt the 96 tpi double
density unit. Firstly, the spindle motor rotates at 360rpm (at least in
high density mode, see below) rather than the 300rpm that all other
drives rotate at. Also the 'write current' (the electric current passed
through the head coil to write on the disk) is higher in high density
mode so as to be able to write on the special high density disks. These
have a higher coercivity than normal double density disks.

There is a signal on the interface connector of high density drives (at
least the properly-designed ones) that, when asserted, reduces the write
current to the value used with normal double density disks. In this mode,
the drive will reliably work as a 96 tpi double density unit. In some
drives, asserting this signal will slow the spindle motor down to 300rpm,
in others, it continues to turn at 360rpm and the controller has to
handle a data rate of 6/5 times times the standard 250kbps double
density rate (=300kbps). The IBM PC/AT disk controller is capable of doing

3) What types of disks exist?

All 5.25" disks that I have ever seen are coated with the magnetic oxide
on both sides.

A double sided disk means that both sides have been tested and shown to
be reliable for storing data. A single sided disk may be one where the
top side has failed this test (and the bottom side is good) or one which
simply hasn't been tested on both sides. It is often claimed that single
sided disks are those that have certainly failed the test on the top
side, but in fact, the reliability of the disk manufacturing process was
such that very few disks failed, and that therefore there simply wouldn't
be enough made to sell as single-sided ones. Most single-sided disks are
therefore good on both sides.

I (ARD) have used a number of DEC RX50 disks that have been bulk-erased
as double-sided 96 tpi disks. Since the RX50 is a single-sided 96tpi
drive, there would be no reason for these disks to work reliably as
double sided ones, but I have yet to have one fail.

'80 track' -- 96 tpi -- double density disks do seem to be different from
'40 track' -- 48 tpi ones. I suspect, without proof, that the 96 tpi ones
are lower 'noise' which is important for the narrower tracks used on such

The original 5.25" disks were designed to be used in 48tpi drives, since
that's all that there was at the time. Once 96 tpi drives became popular,
many manufacturers starting making all their disks suitable for use in such
drives (it was cheaper for them to have one production line) and sold
them as 'universal' disks, suitable for use in 48 or 96 tpi drives,
single or double sided.

However, once the IBM PC and PC/AT became the only common computers to
have 5.25" drives, many manufacturers went back to making 48 tpi disks
only, since that was the only double density drive in common use.
Therefore many modern double density (known as '360K disks') are _not_
reliable in 96tpi drives.

High density disks are different. Period. The magnetic media has a
different coercivity (600 oerstedt for high density disks, 300 oerstedt
for all other 5.25" disks), and it can only be used in the high density
drive _at the high density_.

4) What sorts of blank disks can be used in what drives?

Double sided disks can always be used in single sided drives. The fact
that the unused side is also perfectly good for storing data doesn't
matter, of course.

Some people modified the disk jacket of double sided disks by cutting an
extra write protect notch and index hole (for those systems that used it,
which is basically everything apart from Apple and Commodore) in it. Such
disks could be used either way up in the drives so that both sides of the
magnetic disk could be used, and were often known as 'flippy' disks. Some
manufacturers even sold disks with the extra notch and hole pre-cut at
the factory, and I've even seen the service manual for a Siemens drive
with 2 write protect sensors and 2 index sensors so that a normal,
unmodified disk could be used either way up.

Note that doing this means that the data on one side of the disk is
recorded 'backwards' (the disk is effectively spinning in the opposite
direction), and thus the result cannot be read in a 2 head drive simply
by selecting the other head. Even with a 2 head drive you still have to
flip the disk over manually.

96 tpi double density disks can be used in 48 tpi double density drives.
Again, the disk is 'better' than it needs to be, but that doesn't matter.

This means that these disks can be used as follows :
disk works in
96 tpi DS : 96 tpi DS, 96 tpi SS, 48 tpi DS, 48 tpi SS
96 tpi SS : 96 tpi SS, 48 tpi SS
48 tpi DS : 48 tpi DS, 48 tpi SS
48 tpi SS : 48 tpi SS

For that reason, many manufacturers sold 96 tpi double sided disks as
'Universal' disks. They could be used in all types of (double density)

High density disks are special. They can _only_ be used in high density
drives at the high density format. Similarly, high density drives will
only reliably work in high density mode on such disks. But if the
appropriate signal is asserted, then the high denisty drive behaves like
a 96tpi double sided double density unit, and can use double density disks.

5) What combinations may work under some circumstances?

Single sided disks may work in double sided drives. Firstly, some systems
(many systems?) allow you to format them as single-sided, for which they
are (obviously) suitable. And in many cases the 'other' side of the disk
is perfectly good and the disk can be formatted as double sided.

48 tpi disks may be good enough to work in 96tpi drives. My experience
suggests this is not reliable, though.

6) What about data interchange? What (already recorded) disks can be read
in what sorts of drives?

Let's deal with the obvious cases first. A double sided drive can read a
single sided disk. The upper head is simply not used. Similarly, a double
sided drive can write to an already-used single sided disk.

Another obvious case is that the high density disks can only be used in
high density drives.

The less obvious case is the 48 tpi versus 96tpi issue. The drives were
designed so that the centre line of alternate 96 tpi cylinders is the same
distance from the spindle as the centre line of each 48 tpi cylinder.
Thus 48 tpi disks can be read in 96 tpi drives if the drive 'double
steps'. Some drives can do this in hardware (there may be a switch marked
40/80 on the drive casing), some operating systems can handle this.

Since a high density drive can be 'turned into' a double density drive by
asserting that signal I keep mentioning, a high denisty drive can also
reliably read 48 tpi disks.

96 tpi drives writing to 48 tpi disks is a cause of many problems, which
deserves its own section. Erasing a file involves writing to the disk
directory, of course, and thus counts as writing to the disk.

7) What's all this about writing to 48 tpi disks in 96 tpi drives?

This is perhaps the biggest cause of problems with 5.25" disks. People
write a file to a 48tpi disk using a 96 tpi drive and find that the
result is readable on 96 tpi drives but not on 48 tpi drives.

Remember that the 96 tpi drive has a narrower head than the 48 tpi drive,
so it writes a narrower track to the disk.

A blank disk is one that has just been manufactured, or that has been
bulk-erased. Reformatting a disk in a normal disk drive _does not_
produce a blank disk in this sense -- it writes the track pattern to the
disk, with whatever width of head the drive doing the formatting uses.

Suppose we take a totally blank disk and format it on a 48 tpi drive.
This writes 40 tracks on each side of the disk. They may be 'empty' in
the sense that they contain no user data, but they're still recorded.

Then we write to it on a double-stepping 96 tpi drive. The narrower head
overwrittes the middle band of some tracks, but the edges remain unchanged.

A 96 tpi drive can read that perfectly well. Its narrow head only 'sees'
the 'new' data down the middle of each track.

But a 48 tpi drive with its wider head, sees both the old and new data.
The result is a mess that the controller can't decode. So the disk is not
readable on a 48 tpi drive.

A similar argument shows that the same problem occurs if you take a blank
disk, format it on a double-stepping 96 tpi drive, write some files to it
there, write to it with a 48 tpi drive and then write to it with the
double-stepping 96 tpi drive again. The result is not readable on a 48 tpi

And if you take a disk that has been formatted and used on a 48tpi drive
and then reformat it on a double-stepping 96 tpi drive, the result may
well not be readable on a 48tpi drive. Each track will have the
newly-formatted narrow track down the centre and the remains of the old
wider track along the outsides.

If you must format a disk in a double-stepping 96 tpi drive, then either
use a brand new disk (and not a preformatted one, of course), or
bulk-erase the disk.

In general, if you take a blank disk, format it on a double stepping 96 tpi
drive and write to it there only, the result is readable both 48 tpi and
96 tpi drives. The narrower tracks generally do provide enough signal for
the wider head on the 48 tpi drive provided there is nothing in the
'blank' spaces between the tracks.

The simple rule is :

* If you ever write to a disk in a 96 tpi drive that has already be *
* written to (including formatting) in a 48 tpi drive then the result *
* may well not be readable in 48 tpi drives. *
Received on Mon Jan 17 2005 - 17:48:40 GMT

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