5.25" FAQ (was: DD disks and HD drives

From: Fred Cisin <cisin_at_xenosoft.com>
Date: Sun Jun 25 18:44:33 2000

On Sun, 25 Jun 2000, Tony Duell wrote:
> We went through this shortly before you joined the list ... You've just
> prompted me to throw a FAQ together on this. Here goes :

Good job, Tony!

A few interleaved suggestions (NOT disagreements) for your FAQ:

> 5.25" floppy disk Frequently Asked Questions
> --------------------------------------------
> 2000 A. R. Duell. 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

"Exact capacity, of course, varied according to certain formatting
choices, providing a range from 1600K to 200K.

"On some (early) systems particularly those based on the Shugart
SA400, only 35 of the tracks were available/used.

> 48tpi double sided, double density. These have 40 cylinders (80 tracks
> total, one foe each cylinder on each side of the disk). This is the
> common PC 360K drive

"Exact capacity, of course, varied according to certain formatting
choices, providing a range from 320K to 400K.

> 96 tpi, single sided, double density. These have 80 cylinders (and being
> single-sided, 80 tracks). These are not common on PCs, but if used there
"(not supported by most versions of MS-DOS)
> would store 360K. The DEC RX50 is a (double) drive of this type.
> 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.
"IBM PC-JX appears to be the only IBM model to use them. They were quite
common in CP/M and non-IBM-compatible MS-DOS machines.

"Exact capacity, of course, varied according to certain formatting
choices, providing a range from 640K to 800K.

> 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.

"The 1.2M mimics an 8", even to the extent of a 360RPM motor speed,
although 8" normally used 77 cylinders and 1.2M uses 80. In some cases,
1.2M 5.25" and 8" drives can be interchanged by making an appropriate
cable, however 1.2M often also supports a mode for access of 360K, whereas
8" does not.

> 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. The IBM PC/AT disk controller is capable of doing this.

> 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.

"But the percentage of rejects on decent quality diskettes is low enough
that few, if any, manufacturers followed the apocryphal procedure of
keeping DS diskettes that failed one side in order to peddle them as
single sided.

> '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
> drives.
> 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 coercivitiy, and it can only be used in the high density drive
> _at the high density_.

600 v 300 Oerstedt.

> 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
> 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)
> drives.
> 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.

"When the disks were expensive, some people would modify the jacket to
make it symmetrical so that the disk could be flipped over to use the
other side as an additional single sided disk. Those were sometimes known
as "flippies". On PC type systems, that required punching an additional
access for the index sensor; on Apple ][ and Commodore, only the
write-protect needed to be modified.
> 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.

"Note: remember that "ERASING" a file constitutes a write operation.

> 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.
> 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
> drive.
> 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. *

"Note: blank does NOT mean formatting. The disk must be degaussed, either
by bulk erasing, or using a virgin disk.
Received on Sun Jun 25 2000 - 18:44:33 BST

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