The mystery of HD floppies resolved ?

[eslapion]

Hi people,
I wanted to find out why HD floppies recorded on a 1541 either don't format at all or are highly unreliable so I did some research on this. The same is true with 3.5" (now I know it's really 90 mm disks) diskettes and the 1581 drives.

This is what I found : http://web.ukonline.co.uk/freshwater/floppy1.htm

This little piece of text finally resolved the mystery of quad density disks required for the SFD-1001, 8050 or 8250 drives. You CAN use good quality double density floppies as a replacement.

But it also tells why HD floppies are not reliable when recorded by DD drives.

This brings some new and interesting questions.

First, is it possible to modify a 1541 so it records disks at a recording field strength of 600 oersteds?

You can easily buy 5.25" HD floppy disks for as little as 5$ for 100 nowadays.

Second, now that we know that it is possible to use a PC HD floppy drive in a 1581, is it possible to fool this drive into recording with a HD level of recording field strength even if it writes data with a density equivalent to DD? There is little need to say that in the 3.5" format, HD floppies are much easier to find than DDs.

Is it possible to modify a 8050, 8250 or SFD-1001 so they too record disks at a recording field strength of 600 oersteds to make them compatible with HD floppies which it should always have been from the start ?

[carlsson]

I learned about the different Oe values a couple of weeks ago, and there was an attempt of a serious (haha!) thread on comp.sys.cbm on the exact same topic as you present, but it didn't lead anywhere.

On the topic, I have a box of SKC MD2HD, 96 tpi. Those appear to really be high density, and the 8250 refuses to format them. However, I have a Chinon FZ-502 floppy drive which is a 5.25" 360K drive for the PC. Yesterday I connected it, and while it doesn't function in 1.2 MB mode (obviously), it quite happily formatted those HD floppies as 360K and let me read and write files without error messages. Maybe the floppies will be unreliable over time, but it makes me wonder what kind of floppy drive this is, if it is a DD drive that only handles 300 Oe, but a PC is less picky than other systems about wrong disk formats.

The QD disks were used in a number of other systems, not only Commodore. They were probably quite standardized, but never popular enough to make it big, and at the time IBM decided to upgrade their 5.25" technology, even better stuff was available.

[Mike]

Last year we discussed this within the topic 'DD vs. HD 5.25" Disks for a 1541'.

Especially I used the picture of a row of water buckets. I also wrote there (emphasis added):

even reading a HD disc written as DD in a HD drive can cause problems in a DD drive, as DD heads can't handle the high stored volumes in the water buckets very well.

i.e. you will overdrive the read amplifiers.

Greetings,

Michael

[eslapion]

even reading a HD disc written as DD in a HD drive can cause problems in a DD drive, as DD heads can't handle the high stored volumes in the water buckets very well.

In the case of the 1581, what I would have liked to do is write a HD disc as DD and read it back in the same HD drive. Since the HD head can handle the "high stored volumes" then it should work but it does not.

What I don't know is if there is a way to force the PC HD to write "strong".

When I put a HD disc in the PC HD drive connected to my 1581 board, the formatting always fails. This indicated that somehow the drive is "told" by the 1581 to write with the intensity required for DD discs.

Also, what you say seems to contradict what I had found because if a magnetic material is less sensitive then why would the signal be too strong when reading back ?

[Thomas Hechelhammer]

Probably this old article from comp.sys.ibm.pc.hardware will cover the subject:

Timothy Sipples writes:
> High density diskettes (1.44 MB) may not be formatted to 720K without
> the risk of errors, like the ones you describe. The magnetic material
> on the diskette has different characteristics.

Theoretically, however, I don't understand why it shouldn't be possible
to format an HD diskette to a capacity less than it's rated maximum, if
done in an HD drive. If the software automatically assumes that a DD
diskette is in the drive whenever a 720 kbyte format is requested, then
it would probably adjust the drive to handle the lower coercivity of the
DD magnetic media. If, however, the user had really inserted an HD
diskette, then the drive would not be using the appropriate head currents
for the higher coercivity media. The result would be problems.

On the other hand, DD diskette shells do differ from HD diskette shells
(the extra hole in the corner), so software need not assume that a DD
diskette is in the drive if a 720 kbyte format is requested; it can and
should utilize the media sensing capabilities of the drive (though not
all manufacturers have provided such capability, perhaps as a cost-saving
measure?).

Going the other direction (formatting a DD diskette to HD capacity) is not
recommended. If a DD drive was used to do the format, it would not be
designed to handle the higher coercivity material; if an HD drive was used,
it would either complain about improper media (if media sensing was utilized)
or use write characteristics appropriate for HD media, which some DD diskettes
aren't designed for. Some people get away with this strategy, however,
because the difference is small, and some diskettes have a wider latitude.
If I remember corrected, the difference between DD and HD diskettes is 600
versus 700 oersteds. Compare with the 5.25 inch 360 kbyte and 1.2 Mbyte
capacity diskettes, which used 300 and 600 oersteds -- a much bigger
difference.

AFAIK the material used for 5 1/4" DD and HD-disks is different, which can physically damage the R/W head of the drive.


-- Thomas

[carlsson]

Maybe this table is useful?
http://www.softwolves.pp.se/misc/arkiv/ ... /2006.html

[eslapion]

Ok, I forget everything I tought I knew about this...

In my 1581 with a PC floppy drive, formatting a 3.5" HD floppy:
-Always fails if the density hole is not covered with tape
-Always works if the density hole IS covered with tape

Wow!!!

Now, of course, this doesn't say anything about how long these disk's data retention might last.