The difference in coercivity between DD and HD 3.5" diskettes is actually
relatively small, ~660 vs. ~720 Oersteds, compared to the much larger 290 vs.
660 difference of 5.25" diskettes.
But the thickness of the coating is also different, HD being less than half
that of DD, and different materials are used with the HD having a finer
granularity.
Nevertheless, you can usually get away with using HD diskettes at DD (with the
hole covered if using an HD drive), although it's not quite as reliable as
using the proper media, especially regarding long-term data retention (probably
because the lower DD write current makes them more susceptible to being
erased); even the other way around often works, i.e. a DD diskette with a hole
punched at HD, although this is even less reliable.
Anecdotally it seems that 3.5" HD diskettes in general have become somewhat
unreliable, even used at HD, which may at least partially account for folks
having issues using them at DD.
But I think we're missing the more important issue; the reason for all these
differences is to squeeze more bits per inch on a track; unlike 5.25", 3.5"
diskettes all have 80 tracks and run at the same 300 RPM, so of course this
means that data will normally be written and read at twice the rate in HD mode
(500Kbps) than at the DD rate (250Kbps).
I assume that the CoCo controller only transfers data at 250Kbps whereas with
the HD hole uncovered the drive will expect to see data at 500Kbps (and use HD
write current) and I suspect that's why Steve's having trouble.
3.5" DD diskettes are still around; why not use the right media in the first
place.
m
----- Original Message -----
From: Kurt McCullum
To: m100@lists.bitchin100.com
Sent: Monday, March 25, 2019 11:02 AM
Subject: Re: [M100] question regarding floppy disks.
Yeah that does sound strange. And I agree, the drive 'should' switch based on
the hole in the disk. Does it format to 720 or 1.44 when the hole is covered?
On Mon, Mar 25, 2019, at 7:37 AM, Stephen Adolph wrote:
Kurt, agree with everything you have said.
The odd thing is-
* using an HD disk in an DD/HD drive, and covering the hole with tape,
would seem to be BAD
---> because you are telling the drive to use the wrong current settings
for the actual disk media.
However, this is apparently the way to make my system functional.
so, strange. I would have thought it would be the opposite - let the drive
decide what current to use, matched to the "cookie".
On Mon, Mar 25, 2019 at 10:19 AM Kurt McCullum <ku...@fastmail.com> wrote:
The magnetic coercivity on HD disks is different than on regular disks.
It requires more energy to lay down the tracks. If you start with a blank HD
disk rather than a pre-formatted disk then you have a better chance. That's
because once the HD tracks are laid down, you need to erase them for your new
format. If your drive doesn't have enough energy to completely erase the
existing track, it wont work. 720k disks have a lower coercivity and therefore
work with either a 720k or 1.44mb drive. A 1.44 drive has a sensor for the open
hole and when it sees that hole, it will use a higher level of write energy to
properly work with the media. When that hole is covered, it will use a lower
level which is what the 720k media is looking for. Though I do remember that
formatting a 720k disk in a 1.44mb drive didn't always work when going back to
a 720k drive.
Not sure about the Coco drive, but my TPDD2 does not work reliably with
HD disks. I have only been able to format one properly and it had data failure
shortly after.
Kurt
On Sun, Mar 24, 2019, at 6:32 PM, Stephen Adolph wrote:
interestingly,
Yes, if I take an HD disk, and tape over the hole to make it appear to
be a DD disk, then it works.
But why?
the floppy is capable of both formats...
On Sun, Mar 24, 2019 at 7:39 PM Mike Stein <mhs.st...@gmail.com> wrote:
Have you tried closing the HD sense hole with a piece of tape or
similar?
----- Original Message -----
From: Stephen Adolph
To: m...@bitchin100.com
Sent: Sunday, March 24, 2019 6:08 PM
Subject: Re: [M100] question regarding floppy disks.
the Coco is using it's standard controller
When issuing the DSKINI 0 command the coco tries to format for
180kB.
The combination of
(Coco, std controller, PC 1.44MB drive + a 720kB dd floppy) works
whereas
(Coco, std controller, PC 1.44MB drive + a 1.44MBB hd floppy) does
not work
this is something I don't understand!
On Sun, Mar 24, 2019 at 5:42 PM Gregory McGill
<arcadeshop...@gmail.com> wrote:
likely the floppy controller doesn't support 80 tracks or high
density.. most of the controllers of the era are ds/sd 40 track or dsdd 40
track.. are you able to format 720k? ds/dd 80 track?
Greg
On Sun, Mar 24, 2019 at 2:38 PM Stephen Adolph
<twospru...@gmail.com> wrote:
I'll start by saying this isn't an M100 or TPDD discussion, but
just looking to understand something.
I have a Tandy Coco3 with a 3.5 inch floppy drive. The drive
is a standard PC drive and it is working well.
Seems though that I cannot use 1.44 MB floppies in that drive.
They don't seem to want to format.
I really don't understand where the problem could be.
- the drive and the floppy are compatible
- the disk is known good and formats at 1.44MB in a PC
- if it can support 135 TPI, why can't it support 35 TPI?
Does anyone know what's going on?
thx
Steve
