[Coco] 720kb Diskettes

[Gene Heskett]

On Saturday 27 February 2016 23:17:24 Barry Nelson wrote:

> If you can provide a link to at least one disk I can examine it and
> tell you exactly how to get it to mount. There is also a density
> setting you may need that has to do with if the disk is 48 tpi or 96
> tpi.
>
That is no longer a valid setting when the disk has been converted to a 
file.  Which is precisely why I didn't mention it.  However I get the 
impression that Ed has the physical disks, and that would then be 
applicable.  IF he has to use a dns=3 setting to read the file as a disk 
image, then ded should be used on the disk image file to set that 
particular bit back to zero.  It makes no sense to double step the heads 
of a non-existent drive when its actually a file.

The dns setting of 3 is intended to cause a 96 tpi drive to be double 
stepped when it finds itself with a 48 track per inch, 40 track disk has 
been inserted.  That way the heads stay somewhere near the middle of the 
track on a 40 frack 360k disk, and because the heads are too narrow to 
properly erase and rewrite the track, such disks should be treated as 
READ-ONLY, you WILL trash the diskette if a write is attempted.  Your 
data is gone as the only salvation for that disk is to reformat it.  
This dns=3 ONLY applies to 5.25" drives, no other drive size has more 
than one tpi so this setting is properly left at dns=1 for all other 
drives.

This means that dns remains as 1 for a 3.5" diskette as it has only 135 
tpi track density.

Sure there are HD diskettes in 3.5 form factor, but they get that 1.44 or 
2.88 megabyte capacity by useing a 500 kilobit or one megabit data rate, 
which the coco's controllers cannot do with one or two exceptions that 
require hardware mods to the controller to achieve.

In fact, Robert Gould and I worked out a patch to the floppy driver that 
denies a write to such a miss-matched 48 tpi disk in a 96 tpi drive 
combo, and its part of the official Nitros9-V3.3.0 release, see the 
ChangeLog. IIRC, its now been a while, that an attempted write in that 
scenarion returns a E$WP error, the driver has automatically write 
protected the diskette.  But if the disk is moved to a true 48 tpi 
drive, it will become writeable again.

Its easy enough to forget it, for those of us who move cables between 
3.5" drives and 5.25" drives, forgetting to also set the dmode dns=3 
when plugging the cable onto a 96 tpi drive.

> > Gene Heskett gheskett at wdtv.com
> > Sat Feb 27 19:24:55 EST 2016
> >
> > On Saturday 27 February 2016 18:46:10 Ed Orbea wrote:
> > >    I'm finally getting around to accessing via my SDC) the
> > > Australian OS9 POD library files graciously provided by Bob
> > > Devries in Feb 2015 (yeah, I know it's taken me a year).
> > >    Bob says that they are 720k diskettes and I am having problems
> > > getting dmode properly set (either in VCC or SDC) to access the
> > > drives. Whatever I try just gives error 249.
> > >    Can I get a  listing of the proper dmode settings?
> > >    Thanks
> > >    Ed
> >
> > Depending on whether or not the SDC honors the side bit, a 720k disk
> > would be sid=2 cyl=50 if it does, otherwise I'd try sid=1 cyl=A0.
> > Either way is a 720k, 160 track for sid=1, and 80 track if sid=2
> >
> > Cheers, Gene Heskett


Cheers, Gene Heskett
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