[Coco] 720k 3.5" floppies

[Gene Heskett]

On Sunday 04 November 2007, Joel Ewy wrote:
>Gene Heskett wrote:
>> On Sunday 04 November 2007, Bruce W. Calkins wrote:
>>> ----- Original Message -----
>>> From: "Derek" <dml_68 at yahoo.com>
>>>
>>>> If you put something over the hole on the upper left side
>>>> of the 1.44 MB floppy disk you will have better luck.
>>>
>>> That is precisely what I did.  1.44M media is different than 720k media.
>>
>> It (the HD diskette coating) is about twice as hard magneticly, and the
>> 720 drives don't have enough write power to fully magnetize the coating.
>
>The question I've always had WRT this is "how does a 1.44M drive treat
>media it identifies as 720K differently from media it identifies as
>1.44M, other than expected bit density (how fast it reads/writes the
>disk)?"  It stands to reason that using a high density disk as a faux
>low density disk might not work well when you're trying to write (or
>possibly read) it in a low density drive.  But the high density drives
>have exactly the same interface as the low density drives.  I've used
>1.44M drives as 720K drives on the CoCo for years -- with actual 720K
>disks.  Does the presence of the HD hole in the disk alter alter the
>write current of the disk heads,

yes
>or does it just signal the disk 
>controller to jump up to the higher data rate?

That too in some os's, and in one family of 3.5" drives it also 'tunes' the 
data recovery in the drive for the higher rate.  These drives are worthless 
unless only DD disks are used in them, I've bought a couple of them that were 
like that.  But they do work great with DD disks here, so my stash of old 
amiga DD disks is slowly becoming coco fodder.

>If only the latter, then 
>it should be possible to use HD disks to stand in for low density disks
>-- if you use only HD drives with them.  But if the HD sense hole
>actually causes the drive to 'pump up the volume' then I could see why
>this wouldn't work.

This is why.  Write currents for HD disks are usually 1.75 to 3 times that for 
DD disks.  If one knew where the DD write current limiter was in these, it 
might be possible to jumper a smaller limit resistor and fix them.  Bear in 
mind that the head width differences often quoted are in reference to 5.25" 
drives only, all 3.5" drives are 135 tpi, and all tracks are the same width, 
only the data rate (and write currents) changes from DD (250 kilobyte) to HD 
(500 kilobyte) or QD (1 megahertz).  There were some odd-ball single sided 
drives but they saw only limited service in "Word Processors" AFAIK.  Even 
those are 96 track drives just like the double sided ones.

[...]

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
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