[Coco] 5 1/4" drive

[Gene Heskett]

On Tuesday 07 April 2009, Christian Lesage wrote:
>Gene Heskett wrote:
>> I have never, and I've dealt with a lot of oddball storage media over the
>> years I've been a broadcast engineer, heard of a 300 kilobaud data rate
>> being used, in anything.  Whenever there was a shift in data rates, it was
>> to 500kb, and then to 1 megabaud for the 2.88 meg 3.5" floppies.
>>
>> If 300 kbaud was what it took to make it work, the 1773 family CAN do
>> that, but ALL the controller conversions I've seen use a different chip
>> and run it at 500 kbaud. Std DD floppies turn 300 rpm, the special 1.2
>> meggers turn 360 rpm, which is why the otherwise identical 96 tpi drives
>> will not give you 1.44 megabytes of storage, but only 1.2 megs.  Driving
>> them at 500 kilobyte data rates, at 300 rpm, should give you the full 1.44
>> megs.  And that is a direct scale.  If I had ever built one of the hi-dens
>> I could easily prove that. Running the math the other way, if the data
>> rate for a 720k, 250 kilobaud disk was switched to a 300 kilobaud rate,
>> the disk capacity would only be 864 kilobytes, not 1200k.
>>
>> I don't know who this Dave Dunfield is, but he is flat wrong on that
>> point.
>
>He is not wrong at all, and neither are all the people who wrote about
>this fact. Please do some googling. Let's put it that way:  You format
>and write to a double-density disk in a 360KB disk drive. The drive
>spins at 300RPM and the floppy disk controller feeds it with a 250kbps
>data stream. Now, you take the same disk, and put it in a 1.2MB drive,
>which spins at 360RPM. What happens to the data rate? The RPM is
>increased by 20%, and so is the data rate. 250kbps x 120% = 300kbps.

That on the face of is true.  But, at 360 rpm when the data rate is then 300 
kilobaud, it would still take 300 kilobaud to read and write exactly 720k on 
the faster rpms just to keep up with the higher, 360 rpm disk speed.  Crank 
the data rate up from 300 to 500 and you have the 1.2 megs on that same disk.

Like I said, there are NO disk formats that I am aware of (and I'll plead 
guilty to not knowing anything about the original apple with its variable 
speed drives) that have actually used a 300 kilobaud data rate.

>You are absolutely right when you say that a 5.25" HD drive would be
>able to format a 1.44MB disk if it spun at 300RPM, but that's not the
>point we're discussing. You are also right when you say that switching a
>720KB drive from 250kbps to 300kbps would result in a 864KB format...
>However, it is irrelevant because (as I already mentioned twice, and we
>both agree on this fact) 5.25" HD drives use 500kpbs for high density
>(and 300kbps for double density).

If they can't switch the drive speed in software (and I think they could in 
the pc's but will not bet on it), that may have happened.  I can see that 
scenario, but I've had a couple of machines (I've Been Moved mostly with an 
AT&T 3B1 (unix) thrown in to confuse the issue) that had 1.2 meg 5.25" drives.  
I suppose they could have diddled the clock speed for the FDC chip in order to 
write/read a 720k compatible disk in those drives, but as we both know, that 
format (in those 1.2 meg drives) was largely stillborn.  Which is why there 
was such a glut of those drives available 10+ years ago I suppose.  Teac bet 
the farm on 55-F's & G's at the time.

_WE_ have no controllers capable of that without hardware modifications.  And 
if the hdwe mod to make a 500kb controller was done, I'd sure slow the drive 
down to 300 rpm and have a 1.44 meg drive instead of a 1.2.  With HD disks I'd 
think it would be very dependable.  DD disks, we might have to test a few, but 
90+% should Just Work(TM) as 1.44MB disks.

So we are back to what we can do with 250kb as the default fdc speed, and a 
500kb hack that will give 1.2 megs on the faster spinning disk. 300kb has 
never happened in the coco arena. When someone put a pc formatted 1.2 megger 
in his 720k drive that was only turning 300 rpms, then the data rate would be 
416.666666667 kilobaud.  And with that big a diff in the data rates, (it was 
expecting a 250 kilobaud rate remember?), I have doubts a good read could have 
ever been obtained.

300kb could be done, but I don't believe that in the real world it was done on 
a daily basis, if indeed ever outside of the test labs at Teac et all.

-- 
Cheers, Gene
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