NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/sys/cbm/4444

Internet Message Format | 1996-08-05 | 4.5 KB

Path: news.nstn.ca!news
From: George Taylor <[e-mail]>
Newsgroups: comp.sys.cbm
Subject: Re: Smart Drives and HD disks...
Date: Sat, 23 Mar 1996 15:42:42 -0800
Organization: Interconnection
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Edwin wrote:
>
>      What was this I read about reading HD MSDOS disks in a FD-2000 with
> LRR 128?  Do I have an old copy of it or something?  Mine only supports
> DD 3.5s and DD 5.25s.
>
>    Also, I long read, and vaguely understand, that Commodore drives are
> "smart".  I understand this is due to the fact that a processor is in
> the drive, and it has it's own memory-- but how is this related to the
> alleged fact that this is what enables a 1571 and 81 to be capable of
> reading foriegn format disks (not just foreign file systems) while PC
> drives, being "dumb" (allegedly) can not.  Any of you hardware hacks
> have any details on this?
>
>     - BoWell, I can answer half this question.  I know that pc floppies
can read high density MAC disks, and can also use custom formats
like unix filesystem, or 2Mb format.
There's about 4 things that enable you to read a variety of disks:
the size of the 'head gap', the rotation speed, the read clock, and the
disk
controller
First let's talk about 5.25" disks cause I know most about them.
The 64 format disks use 4 speed zones, where each is a different
density..
what does this mean? You can compare it to a baud rate, because disks
are basically a serial device internally (even if they have a parrallel
port to the host).
And why are there different densities?  Just a fact of life, that
the outer edges of a circle have more circumference than the inner.
Therefore, the design of the 64 disks used this to put more data
on the outer edges.  Dumb PC floppies always use the lowest common
denominator, in other words only the density you can fit on an inner
track.
MAC drives also did something like this.  I believe apple drives
are more like PC drives.
So this is where the adjustability of the clock rate comes in.  In order
for a 1541 to read PC floppies, it needs a higher clock rate (for the
higher density) than it already has the ability to do.
Next is rotation speed.  Normally it's 300rpm, but for example for
an Amiga drive to read a 64 floppy, you need to adjust the speed
slightly, because the Amiga drive is too slow.  (but perhaps this is an
odd hack to fudge the density close enough to be read, I don't recall).
Next is the head gap.  The 1541 actually does have 80 tracks and more,
and there is only one reason you can't use them reliably, and that is
that the read/write head is too 'fat' and thus spreads it's recording
over more than 1 (80 track sized) track.  I think if you just replaced
that head you could enable 80 track drive ability.  (You need a 2um head
gap).
Then next is the controller chip.  In a 1571, is a WDxxxx chip, which is
used also in some PC's, so naturally it has ability to read pc disks.  It
has the clock, the rotation speed, and head gap to enable a wide variety
of drives.  It can also automatically decode GCR and MFM formats.
So this is a great drive.
So what is the difference to a PC drive, and to finally answer your
question, does the 'smart' ability help?
Well, this is where I only know half the answer.  You don't have as much
control over a PC drive I think.. I know that 1mb formatted 720k disks
were hard to achieve and used some kinds of trick to control the floppy
disk controller directly.
However, I guess that implies that if you can affect the registers
directly, then you have the same control as a 1571 can, so then it's just
a matter of which floppy controller chip is more flexbible.. in other
words, the 'smart' ability has no benefit in the end.
The 1541 however is an exception to this.. is has no automatic controller
chip, it's mostly some basic parts controlled by software.  You can
program the 1541 to read GCR or MFM theoretically, or any possible
combination the hardware will allow.. and in fact, by changing the head
and increasing the density clock, you could read any type of disk from
pc, apple, 1571 etc.
In this case, the smart drive is an advantage, because if you couldn't
program it, there would never be any fast loaders, 220k disk formats, or
hacks to read pc disks (yes, with just a few wires your 1541 can also
read pc disks).