Date   : Mon, 24 Oct 2005 21:28:08 +0100 (BST)
From   : Pete Turnbull <pete@...>
Subject: Re: Collected notes on floppy drive butchery?

On Oct 24 2005, 10:10, Richard Gellman wrote:

> * Note for the terminology-criticising Pete Turnbull: Depsite what
you
> may say, floppy drives *do* have a write clock - this is generated
> internally on the drive itself, and synchronised with the index hole.
If
> there wasn't a write clock the data would be out of sync. A read
clock
> may also be used internally, but not so much relied upon, as the
timing
> of the data can be gathered from the disk.

What?  If you mean the one-pulse-per-rotation index pulse, yes there
is a clock.  If you mean a separate clock signal that gets recorded
with, or in some way matched with, the data, in order to syncronise the
bits, then I'm afraid you are completely WRONG.  There is no clock in
the drive.  There are clock bits in the data stream from the
controller, generated by the controller.  There are also sync blocks
between sectors, written there by the controller.  There is no write
(or read) clock in the drive.  The clock used in the data recovery is
the stream of clock bits recorded with the data.  Once again there is
no clock signal created by the drive electronics.

> The write clock will
> typically be generated in accordance with the density selected -
indeed
> the pinout for the shugart interface (at some sources) shows a
density
> select line - high density has a higher clock speed than double
density.

Well, yes, it is typically generated in accordance with the density
selected -- because the data rate and hence the clock bits *generated
at the same time by the controller* are in accordance with that.

The density select line, however, does not do anything to any clock,
although on some 5.25" drives it can be jumpered to alter the speed of
rotation - which may have a bearing on the rate at which the controller
must send data (300rpm vs 360 rpm, requiring 250kHz vs 300kHz data
rate).  What it *does* do is select the write current, which it is
necessary to change to accomodate the different magnetic coercivities
of single/double density and high density disks.  For 5".25" media, the
coercivity of HD media (650 Oersteds) is just over double that of SD/DD
media (300 Oesrsteds).  For 3.5" media, the difference, while
significant, is less pronounced (it's 650 for SD/DD, and 700-750 for
HD).

In fact on some 3.5" drives, the density select is an output from drive
to controller, to indicate to the controller when the drive has
detected an HD disk (with the extra hole), and therefore that the
controller should write in high denisty, not double (or single)
density.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York