MFM vs RLL or ST-225 vs ST-238R (was Re: 3.5" floppy drive in

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Tue Apr 8 17:19:07 2003

> --- Joe <rigdonj_at_cfl.rr.com> wrote:
> > At 10:47 PM 4/7/03 -0700, Don wrote:
> > >
> > >Interesting question is whether it is being used as a 21mb MFM
> > >drive or as a 33mb RLL. My guess would be 21mb.
> >
> >
> > It's an RLL drive and is 32 Mb according to my sources.
>
> Does that mean that you confirmed that your controller is an RLL
> controller? There's no substantial difference between an ST-238R
> and an ST-225 except a) the label, and b) some manufacturing confidence
> that a tighter encoding scheme will not drop bits.
>
> The data cable holds analog signals. This is significantly different

Actually, all the signals on the data cable are digital. The Read/Write
data signals are differential pairs of similar-to-TTL level signals,
normally using a 26LS31 driver and 26LS32 receiver.

But in a sense they're analogue in _time_. In write mode, the drive
writes a flux transition to the disk for every pulse on the write data
line. In read mode, the drive sends a pulse on the read data line for
every flux transition on the disk. There are some restrictions on
allowable frequencies of transisitions, of course.

Therefore it's up to the controller to handle the raw data from the disk,
to sort it out into sectors, check the sector headers and checksums, and
so on. And when writing, the controller has to pack the user's data into
sectors, add the correct headers, etc. The choice of encoding scheme
(FM/MFM/RLL) is up to the controller.

RLL drives should really be called RLL _capable_ drives. They are capable
of handling the somewhat tighter timing requirements of the RLL encoding
scheme. They'll work fine with MFM encoding too (just as double-denisty
-- MFM -- floppy disks will work on a single density -- FM -- controller).

-tony
Received on Tue Apr 08 2003 - 17:19:07 BST