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

From: Ethan Dicks <>
Date: Tue Apr 8 10:13:00 2003

--- Joe <> 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
than an IDE drive (whose cable is all digital). The density of storage
is entirely dependent on the nature of the controller. The quality
of retention is dependent on the nature of the drive.

People did attempt to stick an ST-225 on an RLL controller. Results
were mixed. It was a "cheap" way to increase storage 50%, but if
your drive had some weak spots, you would develop lots of data errors.

Others chose to put ST-238Rs on MFM controllers. Results there were
much better. You only got 21.4MB of storage (marketing MB, AFAIK),
but you weren't taxing the surface as much as Seagate said you could.

So, yes, your sources are correct - the ST-238R *does* do 32MB, but
only when attached to an RLL controller (which you might very well have).


P.S. - there were other arrangements than 614 x 4 x 17 when ST-506/ST-412
drives were used in non-PeeCee applications. One that immediately comes
to mind is the Tandon drive (TM602S or TM603S) used in the Commodore
D9060 and D9090 IEEE-488 hard drives. Due to the nature of Commodore
"DOS" and its floppy origins, they chose not to change the block size
of 256 bytes per block. Its internal OMTI controller did tracks of
32 sectors of 256 bytes for a total of 8192 bytes per track, not
8704 (17 x 512). Yes, to keep it on topic, the ST-225 will work in a
D9060, but will only use the first 154 tracks).

(see ,
among other things you can find googling for D9090)
Received on Tue Apr 08 2003 - 10:13:00 BST

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