Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!usc!orion.oac.uci.edu!mrichey
From: mrichey@orion.oac.uci.edu (Mike Richey)
Newsgroups: comp.sys.ibm.pc
Subject: Re: MFM, RLL, SCSI drives
Keywords: RLL SCSI
Message-ID: <3697@orion.cf.uci.edu>
Date: 22 Nov 89 23:46:23 GMT
References: <286@marvin.moncam.co.uk>
Reply-To: mrichey@orion.oac.uci.edu (Mike Richey)
Organization: University of California, Irvine
Lines: 62

In article <286@marvin.moncam.co.uk> emmo@moncam.co.uk (Dave Emmerson) writes:
>
>I tend to read this group rather than post to it, but I can't resist
>this one, it tickles me pink..
>
>There's been a lot of slagging-off of RLL drives, and praising of SCSI
>(and acceptance of MFM as cheap but reliable). I've just re-read the
>much vaunted Quantum PRO80S's manual to confirm my suspicions, and it's
>TRUE, section 2, page 1 of the Installation Manual for this Embedded
>SCSI drive has an entry 
>Encoding scheme		RLL 2,7
>
>Pick the bones out of that one then!

RLL 2,7 and 1,3   MFM are encoding methods. How magnetic flux is placed onto 
the surface of a platter. On an RLL 2,7 encoding scheme, you will about
fifty percent more data onto the disk, however the amount of magnetic flux
is the SAME between MFM and RLL.

Now, ESDI, and SCSI are electrical and command specifications for a particular
interface type. ST506/412 is another interface specification.

As an example the Seagate ST225 is an MFM encoded, ST506 drive.
The CDC (Imprimis) 94166-155 is an ESDI interface, RLL encoded 150Mbyte
drive. The ST238, is an ST506, RLL encoded drive, and so on.

Encoding and interface are two different things.

Please consider the following an opinion......
The 

What has made RLL seem like a bad scheme to some people is because of
failures of some of the less expensive drives sucha as the Seagate ST238s.
Now I know lots of people that have ST238s and swear by them. But I also know
lots of people that swear at them. There are dealers about that refuse to
sell the Seagates. I spoke with an engineer (not a phone support person)
at a company called Silicon Systems. They produce the part that Seagate
uses on the HDA to detect the magnetic flux as it passes under the heads. It 
form an electrical signal from these magnetic flux patterns.

Okay, the one Seagate uses apparently is an old design. SSI has recommennded
that Seagate switch to a newer (read more expensive) design, because of
potential problems with the older part. 

Well they're still using it. SSI makes them, but has not improved the part
for years, and won't. Let's just say I ran across this information while
working on a proto type of a non standard hard disk controller. Seagate
ST238s and the ST225s for that matter, they use the same pulse detector,
won't work on this controller. A Kalok, Fujitsu, and other newer designed
drives will. Any drive that uses this pld SSI design didn't work. The other
drive that didn't work was the miniscribe 8438 and 8425 (3 1/2 inch RLL and
MFM encoded drives). Guess what? SSI told us that the same poulse detector
is used in that set of drives.

Well take it for what it's worth.

End of opinion....

Michael S. Richey
University of California, Irvine - Network & Telecommunications Services
InterNet:   mrichey@orion.oac.uci.edu    BitNet:  MRichey@UCI
CompuServe: 71650,3132                   Voice:   (714) 856-8374