Ref: 16560005
Title: Basic Information About Hard Disk Controllers
DATE: 12/5/89

Copyright 3Com Corporation, 1991.  All rights reserved.

This article provides general information about hard disk
controllers and different coding schemes.  The information is based
on the report "All About Mass Storage" produced by Datapro Research
Corporation.

CONTROLLERS
-----------
Controllers fit into a microcomputer's expansion slot and
transfer the data between the microcomputer and hard disk.  Different
controllers support different hard disk capacities and data transfer
rates.  Some provide error checking.  Following is a list of the
different controllers that are available:

ST-506     Used on:  IBM PC/XT/AT and compatibles
     Data supported:  Up to 127.5 MB
     Throughput rate:  510 KB/second

Additional notes:  This is the oldest controller.  Most hard disks under
40 MB use it.  Two hard disks can be daisy-chained to one ST-506
controller.


ST-506RLL    Used on:  IBM PC/XT/AT and compatibles
     Data supported:  Up to 200 MB
     Throughput rate:  750 KB/second

Additional notes:  This is an RLL version of the ST-506 controller.
(See Coding Schemes below for an explanation of RLL.)  It packs 50
percent more data onto a disk than the ST-506 controller, but it also
requires more accuracy from the hard disk's electronics, heads, and
media.  Be careful when trying to configure a combination of hard disks
and a ST-506RLL controller.  Incompatible configurations will cause data
to be lost from the hard disk.


ESDI         Used on:  IBM PC/XT/AT and compatibles
     Data supported:  Up to 544 MB
     Throughput rate: 1 MB/second

Additional notes:  The ESDI controller uses a data buffer to
hold data during transfers, enabling data to be recovered that could
be lost with an ST-506 controller.


SCSI         Used on:  IBM PC/XT/AT and compatibles; Apple Macintoshes
     Data supported:  Unlimited
     Throughput rate:  Variable rates, from 300 KB to 4 MB
               depending on the computer

Additional notes:  Small Computer System Interface (SCSI) devices are
daisy-chained along a parallel bus, and data flows through this bus in
a parallel instead of a serial bit stream.  A SCSI controller performs
parity checking on the data that traverses the bus to further increase
the accuracy of the data transfers between disk and RAM.


CODING SCHEMES
--------------
Manufacturers use either the Modified Frequency Modulation (MFM)
technique or the Run Length Limited (RLL) technique to increase the
amount of data that can be stored on disk.

MFM uses sensitive heads to squeeze more bits, represented by
magnetic stripes known as flux changes, onto the disk surfaces.  The
density of these flux changes is increased by plating the disk surfaces
with a thin film of alloy in place of traditional iron oxide.  Putting
more bits of data onto the surfaces, however, strains the ability of the
heads to recognize where one bit ends and the next begins, so data
integrity is decreased.

RLL increases the amount of data on a disk by one-half by packing three
bits of data into two bits on the disk surface.  For example, Priam's Model
ID62 is a four-disk system that stores 62 MB of data; with the RLL scheme,
it can store 103 MB.  In principle, RLL can even increase the density of
installed disks that use the MFM scheme.  However, low-end hard disks cannot
support the RLL controller because they do not have the necessary
electronics.

