DP Tool Club 24/CD_ASCQ_24_0995.iso/vrac/eidefaq.zip/HDTECH.HTM

Text File | 1995-08-02 | 36KB | 731 lines

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TECHNICIANS' GUIDE TO PC HARD DISK SUBSYSTEMS

========================================================================
copyright (c) 1992, 1995

Nicholas Majors,

DATA RECOVERY LABS
(division of Data Recovery Services Inc) Voice : 1-416-510-6990
1315 Lawrence Avenue East - Unit 502 FAX : [phone]
Don Mills, Ontario, Canada M3A 3R3 Email : [e-mail]
========================================================================

Table of Contents

Introduction
Chapter 1 - History and Overview
Clapter 2 - Inside Hard Drives and Controllers
Chapter 3 - Installing and Configuring Hard Drives

Appendix - Additional Reading
Appendix - Acronym Definitions
Notices & Personal Rants
========================================================================

INTRODUCTION :

This booklet (now electronic) is published as a service of Data Recovery
Labs. Its purpose is to provide the fundamental knowledge of concepts and
terminology that is necessary to deal with the complexities of hard disk
subsystems. It is not a technical reference guide and should not replace
original documentation made available by manufacturers.

Every day we meet and deal with technicians and support personnel who
install, upgrade and maintain hard drives, without knowing exactly what
they are and how they work. Gaining a working knowledge of disk subsystems
is not a simple task and requires much study - and - experience. Don't
expect any book, training course, or manual to make you into an expert.
However, if you read and understand most of this material, you will be off
to a fine start.

The principal author is Nick Majors. He has been in the industry since
1979, designing and developing hardware and operating system enhancements
and performance tools. He is an experienced machine language programmer and
has provided technical training to support personnel for some of Canada's
leading banks, corporations, government departments and PC service
organizations. He is Manager of Technical Services for Data Recovery Labs
which he founded in 1989.

While this booklet deals primarily with PC and PC compatible platforms,
there is much information to benefit support personnel with other hardware
platforms.

Every effort has been made to ensure accuracy, but misprints and
ambiguities may still remain, so please use caution.
========================================================================

HISTORY AND OVERVIEW :

The original IBM PC, introduced in 1981, did not support any type of hard
drive. Program code in the BIOS did not recognize any such device and early
versions of DOS precluded mass storage by limiting the maximum number of
directory entries. This is not surprising when you consider that the
original 4.77 MHz PC came with 16 K of RAM, expandable to 64 K on the
motherboard. Even floppy drives and a disk operating system were options to
upgrade the built in cassette recorder port and ROM basic.

To utilize a hard drive in a PC requires:

   *  a Hardware IRQ (interrupt request)
   *  I/O port addresses for the controller
   *  a DMA channel (now optional)
   *  low level program code
   *  a physical interface to the bus (expansion slot or built-in)
   *  operating system support
   *  sufficient power and cooling

After DOS 2 introduced the sub-directory system and support for mass
storage devices, companies started selling hard drives for the PC. These
were external cabinets that contained the disk itself, a controller card,
and a dedicated power supply (since the PC's 63.5 watts were insufficient).
Migrating from other architectures, these units connected to the PC through
cabling to an 8 bit adapter card that plugged into an available expansion
slot. These subsystems were configured to use one of the available IRQ's,
DMA channels, a range of I/O ports, and program code that was made
available through a device driver loaded into memory after startup (booting
from a floppy).

In 1983, the IBM XT (eXTended) was unveiled with its built-in 10 MB fixed
disk. IBM worked with a company (Xebec, I think) to take the controller
components normally located in the external cabinet and build them right
onto a bus interface card, creating what we commonly call the "hard disk
controller".

Program code was provided by a ROM chip on the controller card which
supplemented subroutines in the BIOS, and the internal power supply was
upgraded to 135 watts to provide power for the internally mounted drive.

The XT design utilized :

   *  IRQ 5
   *  I/O port addresses 320-32f
   *  DMA channel 3
   *  Program Code from adapter ROM at c8000
   *  DOS 2.0 or higher

Several companies started manufacturing and selling similar
drive/controller packages with various improvements including higher
capacities, superior performance and built-in floppy controller components
on the same card (to save an expansion slot). These third party subsystems
could even be added to an original PC, so long as the internal power supply
was also upgraded.

In 1984, the IBM AT (Advanced Technology) brought a complete overhaul to
hard disk systems. Program code was included in the motherboard ROM BIOS,
eliminating the need for a ROM chip on the new 16 bit controller card, and
a higher quality drive improved access times. The system included use of
newly added higher IRQ's, eliminated the use of DMA for hard drives, and
changed the range of I/O addresses.

The AT design utilized :

   *  IRQ 14
   *  I/O port addresses 1f0-1f8
   *  DMA channel - none
   *  Program Code from motherboard ROM BIOS
   *  DOS 2.0 or higher

Hardware configuration details for the AT, including hard disk parameters,
were stored in a low power CMOS chip connected to a small battery,
eliminating many of the motherboard configuration dip switches and jumpers.
The battery allowed information to be maintained while the computer was
turned off and information could be changed by running a setup program from
disk.

The original AT supported 14 different drive types, recognizing specific
hard disks ranging from 10 to 112 MB. Any drive with physical parameters
that did not match one of these types needed a ROM extension on the
controller card or a device driver loaded during boot-up.

DOS versions prior to 4.0 (or 3.31) did not support partitions larger than
32 MB no matter how big a drive was. This was because of sector numbering
that could not exceed 16 bit values (up to 65,536 sectors). To make a
larger partition required the use of special software like Ontrack's Disk
Manager. This software was so popular that many drive manufacturers shipped
it with their product. Unfortunately, while this offered one of the better
solutions, it did pose compatibility problems for many disk utilities
because, in effect, you had a non-DOS partition.

Many people opted instead to divide their drives into 32 MB partitions
creating a C: D: E: etc. up to the physical capacity. Prior to DOS 3.3,
even this was not possible, because DOS did not recognize extended
partitions!

The number of drive types supported by CMOS has expanded to over 40 and
most current BIOS's provide a user definable type to allow parameters to
match any drive. Most PC's today still rely on this original CMOS drive
type scheme to configure and control hard disks, though many architectures
and BIOS's have changed how the information is stored and updated.

This inherently creates certain limiting factors, including a problem with
more than two hard drives and BIOS/OS limits to recognizing more than 1024
cylinders, 16 heads and 63 sectors per track. Various technologies must be
used to translate non-compliant physical geometries into logical parameters
recognized by the system to maintain compatibility with operating system
and utility programs that are tied to these limits.

With each sector holding 512 bytes of data this allows for drives no larger
than 504 MB.
(1024 X 16 X 63 = 1,032,192 sectors X 512 = 528,482,304 bytes)

To understand this limit, we have to examine how hard drives are accessed
by a PC.

Primarily, I/O commands are sent to the controller via a range of reserved
port addresses. This is a very complicated and tedious process and the ROM
BIOS includes a subroutine (INT 13) to perform these tasks for you. The DOS
operating system then has certain functions that further simplify the
process. They include INT 25/26 functions to read and write absolute
sectors to the drive as well as higher level functions (INT 21) to Open
files, Close files, Write data to files, etc. Most programs rely on these
DOS functions to control access to hard drives and DOS in turn calls the
INT 13 BIOS subroutines which do the actual I/O commands.

All of these subroutines are assembly language code that are loaded into
memory from either your BIOS chip(s) or Operating System files at startup.

Assembly language routines store and manipulate values in registers within
the CPU. The lowest common denominator for INTEL based PC's is 16 bit
registers. Any program or routine that wants to be backward compatible,
must use these base registers whether run on an 8088 or a Pentium. This
basic INT 13 is the backbone of PC compatibility and uses 16 bit registers
in the following way:

*  DX - 8 bits for head number - 8 bits for drive number
* CX - 10 bits for cylinder number and 6 bits for sector number

The largest 10 bit number possible is 1023 hence the limit on cylinder
numbers (from 0-1023) and the largest 6 bit number is 63 (from 1-63)
allowing 63 sectors per track.

But note - the DX register allows a maximum of 255 heads, not the 16 that
original specifications called for. This is what allows various translation
schemes to deal with drives up to 8 GB while remaining INT 13 compatible.

If the DX register allows 8 bits for drive number, shouldn't it be able to
control 255 drives instead of only two?

When the INT 13 sends commands to your controller, it must know the
physical geometry of the drive in question. During initialization of the
PC, values for disk types are read from CMOS and stored into an area of RAM
called the BIOS DATA AREA. Pointers to those entries are stored in the
Interrupt Vector Table (at addresses 0:104h and 0:118h). The table only
reserved pointers for two drives, so even if your CMOS held more values,
the standard routines wouldn't know how to deal with them.

Why were the INT 13 routines so limited? They were written at a time when
10 and 20 MB drives were the norm, and 120 MB was unbelievable. They were
designed to communicate with a specific controller interface, the ST412/506
standard (WD1003 controller), and the specs for passing parameters included
only 10 bits for cylinders number, 4 bits for head number and a single bit
for drive number.

Some of these limitations have been overcome in the past by replacing the
PC's INT 13 sub- routines with code that could recognize and deal with
different devices. That's why virtually all SCSI adapter cards include a
ROM chip and you must set the drive type to ZERO. In effect, no standard
drives installed for the motherboard BIOS to control.

NetWare and other advanced Operating Systems use the drive type only long
enough to boot- up and then replace the INT 13 code with their own device
drivers. You then need to load a different .DSK file written specifically
for each type of controller.

In the past couple of years, many motherboard and controller BIOS's have
been enhanced to deal with ever increasing demands, but the process of
establishing new standards has led to continuing confusion and
compatibility problems.
========================================================================
END OF CHAPTER 1
Return to Table of Contents
========================================================================

HARD DRIVES AND CONTROLLERS:

Before we consider how to install, configure and maintain hard drives, we
need a basic understanding of drive construction and design concepts. This
chapter examines in some detail the parts and functional components of hard
drive subsystems.

(Note : A number of acronyms are used throughout this chapter and the
glossary for this booklet is not yet available. Therefore, I have attached
a brief set of definitions for some of the terminology.)

A hard drive subsystem is comprised of the following components:

  1.  The Hard Disk, with one or more boards (PCB) attached.
  2.  A Controller Mechanism, either on the hard disk PCB or on the bus
     adapter within the PC.
  3.  Bus Adapter for interfacing the controller to the host PC.
  4.  Cables and Connectors to link it all together.

========================================================================
THE HARD DISK:

Within a sealed enclosure (Head Disk Assembly or HDA) are one or more rigid
platters that are "fixed" or non-removable. These are coated with
magnetically sensitized material and data can be written to and read from
the surface by means of electromagnetic read/write heads. When powered up,
the platters are constantly rotating (except for certain pre-programmed
sleep modes) and the heads are moved back and forth across the surface to
access different locations. This is a sealed unit which should not be
opened, except by qualified personnel in a controlled, dust free
environment.

The circuit board(s) attached to the outside of the HDA provide the
electronics needed for physical control of the motors within the sealed
unit. They interface the source of electrical power to the disk assembly
through varied connectors and cables. Most boards have some jumpers, dip
switches and/or resistors that are used for configuration purposes.

Functionally, these PCB's are separate from the Hard Disk Controller, but
many of the newer drives (IDE and SCSI) embed the controller chip directly
onto this board (as opposed to having it on the Bus adapter).

INSIDE THE HDA - PARTS OF A HARD DISK:

  1.  Disk Platter(s), separated by spacers and held together by a clamp.
  2.  Spindle shaft onto which platters are mounted.
  3.  Spindle motor for rotating the platters.
  4.  Electromagnetic read/write heads (one per surface).
  5.  Access arms or armatures from which the heads are suspended.
  6.  Actuator for moving the arms (with heads attached).
  7.  Preamplifier circuitry to maximize read/write signals.
  8.  Air filter and pressure vent.

The Platters:

Most platters or disks are made of an aluminum alloy, though ceramic or
glass platters can also be found. The diameter is normally 2 1/2", 3 1/2"
or 5 1/4" with a hole in the center for mounting onto the spindle shaft.
Thickness of the media can vary from less than 1/32 of an inch to about 1/8
of an inch.

During manufacture the platters are coated with a magnetizable material.
Older drives used a ferrite compound applied by squirting a solution onto
the surface and rotating at high speeds to distribute the material by
centrifugal force. This process left a rust colored ferrite layer which was
then hardened, polished and coated with a lubricant.

Newer drives apply the magnetic layer by plating a thin metal film onto the
surface through galvanization or sputtering. These surfaces have a shiny
chrome-like appearance.

Spindle and Spindle Motors:

Most drives have several platters that are separated by disk spacers and
clamped to a rotating spindle that turns the platters in unison. A direct
drive, brushless spindle motor is built into the spindle or mounted
directly below it. (Sometimes this motor is visible from outside of the
sealed enclosure.) The spindle, and consequently the platters, are rotated
at a constant speed, usually 3,600 RPM, though newer models have increased
that to 4800, 5400, or 7,200.

The spindle motor receives control signals through a closed loop feedback
system that stabilizes to a constant rotation speed. Control signals come
from information written onto the surface(s) during manufacture or with
older drives, from physical sensors.

Read/Write Heads:

Since both sides of each platter are coated to provide separate surfaces,
there is normally one electromagnetic read/write head for each side of each
platter. Therefore, a drive with 4 platters would have 8 sides and 8 heads.
Some drives use one side as a dedicated surface for control signals leaving
an odd number (5,7,etc.) of heads for actual use.

Each head is mounted onto the end of an access arm and these arms (one per
surface) are moved in unison under the control of a single actuator
mechanism. When one head is over track 143, all the heads on all other
sides should be at the same location over their respective surfaces.

Generally speaking, only one of the heads is active at any given time.
There are some drives that can read or write from two or more heads at a
time, but this represents a major design change and the technology is not
yet widely used.

The spinning disk(s) create an air cushion over which the heads float.
Depending on design, this air buffer ranges from 2 to 15 microns. By
contrast, a smoke particle or finger print is about 30 microns in size!

The heads are not supposed to come into contact with the surface during
rotation. Only when powered off should the heads come to rest on the
surface, but this should be over a specific area of the surface, reserved
for that purpose. Most drives built since the late 1980's employ an
automatic parking feature which moves the heads to this designated region
and may even lock the heads there until powered up.

Head Actuators:

The head actuator is the positioning mechanism used to move the arms and
consequently the heads, back and forth over the surface. Once again,
earlier drives used a different method than is now common.

Originally, head positioning was controlled by a stepper motor that rotated
in either direction by reacting to stepper pulses and moving the head
assembly back and forth by means of a "rack and pinion" or by spooling and
unspooling a band attached to the actuator arms. Each pulse moved the
assembly over the surface in predefined steps or detents. Each step
represented a track location and data was expected to be under the head.
This design, still used for floppy drives, is not suitable for current
drive densities and is prone to alignment problems caused by friction, wear
and tear, heat deformation, and lack of feedback information needed for
correcting positioning error.

The more common voice coil actuator controls the movement of a coil toward
or away from a permanent magnet based upon the amount of current flowing
through it. The armatures are attached to this coil and move in and out
over the surface with it. This is a very precise method, but also very
sensitive. Any variation in the current can cause the head assembly to
change position and there are no pre-defined positions. Inherently this is
an analog system, with the exact amount of movement controlled by the exact
amount of current applied.

The actual position of the coil is determined by servo (or indexing)
information, which is written to the drive by the manufacturer. Location is
adjusted to different tracks by reading and reacting to these control
signals.

Internal Electronics:

There is surprisingly little circuitry found within the sealed HDA. There
are electrical and control wires for the spindle and head actuator motors
and the head assembly has flex cables with a preamplifier chip often built
onto it. This chip takes pulses from the heads (as close to the source as
possible) and cleans up and amplifies these signals before transmission to
components outside of the housing.

Air Filtering and Ventilation:

Minor wear of internal components and occasional contact of the heads with
the surface can cause microscopic particles to be loosened within the HDA.
A permanent air filter is mounted within the air stream to remove these
particles before they can cause damage to delicate mechanisms.

Most drives also have a small vent to allow for minor air exchange from
outside of the housing. This allows for equalization of air pressure so
drives can be used in different environments without risk of imploding or
exploding.

========================================================================
CONTROLLERS AND BUS ADAPTERS:

The hard disk controller provides the logical link between a hard disk unit
and the program code within the host computer. It reacts to requests from
the computer by sending seek, read, write, and control signals to the drive
and must interpret and control the flow of data.

Data moving to and from the drive includes sector ID's, positioning
information and timing or clock signals. The controller must encode, decode
and separate this control information from actual data written to or read
from the drive.

Also, data is sent to and from the drive serially, in bit format, but the
smallest unit that a CPU can work with is a byte (8 bits). The controller
must take bits (8 - 16 - or 32 at a time) and assemble them into bytes,
words, and doublewords that can be transferred to/from the computer.

"OUR INDUSTRY MUST LOVE STANDARDS - WE HAVE THOUSANDS OF THEM!"

And so it is with hard disk controllers.

Controllers can be categorized in several different ways, by :

Basic computer design (PC/XT vs AT-286-386-486,etc)
- as mentioned in the first chapter, standard AT controllers use different
I/O addresses, IRQ and employ PIO as opposed to DMA.

Bus Architecture (8-16 bit ISA, 32 bit MCA/EISA/VLB/PCI, etc.)
- The adapter must be designed to interface with and use features of
available expansion spots in the host computer.

Controller Card vs Adapter
- The expansion board that plugs into the PC is commonly referred to as a
controller card, but for many drives (primarily IDE and SCSI) the
controller mechanism is built directly onto the drive PCB and the expansion
board in the PC (or built into motherboard) is actually a Host/Bus adapter.

TROUBLESHOOTING TIP - If the BIOS reports "HDD CONTROLLER FAILURE" don't
assume the problems is with your AT/IO board. It might well be the drive
PCB that has failed.

Controller/Drive Interface
- Both drive and controller must communicate in the same 'language' and
several different standards have been established. These include ST506/412,
ESDI, SCSI, IDE(ATA/XTA) and EIDE(ATA2).

Data Encoding Method
- Determines how densely data can be packed onto a track. MFM encoding is
sufficient for only 17 x 512 byte sectors per track. RLL permits up to 27
and variations of ARLL allow 34 or more sectors per track. This recording
density is a major determinant of storage capacity, and with rotation speed
and interleave are critical factors for true data transfer capability.

Support for Translation
- Some controllers present different logical parameters to the PC than the
actual physical geometry of the drive.

Need for ROM Extension or Software Device Driver
- Additional program code is used to provide support for hard drives when
none exists (as in PC/XTs), to implement translation schemes (as in
ST506/RLL and ESDI designs), allow for non-standard devices or features
(SCSI), or for a combination of these (EIDE).

Below is a quick list of the major combinations that have been used in PCs
past and present. While I am sure many others could be added, these are the
ones I have come across over the years.

------------------------------------------------------------------------
Computer   Bus Connection        Interface    Encoding   Translate  ROM
------------------------------------------------------------------------
 PC/XT   8 bit ISA Controller    ST506/412      MFM       NO       YES
 PC/XT   8 bit ISA Controller    ST506/412      RLL       OPTION   YES
 AT     16 bit ISA Controller    ST506/412      MFM       NO       NO
 AT     16 bit MCA Controller    ST506/412      MFM       NO       NO
 AT     16 bit ISA Controller    ST506/412      RLL       OPTION   YES
 AT     16 bit MCA Controller    ST506/412      RLL       YES      YES
 AT     16 bit ISA Controller *  ESDI (10 Mbps) RLL       OPTION   YES
 AT     16 bit ISA Controller *  ESDI (24 Mbps) ARLL      OPTION   YES
 AT     16 bit MCA Controller ** ESDI (PS/2)    RLL,ARLL  YES      YES
 PC/XT   8 bit ISA Adapter       SCSI           RLL       YES      YES
 AT     16 bit ISA Adapter       SCSI           RLL,ARLL  YES      YES
 AT     ?? bit MCA Adapter ***   SCSI           RLL,ARLL  YES      YES
 AT     32 bit EISA Adapter      SCSI           RLL,ARLL  YES      YES
 AT     32 bit VLB Adapter       SCSI           RLL,ARLL  YES      YES
 AT     32 bit PCI Adapter       SCSI           RLL,ARLL  YES      YES
 PC/XT   8 bit ISA Adapter       IDE  / XTA     RLL       OPTION   YES
 AT     16 Bit ISA Adapter       IDE  / ATA     RLL,ARLL  OPTION   NO
 AT     32 Bit VLB Adapter       EIDE / ATA2    ARLL      OPTION   YES
 AT     32 Bit PCI Adapter       EIDE / ATA2    ARLL      OPTION   YES

* ESDI drives have some of the controller logic built onto the hard drive
PCB and some on the controller card.
** PS/2 ESDI uses the same physical interface as other ESDI devices, but
supports additional features specific to their implementation.
*** 16 bit? 32 bit? Who knows? I have never been sure.
------------------------------------------------------------------------

Not to mention hundreds of other combinations to support different
interleaves, track buffers, hardware caching, bus mastering, error
correction schemes, SCSI I-II-III, optional floppy control, ESDI to SCSI
converters, ST506 to SCSI converters (etc., etc., etc.).

So, what does all this mean to you?

Specifically, don't be surprised if the drive you have in your left hand,
does not work correctly with the controller / adapter you have in your
right hand. Also, if controllers are changed it may affect performance as
well as the ability to access previously recorded data.

========================================================================
END OF CHAPTER 2
Return to Table of Contents
========================================================================

========================================================================
ADDITIONAL READING MATERIAL:
========================================================================

I have always had difficulty finding appropriate reading material to
recommend, but there are a few sources that I consider a must for
technicians and support personnel.

First, a number of Internet Newsgroups have exceptional FAQ's (Frequently
Asked Questions) which are updated and posted on a regular basis. These
include:

"YET ANOTHER ATA-2/FAST-ATA/EIDE FAQ" by John Wehman and Peter Herweijer
Newsgroup : comp.sys.ibm.hardware.storage

"BIOS TYPES" by Hale Landis Newsgroup : comp.sys.ibm.hardware.storage

"SCSI FAQ - 2 Parts" by Gary A. Field Newsgroup : comp.periphs.scsi

While bookstores are full of titles, most of them simply provide a rehash
of basics. The following two books are an important part of my library
because they cover much more than the usual:

"THE INDISPENSABLE PC HARDWARE BOOK" by Hans-Peter Messmer (1994)
Addison-Wesley Publishing Company,
ISBN - 0-201-62424-9

"THE UNDOCUMENTED PC" by Frank Van Gilluwe (1994)
Addison-Wesley Publishing Company,
ISBN - 0-201-62277-7

Other books well worth the read include:

"THE HARD DISK SURVIVAL GUIDE" by Mark Minasi (1991)
Sybex Inc.,
ISBN - 0-89588-799-1
A little dated, but full of useful information. Hopefully there is a
revised and updated version.

"OFFICIAL SPINRITE II AND HARD DISK COMPANION" by J. M. Goodman, (1990)
IDG Books Worldwide, Inc.,
ISBN - 878058-08-8
Current advances are not covered, but great explanation of drive basics.

"HARD DRIVE BIBLE" by Martin Bodo (1993)
Corporate Systems Center (CSC)
My copy is the sixth edition from April, 1993. The first 50 pages of the
book should be of interest (though not always clearly organized). The
balance of the book (150 pages) is a listing of drive types and jumper
settings. It's quite good, but keeping something like that updated is
virtually impossible.
WARNING - DO NOT BUY - "The Data Recovery Bible" by Pamela Kane.
Poorly organized material, most of which has nothing to do with data
recovery. Waste of a good title if you ask me!

========================================================================
ACRONYM DEFINITIONS:
========================================================================

IRQ (Interrupt Request) - Lines on the bus used to signal hardware
interrupts.

I/O (Input Output) - Peripherals accessible by the CPU through registers at
specific I/O addresses (or I/O ports).

PIO (Programmed Input Output) - Exchange of data between memory and
peripherals by means of Input Output commands.

DMA (Dynamic Memory Access) - Transferring data directly between memory and
peripherals without going through the CPU.

BUS ARCHITECTURES:

ISA (Industry Standard Architecture) - 8 bit and 16 bit expansion slots
used by PC, XT, and AT designs. Often called IBM Standard Architecture.

EISA (Extended Industry Standard Architecture) - Developed by several
independent manufacturers (Compaq, AST, Zenith, Tandy, etc.) to standardize
32 bit operation and combat IBM's MCA.

MCA (Micro Channel Architecture) - Expansion bus introduced by IBM in 1987,
used by some (but not all) PS/2 models.

PCI (Peripheral Component Interconnect) - High speed bus developed by Intel
to support the demands of Pentium and 486 based computers.

VLB (VESA Local Bus) - High speed, 32 bit extension to the ISA bus promoted
by the VESA (Video Electronics Standards Association).

DRIVE INTERFACES:

ST506/412 - Standard interface used on XT and AT drives and controllers.
Originally developed by Seagate Technologies to support their ST506 (5 MB)
and ST412 (10 MB) drives. The entire controller mechanism is located on a
controller card and communications between the drive and controller flow
over 2 ribbon cables - one for drive control and one for data.

ESDI (Enhanced Small Device Interface) - Developed by Maxtor in the early
1980's as an upgrade and improvement to the ST506 design. While the drive
does not have an embedded controller, one of the most critical functions
,encoding-decoding, is performed on the drive. This allows for faster
communications and higher drive capacities. Uses the same cabling as ST506
interface, but carries different signals on each line.

SCSI (Small Computer System Interface) - Based on an original design by
Shugart Associates, SCSI is not specifically a drive interface, but a
method of allowing different devices to communicate with a PC. For hard
drives the entire controller is built onto the drive PCB, allowing for very
high speed transfers to and from the drive. Fully interpreted, parallel
data is then transferred to and from the PC by way of a single cable
through a bus interface that has configured the device as a hard drive.

IDE (Integrated Drive Electronics) - A technology pioneered by Compaq and
Conner that embedded a controller onto the hard disk PCB while maintaining
compatibility with the register level commands sent by the computer's INT
13 routines. IDE drives are configured and appear to the computer like
standard ST506 drives.

ATA (AT Attachment) - Implementation of the IDE design with a 16 bit AT
style controller on board the drive.

XTA (XT Attachment) - Rarely used implementation of IDE with an integrated
8 bit XT controller.

ATA-2 - Enhancement to the AT Attachment standard to provide for
considerable performance improvement and more sophisticated drive
identification.

EIDE (Enhanced IDE) and FAST-ATA - Various implementations of the ATA-2
standard as marketed by Western Digital (EIDE) and Seagate/Quantum
(FAST-ATA).

DATA ENCODING SCHEMES

MFM (Modified Frequency Modulation) - Common technique used to encode the
magnetic fluxes recorded on a drive into data. Still used on floppy drives
and most original XT and AT systems. Notice that most drive types supported
in CMOS have 17 sectors per track. This is the standard density for MFM
encoding.

RLL (Run Length Limited) - Encoding method that allows 50% more information
to be recorded on a track than MFM. Actually accomplished by recording more
fluxes for every byte, but packing them more tightly onto the surface.
Often called 2,7 RLL because the recording scheme involves patterns with no
more than 7 successive zeros and no less than two.

ARLL (Advanced Run Length Limited) - More complex yet powerful derivatives
of the RLL scheme. Include 1,7 and 3,9 encoding.

========================================================================
NOTICES & PERSONAL RANTS :
========================================================================

INTERNET APOLOGIES : The original artiticle, offering this booklet, was
posted Jan. 29, 1995. I have had lots of replies, BUT - all kinds of people
insist on replying by follow-up articles instead of private Mail. This
proved downright embarrassing! I only wanted E-MAIL responses. Sorry for
the clutter!

DISTRIBUTION : Chapters 1-2 will be sent by E-Mail to some 2,700 people who
requested it and is being posted to various News groups. The work to
maintain a mailing list is much more trouble than it's worth, therefore -
future chapters will be posted to a select number of New Groups and
Bulletin Boards.

Files can be obtained by ftp. Look for "hdtech??.txt"

ftp site: archive.umich.edu in /msdos/info/disk
mirror : wuarchive.wustl.edu in /systems/ibmpc/umich.edu/info/disk (Note -
This mirror is not always up to date)
ftp site: ftp.wi.leidenuniv.nl in /pub/faqs

Also Daniel Tauritz has a home page with links to this document plus a
number of hard drive FAQs: www.wi.leidenuniv.nl/home/dtauritz/ata

If anyone specifically requires E-mail, send me a note with "e-mail please"
as the subject. I will be glad to accommodate them, assuming it's a
manageable number. I cannot provide hard copy or S-Mail.

RIGHTS : This material is copyright (C) 1992,1995 by Nicholas Majors and
Data Recovery Labs. Feel free to distribute it as either hard copy,
electronically, or by BBS - with full copyright credit to both.

COMMENTS & CRITICISM : I look forward to receiving suggestions,
improvements, additions, (and yes even compliments) to this material.
Please bear in mind who this has been written for - tech support personnel,
not end-users and not engineers or hardware designers. HTML markup by Roger
Mathews

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Nicholas Majors

DATA RECOVERY LABS
(division of Data Recovery Services Inc) Voice : 1-416-510-6990
1315 Lawrence Avenue East - Unit 502 FAX : [phone]
Don Mills, Ontario, Canada M3A 3R3 Email : [e-mail]

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