From: datarec@inforamp.net (Nicholas Majors)
Subject: Chapter 2 - Technicians' Guide to Hard Disks
Date: 25 Mar 1995 18:40:54 GMT
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Chapter 2 - TECHNICIANS' GUIDE TO PC HARD DISK SUBSYSTEMS
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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 : 1-416-510-6992
Don Mills, Ontario, Canada M3A 3R3 Email : datarec@the-wire.com
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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.)
HARD DRIVES AND CONTROLLERS:
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.
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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 and control signals
to the disk assembly through various 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 (usually, 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 while this has been common with main frame drives, it
represents a major design change for personal computers 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.
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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 CPU wants to recieve at least a byte (8 bits) at a time. 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 problem 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 for electrical properties and
logical meaning of signals 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.
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Computer Bus Connection Interface Encoding Translate ROM
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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.
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END OF CHAPTER 2
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How to deal with some of this confusion will be addressed in
CHAPTER 3 - Installing and Configuring Hard Drives.
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ADDITIONAL READING MATERIAL:
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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!
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ACRONYM DEFINITIONS:
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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 by the motherboard BIOS 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. Accomplished by
recording larger representations for every byte, but able to pack them
more tightly onto the surface, because of fewer actual flux changes.
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. Most
every new drive made today uses some form of RLL or ARLL encoding.
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