<:-? Hard Disk Drive Tech Overview

                  +++++++++++++++++++++++++++++
                  | MAGNETIC HARD DISK DRIVES |
                  +++++++++++++++++++++++++++++

Significant improvements in manufacturing techniques
and technologies has led to the availability of
physically small, high storage capacity disk drives.

In 1967 a state of the art disk drive had 50 rotating
surfaces, platters 24 inches in diameter, an access
time of 600mS and a total capacity of 5MB!  In contrast,
todays most popular products are 2.5-3.5 inches in diameter with
inbuilt intelligence and capacities in the hundreds of
Mega-Bytes.

The advances in technology that has enabled the physical
size to be reduced to todays dimensions have included:



MATERIALS
=========

-  Very powerful magnets, made from rare metals
-  Control of chemical outgassing/contamination
-  Improved adhesives with predictable characteristics
-  Careful choice of materials to ensure compatibility



DESIGN
======

-  High level of intelligence built in through use of Micro-Processor
   controllers
-  VLSI and SMC to provide very compact electronics packages
-  Advanced signal processing techniques for data recording
-  Power requirements minimized by careful circuit design and choice of
   components
-  Huge increase in reliability through predictive modelling, component
   choice and component testing
-  Innovative mechanical design to reduce tolerance dependence
-  Magneto resistive and thin film heads improve the signal to noise
   ratio and increase areal densities



MANUFACTURING
=============

-  Early involvement in product design ensures ease of assembly
-  Consistent quality through rigid process control limits
-  Close cooperation with suppliers to maintain high quality
-  Closed loop failure analysis for defective units ensures continuous
   process improvements





               +++++++++++++++++++++++++++++++++
               | BASIC PRINCIPLES OF OPERATION |
               +++++++++++++++++++++++++++++++++



HEAD POSITIONING
================

Many disk drives have used stepper motors to control
the position of the read/write heads.  But as the
capacity of a drive increases, the spacing between
data tracks invariably decreases.  This means that
the heads have to be positioned more accurately.
Stepper motors can no longer cope with the very small
positional changes required to maintain the data head
over the center of the track--it is an open loop
system; ie, there is no feedback signal to provide
a fine positional adjustment.

Voice coil actuators are used to position the heads
in high capacity drives because they form part of a
closed loop system.  As the head nears the data track
which contains the desired information, a signal is
generated from prerecorded data, which tells the
positioning system how far the head is away from the
center of the track.  The servo loop moves the head
until the position error signal has decreased to zero
and keeps it at zero until a new access to a different
data track is required.



DATA RECORDING
==============

There have been numerous techniques used for recording
data, from the early Non Return to Zero (NRZ) to
Modified Frequency Modulation (MFM) which was used on
many drives during the late 1970s.

Most of the current popular recording techniques use a
form of data encoding based upon a pre-determined set of
bit patterns.  The encoding techniques is known as run
length limited (RLL) and it can be implemented in various
forms.  The one used in IBM OEM drives tends to be the
2,7 RLL code, this means that the data pattern follows a
protocol that allows for a bit stream of 2 zeros (minimum)
of 7 zeros (maximum).

This encoding method provides superior performance in terms
of data recovery but does increase circuit complexity
somewhat.

In a typical drive the very low amplitude signals, picked up
when the read circuits are activated, can easily be affected
by external events such as mechanical shocks or vibration
or electrical noise.  These phenomenon usually show up as
random read errors which become apparent when long data
streams are being read.  To ensure that the read signals are
not corrupted by external means, it is important to examine
carefully the operating environment that the drive will be
used in.

Errors can be introduced into the recorded data as it
is being written in exactly the same way as described
above except that when the data is read back it is
most likely to show up as a hard error.

Careful evaluation of the mechanical mounting
arrangements, grounding system and the power supply will
minimize problems of this sort.



PACKAGING
=========

During the development of IBM OEM disk drives the level
of shock and vibration that each product will withstand
is carefully established to ensure that the product will
continue to function to its full specification for all
of its expected life.

If disk drives are subjected to shocks or vibrations
which are in excess of the levels permitted, errors
can be expected to occur.


OPERATIONAL SHOCK occurs while the disk drive is
powered on, depending upon the severity of the shock
(or vibration), and what the drive is doing at the
time will determine the damage suffered.

For example, if a screwdriver handle is accidentally
dropped onto a drive that is performing a seek
operation, it will quite likely cause the head(s)
to momentarily touch the disk(s), resulting in a small
area of surface damage which in turn will cause data
errors.

The disk enclosure in higher capacity drives usually
has a large mass, which must be allowed to move freely
in any direction.  Shock mounts are designed in provide
a high level of isolation between the disk enclosure
and the frame.  If any of the shock mounts becomes
jammed (for example, perhaps a mounting screw which is
too long has been used during installation) and vibrations,
due to cooling fans etc, will be transmitted through the
body of the drive and will affect the ability of the data
heads to follow the track properly--this will again give
rise to data errors.

On smaller drives, the disk enclosure, in fact the whole
file, has so little mass that it is not necessary to fit
shock mounts.

NON OPERATIONAL SHOCK occurs when the drive is powered
off.  Damage sustained in this way is usually caused by
inadequate packaging or poor handling disciplines.  The
fragile, high tolerance, mechanical components found in
the modern disk drive can be permanently damaged by
someone carelessly dropping a drive onto a work surface,
even from a height of a few inches.  Likewise,
inadequate packaging, which has not been designed to
withstand the rigors of road/rail transportation will
not protect the drive against the shock levels
experienced when a delivery man throws the box
containing the drive into the back of his van.  Once
the parcel delivery service has got the box, it will be
well tested by the time it reaches the customer!

Modern disk drives will work for many years, trouble
free, if they are handled with care and respect.
Damaged sustained by poor handling techniques is a pure
waste of your money!

To avoid using inadequate packaging, it is possible to
purchase single pack boxes from IBM OEM, or have your
packaging tested to ensure it will meet the required
performance.



POINTS TO REMEMBER
==================

-  Do not remove drives from the box until they are
   needed.
-  Do not stack boxes containing drives too high.
-  If you are not sure how good your packaging is,
   have it tested.
-  Conductive rubber mats should be fitted to all
   surfaces where unboxed drives are stored.
-  Only use qualified packaging when transporting
   drives.



ELECTRO STATIC DISCHARGE
========================

State of the Art electronic design has given us
circuits which require extremely low power level to
function.  It is not unusual to find circuits which
will work for years with only a tiny battery to power
them.

When electronic designers are confronted with the
various problems of how to make circuits physically
smaller, one particular aspect occurs time after time;
ie, how to get rid of heat.  The more amps and volts
that a circuit consumes, the more heat there is to
dissipate.  By reducing the thickness of the
semiconducting layers, which each transistor is made
up from, it is possible to use less voltage and current
to perform a given function.

The individual links between each circuit element
can be reduced in size because of the overall
reduction in power requirements and so integrated
circuits keep getting smaller, or the complexity in
a given chip size keep increasing.  Is everyone
happy?...No, we have a problem!

If you build a circuit that is both physically tiny
and of very low power consumption, and then subject
it to a very high voltage discharge, it will suffer
damage.  The individual connections between the
various components within the integrated circuit
will suddenly be required to conduct much more power
than they were designed to.  Power generates heat and
the device will develop hot spots: areas where the
power is concentrated in several components.  Heating
circuit elements beyond the design point reduce the
reliability of the components and the interconnections
which join them together.

So, by applying a high voltage discharge to a modern
electronic circuit we are most likely going to damage
the long term reliability; the individual components
that were stressed will fail long before those that
were not.  If a particular components absorbs a lot of
energy from a discharge it will literally blow apart
on a microscopic level.  The individual layers that
make up a transistor or diode can be punctured by the
high power flowing in the component during the discharge.

The industry has investigated failures due to ESD
damage.  Post mortems have been carried out to see what
physical damage has been done and in most cases it is
very similar to what happens during a lightning
strike--except of course we are looking at a microscopic
level.

REMEMBER, when YOU pick up a modern disk file,
from the bench or out of its' box, you are
handling a device which probably has close to
250,000 internal circuit connections.  If you have
walked across the workshop, you are probably
charged up to 8000-20000 volts.  What do you think
the chances are that you can avoid damaging
something in the drive?