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This document is an attempt to provide as much information as possible
regarding hard disk drives. This document is based on a compilation of
facts from many sources, some of which seemed to have opposite
viewpoints on the subjects in question; therefore all the information
contained here cannot be guaranteed to be 100% accurate. Also, although
this may not be everything that you wanted to know about hard disks,
it's at least a good start and should provide answers to at least the
most common questions.


A) Modulation and Demodulation

1) Although the frequencies are too high to be audible, the heads
on your hard disk receive and transmit data in much the same
way a modem sends and receives data across a phone line. Noise
occurs in bursts in much the same manner as on your phone line.

2) Error correction codes are used to cope with these error bursts.
Data is read into a buffer, corrected, and then passed along.

3) MFM (modified frequency modulation) & RLL(Run Length Limited)
A single error in an RLL bit string results in an error burst
several bits long in the data. Disk locations that were "good
enough" for MFM may not be good enough for RLL. It's not that
additional signal loss occurs on the physical disk. It's that
RLL can't perform as much error correction. Any drive capable
of RLL is capable of MFM, but the reverse is not true. If the
manufacturer hasn't certified the drive for RLL, tracks have not
been flagged as defective if MFM would correct their errors.

4) In much the same way that modems went from 300 to 1200 to 2400
to 9600 baud by increasing sound frequencies(higher pitch) and
sensitivity to phase angles(acoustics), improved densities on
hard disks can also be achieved, limited by the quality of the
magnetic media and the fidelity of the recording head very much
like the limitations of a stereo cassette recorder. If you want
an up-close look at some good magnetic media, look at a VCR tape.

5) The same technology which makes integrated circuits with wires
so close together you need a microscope to see them also makes
read/write heads with equally small gaps. Think of taking an
integrated circuit, turning it upside down, and letting it
glide on the cushion of air above a platter driven by a
synchronous motor. Those disks spin about as fast as your
typical house fan. That platter had better be very smooth
and very dust free.

B) Interleave, the spacing of blocks of data

1) The blocks of data on PC hard disk are called sectors.
Controllers must read and write sectors at disk speed,
but they may take their time in generating and applying
error correction. This must be completed for one sector
before the next sector is read.

2) In order to allow time for this error correction, these
sectors may be numbered in a non-sequential manner, so
that sector 2 isn't encountered until the controller
can be expected to be done with sector 1. A 1:1 interleave
has no spacing. The controller must perform error correction
as quickly as the data is read and recorded. A 1:2 interleave
has one sector between each sequentially numbered sector.
A 1:3 interleave has two sectors between each sequentially
numbered sector, and so forth. The interleave is the
distance between sequentially numbered sectors.

3) For example, an interleave of 1 would appear as follows:

sector: 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17

while an interleave of 4 would appear like this:

sector: 01 14 10 06 02 15 11 07 03 16 12 08 04 17 13 09 05

4) The number of sectors on the disk should be a prime number,
so that any desired interleave can be accomplished.

5) Since software must run to start the next sector transfer,
your choice of interleave is also affected by processor
speed and the length of software paths in the operating
system being used.

C) Interfaces - connecting the drive to the computer

1) ST506 - named after an ancient 5meg Seagate Hard disk

The controller card is free to encode data with
any scheme at any frequency, limited only by the
fidelity(yes, like your stereo) of the read/write
head assembly and the magnetic medium.

2) SCSI - Small Computer System Interface, used by
Apple, Commodore, Atari and Sun, and now
also available for IBM PC Compatibles.

The modulator/demodulator, error correction, and
data buffering are included in the disk assembly.
The manufacturer of the DASD is also responsible
for the recording and playback mechanism to use it.
Whether it is MFM or RLL is not something that the
attached computer has need to be aware of.

3) ESDI - Enhanced Small Device Interface, used on
the PS/2's.

This represents a compromise between (1) and (2).
The attached computer retains some perception of
physical geometry, but much of the modulator and
demodulator logic is on the drive. These tend to
be RLL, but the user has no choice in the matter.

D) Sealed Head and Disk Assembly(HDA)

1) As you may well imagine, floating an integrated
circuit face down against a disk leaves little
room for pot-holes or dirt mounds for the thing
to bump into. The disk must be very smooth and
its enclosure must either be air-tight or have
a good air filtration system.

2) This form of sealed enclosure has come to be known
as "Winchester" technology. This originated as the
name for IBM 3340 disk assemblies for System/370
but is now commonly used to refer to any sealed
head and disk assembly. According to Kenneth Deckert,
the term "Winchester" may also be used to refer to
the small slider and suspension head assembly within
the sealed enclosure. In my own reading, "Winchester"
technology was identified strictly by the labels
warning a user not to unseal the enclosure.

3) Nothing within a sealed head and disk assembly is
serviceable outside a clean room. If something
external to the enclosure has failed, it may be
repaired easily, but if the failure is within the
enclosure, the cost of the equipment needed to
safely unseal and safely reseal the assembly is
very large.

4) Air filtration systems on units which are not completely
air-tight can be overloaded by dust in the environment.
Keep them away from heating vents and out of carpeted
areas wherever possible. Such a unit will fail very soon
after its air filter plugs up. The completely sealed units
which don't use outside air are more expensive.

E) Stepping Motors, Head Parking, Voice Coils, and "Ka-Chunk" Sounds

1) Stepping motors represent ancient history in the mainframe world.
The arm holding the integrated circuit against the disk is very
much affected by the physical force of the motion produced. The
life expectancy measured in time-in-use (not time powered on) is
very limited. But they're very inexpensive to manufacture.

2) Voice coils have much less inertia(weight) to overcome and move
faster as a result. Hard disks with access times below 30ms are
usually voice coils. Aside from being faster, they last much
longer. They are more expensive to manufacture.

3) Allowing the integrated circuit which reads data to literally
fall on its face every time you turn the power off is not good
for the integrated circuit or the surface it lands on. The
circuit will lose its face and the surface will gain some
scratches. Moving the heads before landing is known as parking.
One form of parking simply lands the circuit on its face in the
same gutter every time. The best form of parking uses an electric
magnet acting against a counterbalance for the head to reach the
disk at all. When power is removed, the head "falls away" from
the disk surface, courtesy of the counterbalance. This form
produces an assembly which can be operated in multiple
orientations: horizontal, vertical, or whatever.

4) Stepping motors, having more weight, need an application of
considerable force to move the heads to a designated location.
Counterbalances and stepping motors don't mix. Any parking
mechanism must be a strong one. Expect a loud Ka-Chunk sound.

5) Some voice coil units have auto head parking units intended
for use with stepping motors and also go Ka-Chunk. But voice
coils are delicate and this application of force tends to
damage them.

6) For reasons that have nothing to do with performance, look
for a disk with an access time below 30 milliseconds which
does NOT go Ka-Chunk when power is removed, and if you're
looking for something REALLY good, see about one that can
operate in any orientation. The July 1988 issue of Byte
magazine lists some fast 40Meg drives and tells which are
stepping motor and which are voice coil:

Product Arm Movement Recording Interface Heads
---------------------- ------------- --------- ---------- -----
Seagate 251-1 Stepper Motor MFM ST506 6
Tandy SCSI drive Stepper Motor RLL SCSI 6
Columbia SCSI drive Stepper Motor RLL SCSI 3
Core AT40F Voice Coil MFM ESDI 4
CMS F40-K Voice Coil MFM ST506 5
Miniscribe 3053 Voice Coil MFM ST506 6
CDC Wren II Voice Coil MFM ST506 5
Core AT43 Voice Coil MFM ST506 5
Fujitsu M2242AS2 Voice Coil MFM ST506 7
Micropolis 1333A Voice Coil MFM ST506 5
Microscience HH-1050 Voice Coil MFM ST506 5
Priam ID45-AT-D2 Voice Coil MFM ST506 5
Rodime RO3055 Voice Coil MFM ST506 7
SPC Scorecard 44 Voice Coil MFM ST506 7
Toshiba MK54FB Voice Coil MFM ST506 7
CompuAdd Flashcard 49 Voice Coil RLL ST506 6
Northgate Turbo Kit Voice Coil RLL ST506 6
Plus Hardcard 40 Voice Coil RLL ST506 4

F) Surge and Spike Protection

1) Estimates of damage to personal computers arising from
power spikes and such range from 75% to 95% of all repairs.

2) Spike protectors are much cheaper than computer repairs.
Radio Shack carries a nice $25 6-outlet unit that you
affix to a standard wall outlet. It has an indicator
light to let you see that the critical MOVs are still
working.

3) Spikes are common. Any appliance which needs a "slow blow"
fuse will generate a spike. Heaters, dryers, and air
conditioners are all common sources of spikes.

4) Even if the spike only wipes out your disk's low-level format
information by what is commonly referred to as "spiral write,"
it will take you a great amount of effort to do a low-level
controller format, not to be confused with DOS format or FDISK,
and to restore from your most recent backup.

5) A lot of "surge protectors" consist of nothing more than
a fuse on an extension cord. These won't protect electronic
equipment, but so far as any govermental regulation, this
is all that a surge protector needs to contain. I haven't
encountered any combination spike-and-filter boxes that
are deficient in this manner.

6) Don't buy a spike protector without an RFI/EMI filter.

a) The clipping circuit to discharge power spikes generates
enormous RFI/EMI bursts, which will damage your equipment
just as surely as the spike. Although some EMI/RFI noise
may be inserted between your house and the local power
transformer, particularly if a ham radio operator lives
nearby, the source you should be most concerned with is
the clipping circuit for spike protection.

b) Hard disk drive motors are AC motors run by AC current
which does not have the RFI/EMI filtration provided by
a regulated DC power supply.

c) Anyone who's set up the turntable on a stereo knows the
reception afforded to noise by the magnetic cartridge.
Grounding and shielding are vital to preserve the rest
of the stereo system.

d) The head assembly of a hard disk is similarly sensitive
and exists within the same enclosure as the AC drive motor.

e) A burst of high-frequency energy introduced into the drive
enclosure through the AC for the motor can be picked up
by the head assembly and passed through the amplifier to
overload("fry") the electronics on the disk itself and
associated electronics in the controller card.

G) "Expected" MTBF (Mean Time Between Failure)

There is no government regulation of this figure. A manufacturer
can always "expect" a long life. Once having "expected" it, any
attempt to correct it downward at a later time would produce a
panic in the user community, so there is never any such attempt.
Everything I've read indicates published MTBF figures are totally
worthless.

H) How much is a Warranty Worth?

1) You should be able to locate firms in the "Computer Shopper"
that will rebuild an ST225 for $95 with a 6 month warranty.

2) If it costs you $35 to get service under warranty, then the
warranty was only good for $60 of the cost of the repair.

I) Low Level Formatting and Interleave Optimization Should be Painless

1) Several Share-Ware packages attempt to do this, but the
fixing of problems found in them leaves much to be
desired.

2) For commercial packages, Spin-Rite, Disk Technician,
and Htest/Hformat do this. None of these support ESDI
or SCSI controllers. Only Htest/Hformat supports RLL.
Htest/Hformat is not copy-protected, in contrast to
a very nasty form of copy protection on Disk Technician.

3) Htest/Hformat runs on PC, XT, AT and compatible computers.
It supports Adaptec, DTC, OMTI, Western Digital, and
Xebec controllers. PC Connection(1-800-243-8088) sells
it for $49 plus $3 shipping and handling. It consists
of the following:

a) HOPTIMUM - determines your best interleave value,
and will offer to reformat your disk
to the determined interleave while
preserving your data(if you have any).
You should leave your system powered
on for about two hours before starting
a low-level format, so heat expansion
has taken effect for normal operating
temperatures.

b) HFORMAT - does a low-level format and intensive
examination for faulty track locations.
Contrary to what I read in one review,
it does not preserve your data. It's
an all-night utility. Start it up,
turn off your monitor, and it'll meet
you in the morning. If you're formatting
as RLL a drive not certified for same,
give VERY serious consideration to a
large FT=(verification test count) number
and leaving it running all weekend.
You should leave your system powered
on for about two hours before starting
a low-level format, so heat expansion
has taken effect for normal operating
temperatures.

c) HPERF - determines track-to-track and average
seek times, transfer rates, and interleave.

d) HTEST - attempts to determine bad tracks without
altering your data in any way.

e) GETSEC - saves your boot record with your partition
definition on a floppy somewhere.

f) PUTSEC - restores your boot record from the floppy.

4) Drives with an even number of heads need a low-level
reformat every 3 to 6 months. David J. Thompson explains
in the July/August 1988 issue of Micro Cornucopia:

"As a drive ages, the casting destresses itself, the bearings wear
a bit, and the head assembly shifts. Add to this a bit of heat
(expansion) and you'll see that when the heads are over track 427,
they're not necessarily precisely over the same track 427 they
were over when the drive was last formatted. "Some drives have a
surface that's used only for verifying head position. That works
well, but it's not used a lot because it wastes space. (These
drives have an odd number of read/write heads)...
"So, sector headers and trailers eventually get out of line
from the data.... after six months or a year you'll probably
start losing sectors... The cure is to backup everything you
hold dear(at least whatever's still readable) and reformat
the drive."

This effect is responsible for the notorious stepper transistor
failure on Seagate ST225 drives. Alignment problems prevent the
drive from identifying track -1, so it bangs its head against the
wall(clicking sound) and burns out the stepper motor transistor.
A loud clicking sound(not a chirp) on any Seagate is cause for
concern.

Refer to the list of fast 40Meg hard drives in item 6 of Section
E, above, for the number of heads on each drive. A possible
exception to the rule is the Plus Hardcard, which may be using
cylinder information stored in each sector identifier.

J) Some Controllers Don't Mix

1) Don't use more than one hard disk controller or more than one
floppy disk controller. If you have a combination floppy
and hard disk controller, don't mix it with any other
disk controller. A possible exception to this is the
Plus Hardcard; you may be able to install more than one.

2) Before installing ANY controller in combination with any other
controller, get the approval of your supplier, and be VERY sure
to describe any tape backup units or other DMA devices that
you are using.

3) 8088 machines have four DMA ports and can handle(multiplexed)
up to 4 data simultaneous data transfers. Tony Ellis of
Australia has explained as follows: "channel 2(is used)
for all diskette I/O, channel 3 for all hard-disk I/O
and channel 1 for special adapters." Two hard disks
and two floppy disks have become an unofficial standard.

4) Good AT compatible machines have four 8-bit DMA ports and
three 16-bit DMA ports. There is a fourth 16-bit DMA port
internally which the 8-bit DMA ports go through, and the
8-bit DMA ports of an AT may, as a result, be somewhat
slower than those of a PC or XT. An 8-bit hard disk
controller which achieves 1:1 interleave on a PC or XT
may fail to do so on an AT or compatible computer.

5) You can buy single controller cards for four floppy disks
and two hard disks. Tulin corporation(408-432-9025) sells
16-bit DTC controllers for four floppies and two hard disks
for $219(MFM) and $249(RLL). They say they will have 1:1
interleave versions available by September 1st, 1988.

6) Combination controllers save slots. A two-floppy-disk
and two-hard-disk controller should cost you only
about $10 more than the two-hard-disk-only version
of the same controller. Combination cards are the
most popular.

7) For Western Digital and Adaptek, controller identifiers
ending in "2" include control for 2 floppy disks. The
Western Digital WD1006WAH is a 16-bit 1:1 interleave MFM
card for two hard disks only. WD1006WAH2 is the same card,
but for two hard disks and two floppies. The Adaptek 2371
is an 8-bit 1:1 interleave RLL hard-disk-only controller
for a PC or XT or compatible(not an AT). The Adaptek 2372
is the same card but for two hard disks and two floppies.
A "turbo" cpu card may be required to actually achieve 1:1
interleave using the Adaptek cards.

8) For DTC, the second digit of the ID indicates whether the
controller is hard-disk-only or a combination controller for
hard disk and floppy. The 5180 is a 16-bit MFM card for two
hard disks. The 5280 is the same card but for two hard disks
and two floppies. The price difference between hard-disk-only
and combination hard-disk-and-floppy controllers for DTC seems
to be slightly less than for Western Digital: about $6. This
may reflect Western Digital's reputation for making better
floppy disk controllers.

9) Some tape backup controllers are incompatible with other cards.
Review any tape backup card carefully before installing it.

10) Perhaps the worst aspect of these incompatibilities is
that a combination may appear to work for quite some
time before one of the cards burns out.

K) Special Purpose Controllers

1) PERSTOR, for ST506

This controller is specifically designed to use RLL on
drives which are not RLL-certified, employing additional
error correction and higher frequencies. It boasts a 90%
increase in disk capacity over MFM and an increase of 30%
over standard RLL.

Tulin Corporation, (408)432-9025, advertises a price of $209
for the 8-bit PC/XT card and a price of $219 for the 16-bit
AT card. I presume these prices are for the non-combination
versions without floppy control.

2) KONAN, for ST506

This is a cached 1:1 interleave MFM disk controller. It achieves
1:1 interleave with almost any software because it reads the
next sector(s) into cache in anticipation of your needing them.
It boasts a highly effective cache retention algorithm and a
VERY fast cache-to-memory data transfer capability.

Tulin Corporation, (408)432-9025, advertises a price of $160
for the 8-bit PC/XT card and a price of $177 for the 16-bit
AT card. I presume these prices are for the non-combination
versions without floppy control.

L) Installing a Hard disk

1) Unplug Everything. Turning switches off is NOT good enough.

2) Try to find a non-carpeted area, to reduce static electricity,
before opening anything.

3) Touch the screw on the wall plate for an electric light to
discharge static electricity before beginning, and every
time you return after leaving the room.

4) Never, ever touch anything within your computer without first
touching a metal part of the casing. This equalizes any static
charge between you and your computer. Wrist grounding straps
are available to make this automatic.

5) Never, ever touch any of the electric contacts on your hard disk
without first touching a metal part of its casing, for the same
reason. Wrist grounding straps will do you no good here. You
must remember.

6) If installing a controller card as well, be very certain to
touch the faceplate at one end of the card before touching
any other portion. Again, wrist straps will do you no good
here. You must remember.

7) For the ST506 interface, you will need the following cables:

a) A power plug from your power supply.
b) A 34-pin daisy-chained data cable, which connects to both
hard disks.
c) A 20-pin cable, one for each hard disk.

8) Rails must be screwed to the side of your hard disk before it
is inserted into the your computer. These rails normally come
with the computer. If screws do not come with the drive, use
a flashlight to look inside the cage to make sure the screws
you've used do NOT contact any circuitry.

9) Normally, one side of each ribbon cable is colored. This is an
indication of which side of the ribbon cable is pin number one.
If you're lucky, your connectors also have slots in them, on
the side next to pin number one, to eliminate any possibility
of putting them on in reverse order.

10) The 34-pin and 20-pin connectors on your DASD controller should
have the location of pin number one clearly marked, normally on
the left as you face the connectors. The colored side of the
ribbon must be on the side for pin number one, normally on the
left as you face the connectors.

11) ST506 includes two circuit board connectors on the rear of the
disk unit, each having a slit, or slot, toward one side. If
your cables are also slotted, there will be no way to connect
them incorrectly. If not, the colored side of the ribbon
connector goes on the same side as the slot in the circuit
board connector.

12) The power connector should be rounded on one side but not on the
other. It should be impossible to connect it improperly.

13) If you have even the slightest doubt that you have connected them
correctly, call your supplier for help before applying any power.

M) Where to Find the Manufacturer (Source: Byte Magazine, July, 1988)

CMS Northgate Computer Systems
1372 Valencia Ave 13895 Industrial Park Blvd,Suite 110
Tustin, CA, 92680 Plymouth, MN 55441
(714)259-9555 (800)548-1993

Columbia Data Products, Inc Plus Development Corporation
851 West 436, Suite 1061 1778 McCarthy Blvd
Altamonte Springs, FL 32714 Milpitas, CA 95035
(305)869-6700 (408)946-3700

CompuAdd Corporation Priam Corporation
12303-G Technology Blvd 20 West Montague Expy
Austin, TX 78727 San Jose, CA 95134
(512)250-1489 (408)434-9300

Control Data Corporation Rodime, Inc.
8100 34th Ave. S Peripheral Systems Division
Minneapolis, MN 94043 29525 Chagrin Blvd, Suite 214
(612)851-4131 (216)765-8414

Core International Seagate Technology
7171 North Federal Hwy 920 Disc Dr.
Boca Raton, FL 33431 Scotts Valley, CA 95066
(303)997-6055 (408)438-6550

Fujitsu America, Inc Systems Peripheral Consultants,Inc.
3055 Orchard Dr 9747 Businesspark Ave.
San Jose, CA 95134 San Diego, CA 92131
(408)432-1300 (619)693-8611

Micropolis Corporation Tandy/Radio Shack
21211 Nordhoff St 1800 One Tandy Center
Chatsworth, CA 91311 Fort Worth, TX 76102
(818)718-5117 (817)390-3700

Microscience International Toshiba America, Inc
305 North Mathilda Ave Disk Products Division
Sunnyvale, CA 94086 9740 Irvine Blvd
(408)730-5965 Irvine, CA 92718
(714)380-3000
MiniScribe
1861 Lefthand Cir.
Longmount, CO 80501
(303)651-6000


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