Introduction to Hard
Drive Technologies
By IBM Storage Systems Division
IT Professional Services E-mail Systems 1
___________________________________________________________________________________
© ComputerCORP March 1999. This information is correct at the time of writing and is subject to change without notice.
Introduction to Hard Drive Technology
What is a Hard Drive? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 2
Terms of reference. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 3
Inside the Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 4
Disk Drive Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 6
Recording the data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 7
Embedded and dedicated servo mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . Page 8
Tracking and reading the data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 9
Disk capacities and Areal Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 10
Drive Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 11
Disk performance Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 12
Drive Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 13
Disk Enclosures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 16
Form Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 18
Interfaces Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 19
ATA Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 20
Summary of ATA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 22
ATA and your BIOS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 23
SCSI Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 25
Serial Storage Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 27
Fibre Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 27
Configuration of a Serial Drive System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 28
Cabling & Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 29
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What is a Hard Drive?
A hard disk is a device fitted to computers and other electronic devices to store large amounts
of data.
A full definition of a hard disk is as follows:
A Hard Disk Drive (HDD), often called a "disk drive," "hard drive," or "hard disk," is a device
that stores and provides relatively quick access to large amounts of data on an
electromagnetically charged surface or set of surfaces. Today's computers typically come with
a hard disk that contains several billion bytes (gigabytes) of storage space.
A hard disk is really a set of stacked "disks," each of which, like phonograph records, has data
recorded electromagnetically in concentric circles or "tracks" on the disk. A "head"
(something like a phonograph arm but in a relatively fixed position) records (writes) or reads
the information on the tracks. Two heads, one on each side of a disk, read or write the data as
the disk spins. Each read or write operation requires that data be located, which is an
operation called a "seek." (Data already in a disk cache, however, will be located more
quickly.)
A hard disk/drive unit comes with a set rotation speed varying from around 4200 and up to
10000 rpm or maybe even beyond. Disk access time is measured in milliseconds. Although the
physical location can be identified with cylinder, track, and sector locations, pointing to a
physical location on the surface of the disks within, these are actually mapped to a logical
block address (LBA) that works with the larger address range on today's hard disks and acts
as a kind of postcode to find data easily when there are a large number of cylinders, tracks,
and sectors.
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Terms of reference.
There are many special terms used with disk drives. Below are a few used to describe the disk
as a whole.
Capacity A measure of how much you can store on a drive.
Gigabyte A gigabyte is a measure of computer data storage capacity and is
"roughly" a billion bytes. A gigabyte is two to the 30th power,
or 1,073,741,824 in decimal notation.
Megabyte As a measure of computer processor storage and real and virtual
memory, a megabyte (abbreviated MB) is 2 to the 20th power
bytes, or 1,048,576 bytes in decimal notation.
Kilobyte As a measure of computer memory or storage, a kilobyte (KB or
Kbyte*) is approximately a thousand bytes (actually, 2 to the
10th power, or decimal 1,024 bytes).
Byte A byte is a unit of information that is eight bits long. A byte is
the unit most computers use to represent a character such as a
letter, number, or typographic symbol (for example, "g", "5", or
"?").
Bit A bit is the smallest unit of data in a computer. A bit has a single
binary value, either 0 or 1. Although computers usually provide
instructions that can test and manipulate bits, they generally are
designed to store data and execute instructions in bit multiples
called bytes.
CHS Cylinders, Heads, Sectors - These are the logical parameters
which allow you to map all the areas on the disk.
Logical Block Address Logical block addressing is a technique that allows a computer
(LBA) to address a hard disk larger than 528 megabytes. A logical
block address is a 28-bit value that maps to a specific
cylinder-head-sector address on the disk. 28 bits allows
sufficient variation to specify addresses on a hard disk up to 8.4
gigabytes in data storage capacity. Logical block addressing is
one of the defining features of Enhanced IDE (EIDE), a hard
disk interface to the computer bus or data paths.
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Inside the Drive
The diagram below shows the inside workings of the drive unit. The unit is built of a stack of
disks or platters as they are known that rotate at high speed and are the surfaces that data is
written to and read from. An arm or actuator pivots from the edge of the unit and holds the
head, which reads and writes the data to the platter at any point over the disk.
top clamp/ in hub motor
HGA Suspension
actuator motor magnets
Actuator
Spacers
Actuator motor coil
Disk
At then end of the actuator you will see a tiny assembly holding the head assembly. This is
suspended over the disk on the end of a suspension arm which protrudes from the end of the
actuator.
Swaging Hole
For fixing
suspension to
actuator
Suspension
Gimbal
HGA
{
Head / Slider
The head is suspended over the surface of the platter by a tiny suspension mechanism. The
head flies over the disk at both a very high speed and with a minute gap between the platter
and the head.
Air/ Disk
Movement
TF / MR
head
The head reads the data from the disk and passes it to the drive electronics so it can be
transferred to the computer.
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This is a greatly magnified picture of a head. The precision engineering required to produce a
device such as this means that the head must be kept totally clean. Drives can only be opened
in a clean room environment. The diagram below shows an approximation of the scales
involved.
Human hair
Dust Particle
Smoke Particle
Head / Slider
Finger Print
Disk
Head Fly Height
From the diagram above you can see that, obviously it is not possible to service a hard disk in
a regular home or office environment. If you encounter a hard disk which has a physical defect
then essentially it cannot be repaired. If the data contained on it is important then it should be
sent to a professional data recovery lab such as Ontrack.
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Disk Drive Terminology
Here are the definitions of the disk drive components that we have just seen.
HEAD The magnetic element that reads/writes magnetic signals.
SLIDER The strategically shaped block of graphite that carries the head.
GIMBAL Flexible connector that attaches the slider to the suspension.
SUSPENSION The head is attached to this and it is the "carrier" for the Slider that
allows the head to "fly" over the disk surface.
ACTUATOR A multi-"arm" structure to which the suspension is mounted. It
moves across the disk surface to position all the heads over the
data.
DISK A thin round aluminium or glass substrate that is coated with a
magnetic material. The head writes and reads to/from the disk.
SPINDLE/MOTOR Disks are stacked along the length of the spindle and the motor
spins the disks at a consistent RPM (Revolutions Per Minute).
HDA The Head Disk Assembly is the complete HDD (Hard Disk Drive)
less the electronics card.
HGA The Head / Gimbal assembly
HDD The Hard Disk Drive defines the complete functioning assembly.
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Recording the data
As we saw in the last section the Hard Disk drive is made up of several stacked disks. These
disks are mounted on the motor spindle.
Disk
Head
0
1
2
3
4
5
6
7
8
9
Cylinder
A single stack of single circles on all used surfaces
Each disk or platter contains concentric circles of data known as tracks.
Track
A single circle on
a
surface
Disk
Each track contains sectors. A sector is a section of the track that is referenced in the internal
map of the platters that the disk maintains.
1 Sector
512 Bytes of Data
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Embedded and dedicated servo mechanisms
Hard Disk drives have built in check data stored on the surface of the platters which allows the
drive to check that the head is positioned in the correct place and to ensure that the drive can
read data reliably.
There are two techniques used for working with this data and they are called Dedicated and
Embedded servo
Dedicated Servo
Dedicated servo was the first method. It employed concentric circles of data on a dedicated
platter surface. The main trouble with this method is that if the platters became misaligned
then the drive would believe it was reading correctly when it actually was not. This could
happen during use when the drive warmed and components expanded and therefore thermal
recalibration was required regularly to allow for expansion / contraction between the
dedicated surface and the data surfaces.
Embedded Servo
Sector Servo
The newer method, and probably the only method that you will encounter in your job is
Embedded Servo. In this method servo information is intermixed with the data sectors. This
has a number of advantages. Firstly there is no need for thermal recalibration. In addition the
drive will be measuring its accuracy on the platter that it is reading from.
Diagrams are shown on the next page.
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Tracking and reading the data
Keeping on track is an essential task and one that can be demanding. You have already seen
the scale of the drive and head. Keeping the read head over the correct track at 10000 rpm or
greater requires constant checking. The surface of the disk is shown below with the head
moving over a track. As the platter rotates the head will periodically pass over a servo sector
and if the head is straying from the track it will move back into the centre of the track to carry
on reading. This minimises the number of times that data is read incorrectly and hence the
amount of time that the drive spends re reading data.
Each sector on the track will have a section showing its unique address (LBA/CHS). This
works like a postcode to allow the drive to find the data that it is looking for.
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Disk capacities and Areal Density
Areal Density is the measure of how many bits can be stored on an area of the platter. It is
worked out by multiplying the number or tracks per inch (width) by the number of bits per
inch (length).
As an example the Deskstar 25 GP has 16000 tracks per inch and an Areal Density of up to
3.78Gb/sq.in.
TPI
BPI
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Drive Performance
Drive technology is getting better all the time. There are a few points that play a key role in
making a faster and better drive but there is always a trade off. The table below shows some of
the key points to look for
Modification Benefits Drawbacks
Faster Spindle Read / Write data faster More power, higher temperature, higher
cost
Media Data Rate
Access Time
Latency
Faster, more accurate Finds and settles on the right Higher cost, more power, higher
Actuator track faster temperature
Seek Time
Access Time
Smoother Disks, Smaller Smaller bits, closer tracks, more
Heads disks
Areal Density, Capacity
Interface Type More drives per cable, faster Higher cost, more power, bigger space
speed along the cable, longer
cables
Functions
Interface Data Rate
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Disk performance Terminology
RPM: Revolutions per minute of the motor spindle.
Today: 4000 - 10000 RPM
Seek Time: A measurement used in calculating the access time of a drive.
Latency: Latency is equal to the time required for 1/2 a spindle rotation:
7200 RPM = 60/7200 = 0.008333 Seconds (8.3 mS).
Access Time: Latency + Seek Time
Media Data Rate: Commonly specified as an instantaneous rate at which the drive can
write / read data to / from the disk.
Interface Data Rate: The rate at which the data can be transferred from one device to
another across the bus.
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Drive Handling
Shipping a drive and Electrostatic precautions
The electronic parts from an drive can be damaged or destroyed by small static discharges.
People handling electronic equipment should always ground themselves before touching the
equipment. Electronic equipment should always be handled by the chassis or frame.
Components, printed circuit board edge connectors, should never be touched.
The only bags which are acceptable for packing hard disk drives are ESD shielding bags
constructed with a conductive layer. These bags are usually of a silvery appearance. Use thick
foam rubber to secure the drive during shipment. DO NOT USE foam peanuts, bubble wrap,
or newspaper. See good and bad packaging examples below:
In the example above the drives are just crammed into the box with no spacing. They will bang
against each other and the carton or whatever is shipped next to it in transit. This will cause
significant damage to the drives and could totally destroy their ability to function.
In the example above, proper packaging has been used which will ensure that the drive cannot
bang against other drives or the case itself. There is also room for the box to be slightly
squashed during transit without damaging the unit.
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This is a common yet drastically bad way to ship a drive. Placing a drive in an envelope is
totally unacceptable. If the drive wasn t broken before shipment, then it definitely will be after.
When talking to end users who will ship a drive for return make sure that you inform them
that this is wrong and they should always ship the drive in an ESD bag within a padded box.
In an ideal world everybody would pack their drives like the example above. An electrostatic
protection bag, and foam padding cushioning the drive within the firm cardboard box will keep
this drive safe to its destination.
When shipping multiple drives there are additional considerations. The example above shows
multiple drives in the correct packaging. They are not free to slip and bang into each other or
the edge of the carton. They are also evenly spaced within the box so that weight is evenly
distributed. Try to pack multiple drives in this way so that they are easier to ship and carry.
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These drives, whilst they will be safe from ESD damage and are padded within their shipment
box can slip and damage each other during shipment. Make sure that the drives cannot slip and
knock into any hard objects including each other.
In addition it helps to label the outside of the box to ensure that the shipping agent or carrier
knows to take care with the box and to encourage them not to throw, kick, or crush the
package.
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Disk Enclosures
The enclosure is the outer box that keeps the platters and heads sealed from the outside and is
the frame of the drive. The enclosure keeps out dust and other particles that accumulate inside
the PC. The enclosure will also affect sound (acoustics), airflow, temperature (thermal) specs
and manufacturing cost.
There are a variety of enclosure designs shown below:
This older design now longer in production had a cast top and bottom and a seal around the
middle.
The clamshell design was used on the high end Server drives from the plant in San Jose, drives
such as the DCHS Scorpion have enclosures such as these.
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The new bathtub design is made from a cast bottom section and a flat pressed cover. This is
the design of drive that you will see most often.
The enclosure has important characteristics and will affect the choice of drive the customer
has.
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Form Factors
These are the industry standard that defines the physical features of the drive ie: 5.25", 3.5",
2.5". These are controlled by the SFF (Small Form Factor) Committee. Full High, Half High
also refer to form factors. There is some confusion in the general market place with regards to
drive height. The standards are:
1"(half height), (low profile)
1.6" (full height), (half height)
The definition of a form factor will also include items such as connector types, physical
dimensions, mounting holes and the weight.
PCMCIA and Compact Flash (CF) are extra small size connections for disk drives and other
peripherals.
Interfaces
These are the industry standard that defines the method by which the HDD attaches to the
system. Examples of this are ESDI, SCSI, ATA(ATAPI), SSA, Fibre Channel (FCAL), SCSI
- narrow / wide / SCA / Ultra (SE) / Ultra2 (LVD) / Ultra3, ATA (IDE, EIDE) - PIO / DMA /
UDMA (33/66), SSA 40 / 80 / 160, FCAL.
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Interfaces Overview
The interface is the connection from the computer to the hard disk. There are a variety of
different types in use. This section will introduce you to the ones that you are likely to come
across in your day to day job.
The diagram below shows a computer in block form. The green arrow is the interface.
Obviously there are in addition, components of the interface at either end to connect the hard
disk and computer data flow together.
Peripheral
Micro-
Bus Adapter/s
Processor
Memory
Disk
Drive/s
There are two general types of interface. Parallel and Serial
Parallel
The main examples of this are:
ATA (Advanced Technology Attachment)
SCSI (Small Computer System Interface)
In parallel interfaces, more than one bit of data is transferred
simultaneously. This has the advantage of greater throughput,
but on long cables you can get timing errors when the individual
wires have different lengths.
Serial
The main examples of this are:
SSA (Serial Storage Architecture)
FC (Fibre Channel) (FCAL)
In serial interfaces, only a single bit of data is transferred at any one
time. This gives the disadvantage of smaller throughput, but can be
less susceptible to errors.
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ATA Overview
ATA or IDE is a cheap low tech interface which supports up to 2 devices per channel. It is
very widely used as most motherboards have an onboard controller which for home users
makes it a much more cost effective option than having to buy a SCSI card on top of the price
of a drive.
Most of the calls from home users that come into the TSC will be about ATA drives.
The interface itself is simple with no 'Ground lines' which can protect data integrity in the
cable at high transfer rates, no 'Termination', which would provide the ability to run the
interface in a more demanding fashion. Performance on ATA has been lower than the other
alternatives, but there are attempts to address this and there have been many different versions
of backwardly compatible ATA emerging including versions with transfer rates of:
3.3MB/s - 16.7MB/s - 33MB/s - 66MB/s - ?
Limited speed and # devices limits IDE to small servers and desktop commercial applications.
In addition the maximum permissable length of an ATA cable is 18 inches. Cables longer than
this may cause problems, and in extreme cases compromise data integrity.
Drive 0
IDE Controlle r
ATA devices are either set as a master or slave device. When you have only one device on the
bus then you should always set it as master, and connect it at the end.
Drive 1
Drive 0
IDE Controller
When you have two devices on the bus it is good practice to set the device at the end of the
cable as the master.
Also bear in mind that the drive may be sharing the bus with other devices such as CD ROM
drives and Tape drives.
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Cable Select
Predefined positions
Unique Cable (coloured coded)
Master always at end
2,5  ATA HDD Connector
3,5  ATA HDD-Connector
Some IBM drives have connectors on them as shown above. Because there are no jumpers on
the end of the drive there is a jumper block on the underside of the drive.
The newer drives have a smaller jumper block on the end of the drive in between the power
and ATA connectors. This can be used to set the drive as master or slave device and to force
certain configurations.
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Summary of ATA
ATA is a dumb interface. The computer tells the device how and when to do everything and
controls transfers (except in UDMA).
ATA cannot multitask and in addition uses CPU processing time from the computers main
processor.
No error checking on the data transferred (except in UDMA).
Communication between the drive and system relies on predefined times to wait for replies or
to send further communications.
System designers sometimes ignore these 'rules' and base their design on - "Device 'X'
Responded in time 'Y', so that's how long we'll wait".
These factors can cause problems. This is why ATA is big in the home PC market but in larger
and server systems SCSI is the more usual choice.
However, things can be done to improve the performance and data integrity of ATA.
Add ground lines in the cable:
80 way cables are now widely available. These cables have a ground line in between each of
the data lines of the cable. This reduces the chance that electromagnetic interference from a
conductor can affect the conductors next to it. In fact UltraDMA 66 requires these additional
ground lines to function. A diagram of part of a cable is shown below with the ground lines
coloured blue.
Cable
Data Bit
Conductor 3 DD7
Conductor 4
GND
Conductor 5
DD8
Conductor 6
GND
Conductor 7
DD9
Add termination
ATA-3 Annex C defines values for each signal
The drive can only control one end of the bus ... System / Adapter manufactures must play
their part
Data Bit
DD5
DD9
DD5
DD10
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ATA and your BIOS
The BIOS is software (firmware) that resides on the system motherboard. It is the link
between the system and attached devices. It interprets and translates the device geometry into
a format that the system understands.
Below is a diagram of how the BIOS fits into the computer.
User
Application
File System
Win NT - NTFS -
CHS Translation
4.2GB Operating System
528 MB
Dos - Fat16 - 2GB
8.4 / 7.9 GB
BIOS
ATA Standard
Task File Registers -
Hardware
137GB
Peripheral
There have been a few BIOS issues with regards to the largest hard disk drive capacities that
they can access. SCSI is not susceptible to these as the SCSI card will have its own BIOS.
Basically there are limits to the size of disk that can be addressed from the BIOS. There are
however utilities to get around this issue such as Ontrack Disk manager, available for free
download from the web for use with IBM Hard Disk Drives.
ANSI ATA Standard states:
If the max # LBA's is greater than 16,515,072 then:
Max cylinders is 16,383
Max Heads is 16
Max Sectors is 63
This assumes that the BIOS considers 256 as 255 in binary 1-256 is 0-255
Not all BIOS manufacturers agreed. Phoenix insisted on the drive declaring 15 heads
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To get the full capacity
Upgrade the system BIOS
Must be obtained from the system / motherboard manufacturer
Install IBM Disk Manager
Can be downloaded free from:http://www.ibm.com/harddrive
'Support' section
'Utilities' sub-section
Operating System limits cannot be overcome
Change operating system / version is only solution
As an example Windows 95 basic version and version a only support partitions up to
2GB. Windows 95b will however support partitions up to 8GB.
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SCSI Overview
SCSI is the more usual interface used widely in large systems, workstations and servers. It
come in a variety of configurations including: 50 Pin, 68 Pin and 80 Pin connectors. Various
versions of SCSI can host up to 16 devices on a bus (including the controller). In addition
there are two main types of SCSI: single ended and differential.
SE (single ended) is cheaper but limited to short distances
LVD (low voltage differential) can be as long as 12 metres
SCSI is well suited to demanding applications as the processing for the bus is performed by
the controller card and not the CPU of the host system. In addition, by using features such as
termination it can reach very high data transfer speeds of up to 160 MB/s.
There are basically two types of SCSI bus: Narrow (8 bit) and Wide (16 bit), SCSI variants
are generally backwardly compatible though and wide buses can also transfer data in narrow
mode.
Narrow bus connectors
50-Pin
Wide bus connectors
68-Pin
80 Pin
(SCA - Single Connector Attach) This is a wider version of the 68 pin connector it
also includes power and jumpering pins
removing the need for other connectors
SCSI (Small Computer System Interface) Data Bus Speeds
STA Term SCSI Bus Width SCSI Bus Speed
(bits) (MByte/s)
SCSI 1 8 5
Fast SCSI 8 10
Fast Wide SCSI 16 20
Ultra SCSI 8 20
Wide Ultra SCSI 16 40
Ultra2 SCSI 8 40
Wide Ultra 2 SCSI 16 80
Wide Ultra 3 SCSI (U160) 16 160
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SCSI also allows provision for other features designed to keep data as well protected as
possible. These include:
Data Redundancy
RAID
Dual Controllers
Redundant power and cooling
Battery Backup
Redundant Array of Independent disks (RAID), minimises the impact of a single drive failure
by allowing data to be automatically copied over a bank of drives so that if a drive fails the
data will still be intact.
RAID Definitions:
There are several types of RAID systems described below
RAID Level Name Description
0 Striping Data is written across a bank of disks with no fault
tolerance. This is a dangerous choice as if 1 disk
fails you could lose the information on the whole
bank.
1 Mirroring Data is written simultaneously across two disks. If
one fails then the data is simply read from the other
to reconstruct. Good but wasteful.
5 Striping, Independent Data is written in stripes across a bank of disks,
Access with with checksum information on an additional disk. If
Distributed Parity any of the disks fail then the information can be
reconstructed from the checksum information.
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Serial Storage Architecture
Serial Storage Architecture is an interface produced and promoted by IBM. It has been
available industry wide in 1991. It is an interface with a low cost serial implementation capable
of supporting speeds up to 20MB/s on each link. Essentially it is simple, and reliable reliable,
even supporting long distance operation.
Fibre Channel
IBM began in 1988 to develop an interface with capability beyond anything available at the
time. Fibre refers in general to all the physical media types supported optical fibre, twisted pair
and coaxial cable
Fibre channel is a very high speed interface that can transfer large amounts and varying types
of information. It is a bi-directional, point-to-point, serial data channel that can send and
receive data at the same time Capable of speeds up to 100MB/sec (fibre). It has the following
key advantages:
Transfer speeds are higher than parallel
Reduced cabling/connector costs
Initial implementations are more expensive than parallel
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Configuration of a Serial Drive System
There are three ways that you can configure a system of serial drives.
Control
The String topology above allows the controller to talk to each of the drives along a cable.
This is the simplest method but it will fall over if the cable is broken as the drive units beyond
the break will be isolated from the controller.
Control
The Loop topology is a very good way to connect drives as it provides a layer of redundancy.
If the cable is broken at any point then the controller can still access drives beyond the break
by simply going the other way around the loop.
Switch
Control
Switched topology is more complex as it requires hardware to perform the switching. It can
however be more redundant as each of the drives is electrically separate.
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Cabling & Connectors
As a summary:
ATA drives are connected with a 40 way cable. This has a coloured line along conductor/pin 1
which should always be mounted in the drive connector nearest the power connector. The
connector itself is a square connector with 2 rows of 20 pins.
UDMA 66 ATA cables are similar but with an additional ground line in between each of the
usual conductors. The connector on an ATA UDMA 66 cable for connection to the
motherboard will be coloured blue.
SCSI comes in a variety of flavours but will usually be found as
A 50 way square connector, or 68, or 80 way high density D plug. Converters should be
available for all combinations.
Remember LVD drives do not have termination so it will have to be provided.
For more information on ATA and SCSI devices view their separate checklist documents.
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