History of Computer 计算机发展史 8 Peripheral

Chapter 7
Peripheral

503
Peripheral
• A computer peripheral is any external device
that provides input and output for the
computer. For example, a keyboard and mouse
are input peripherals, while a monitor and
printer are output peripherals. Computer
peripherals, or peripheral devices, are
sometimes called “I/O devices” because they
provide input and output for the computer.
Some peripherals, such as external hard drives,
provide both input and output for the
computer.

504
7.1 Paper Tape
• In 1837, the American inventor Samuel Finley
Breese Morse developed the first American
electric telegraph, which was based on simple
patterns of “dots” and “dashes” called Morse
Code being transmitted over a single wire.
• The telegraph quickly proliferated thanks to
the relative simplicity of Morse’s system.
However, a problem soon arose in that
operators could only transmit around ten
words a minute, which meant that they
couldn’t keep up with the public’s seemingly
insatiable desire to send messages to each other.
This was a classic example of a communications
bottleneck.

505
Uses Paper Tape to Store Data
In 1857, only twenty years after the
invention of the telegraph, Sir Charles
Wheatstone (the inventor of the
accordion) introduced the first
application of paper tapes as a medium
for the preparation, storage, and
transmission of data.

506
Uses Paper Tape to Store Data
• Sir Charles’ paper tape used two rows of holes to
represent Morse’s dots and dashes. Outgoing messages
could be prepared off-line on paper tape and
transmitted later.
• By 1858, a Morse paper tape transmitter could operate
at 100 words a minute.

507
Uses Paper Tapes as Data Presentation
In a similar manner to Sir Charles’ telegraph
tape, the designers of the early computers
realized that they could record their data on a
paper tape by punching rows of holes across
the width of the tape. The pattern of the holes
in each data row represented a single data
value or character. The individual hole
positions forming the data rows were referred
to as “channels” or “tracks,” and the number
of different characters that could be
represented by each row depended on the
number of channels forming the rows.

508
The Original Computer Tapes
The original computer tapes had five
channels, so each data row could
represent one of thirty-two different
characters. However, as users began to
demand more complex character sets,
including the ability to use both
uppercase characters (‘A’, ‘B’, ‘C’, …)
and their lowercase equivalents (‘a’, ‘b’,
‘c’, …), the number of channels rapidly
increased, first to six and later to eight.

509
IBM’s Tape
• This illustration
represents one of the
more popular IBM
standards — a oneinch
wide tape
supporting eight
channels (numbered
from 0 to 7) with 0.1
inches between the
punched holes.

510
Paper Tape Readers
• The first paper tape readers accessed the data
by means of springy wires (one per channel),
which could make electrical connections to
conducting plates under the tape wherever a
hole was present. These readers were relatively
slow and could only operate at around fifty
characters per second.
• Later models used opto-electronic techniques,
in which a light source was placed on one side
of the tape and optical cells located on the other
side were used to detect the light and thereby
recognize the presence or absence of any holes.

511
7.2 Disks
• There are two basic types of disks: magnetic
disks and optical disks.
• On magnetic disks, data is encoded as
microscopic magnetized needles on the disk’s
surface. You can record and erase data on a
magnetic disk any number of times, just as you
can with a cassette tape.
• Optical disks record data by burning
microscopic holes in the surface of the disk
with a laser. To read the disk, another laser
beam shines on the disk and detects the holes
by changes in the reflection pattern.

512
Magnetic Disks
• floppy disk : A typical 5¼-inch floppy disk can
hold 360K or 1.2MB (megabytes). 3½-inch
floppies normally store 720K, 1.2MB or
1.44MB of data.
• hard disk : Hard disks can store anywhere
from 20MB to more than 200GB. Hard disks
are also from 10 to 100 times faster than floppy
disks.
• removable cartridge : Removable cartridges
are hard disks encased in a metal or plastic
cartridge, so you can remove them just like a
floppy disk. Removable cartridges are very fast,
though usually not as fast as fixed hard disks.

513
7.2.1 Floppy Disk
• Floppy disk is called floppy because it flops if
you wave it. Unlike most hard disks, floppy
disks (often called floppies or diskettes) are
portable, because you can remove them from a
disk drive. Disk drives for floppy disks are
called floppy drives. Floppy disks are slower to
access than hard disks and have less storage
capacity, but they are much less expensive. And
most importantly, they are portable.

514
Three basic types
• 8-inch: The first floppy disk design, invented
by IBM in the late 1960s and used in the early
1970s as first a read-only format and then as a
read-write format. The typical desktop/laptop
computer does not use the 8-inch floppy disk.
• 5-inch: The common size for PCs made before
1987 and the predecessor to the 8-inch floppy
disk. This type of floppy is generally capable of
storing between 100K and 1.2MB (megabytes)
of data. The most common sizes are 360K and
1.2MB.

515
Three basic types
• 3-inch: Floppy is something of a
misnomer for these disks, as they are
encased in a rigid envelope. Despite their
small size, microfloppies have a larger
storage capacity than their cousins —
from 400K to 1.4MB of data. The most
common sizes for PCs are 720K (doubledensity)
and 1.44MB (high-density).
Macintoshes support disks of 400K, 800K,
and 1.2MB.

516
Comparisons
• Following are the three types developed, from newest to
oldest, and their raw, uncompressed storage capacity.
Final
Storage
• Housing Capacity Capacity Range Creator
• 3.5″ rigid 1.44MB 400KB – 1.44MB Sony
• 3.5″ rigid 2.88MB (See ED.) IBM
• 5.25″ flexible 1.2MB 100KB – 1.2MB Shugart
• 8″ flexible 500KB 100 – 500KB IBM

517
Images

518
Appraisements
• The floppy drive is a good example of how the
standard itself is more important than the
technology.
• The capacity and data transfer rates of floppy
drives saturated at extremely low levels (1.44
MB and about 0.06 MB/sec). These parameters
could be improved dramatically with today’s
technology but do not expect changes. There
are millions and millions of floppy drives out
there and it is too late to change standards

519
Appraisements
• Today, the only two functions left for the
floppy disks are serving as boot disks in
the case of system conflicts on your PC
and serving as movable storage for those
who still live in the sub-two-megabytesof-
storage world. The main advantage of
floppy drives is still low cost and
universal compatibility.

520
7.2.2 Hard Disk’ Introduction
• The primary computer storage device.
Like tape, it is magnetically recorded and
can be re-recorded over and over. Disks
are rotating platters with a mechanical
arm that moves a read/write head
between the outer and inner edges of the
platter’s surface. It can take as long as
one second to find a location on a floppy
disk to as little as a couple of milliseconds
on a fast hard disk.

521
Hard Disk’ Introduction
• The primary computer storage medium,
which is made of one or more aluminum
or glass platters, coated with a
ferromagnetic material. Most hard disks
are “fixed disks,” which have platters
that reside permanently in the drive.
Removable disks are encased in plug-in
cartridges, allowing data to be taken out
of the drive for storage or for transfer to
another party.

522
Hard Disk’ Introduction
• The disk surface is divided into concentric
tracks (circles within circles). The thinner
the tracks, the more storage. The data bits
are recorded as tiny magnetic spots on the
tracks. The smaller the spot, the more bits
per inch and the greater the storage.
• Modern disks have more sectors in the outer
tracks than the inner ones because the outer
radius of the platter is greater than the inner
radius .

523
Hard Disk’ Introduction
• Tracks are further divided into sectors,
which hold a block of data that is read or
written at one time; for example, READ
SECTOR 782, WRITE SECTOR 5448.
In order to update the disk, one or more
sectors are read into the computer,
changed and written back to disk. The
operating system figures out how to fit
data into these fixed spaces.

524
The First Hard Disk
• In 1956, IBM introduced the RAMAC hard
disk with platters two feet in diameter that held
the equivalent of 100,000 bytes.
• In the 1980s, desktop computer hard disks
were introduced with 5MB using 5.25″ platters
(see ST506).
• Today’s entry level drives have at least 8,000
times more capacity. Platter size was reduced
to 3.5″ for desktops, 2.5″ for laptops and 1″ for
handhelds. In 2004, Toshiba introduced the
0.85″ drive.

525
The First Hard Disk
• In 1956, IBM’s RAMAC
was the first machine with
a hard disk, which was
extraordinary technology
of the times. Each of its
24″ diameter platters held
a whopping 100,000
characters (they were not
bytes then) for a total of
five million characters.

526
The First Desktop Hard Disk
• The first hard disk drive for
personal computers,
introduced in 1979 by
Seagate.
• It was used in drives of 40MB
and less and transferred data
at 625 KBytes/sec, using the
MFM encoding method.
• This 5.25″ full-height drive
held 5MB and became an
industry standard used in
CP/M machines and, later,
the IBM PC and its
successors.

527
Tracks and Sectors
• Tracks are
concentric circles on
the disk, broken up
into storage units
called “sectors.”
The sector, which is
typically 512 bytes,
is the smallest unit
that can be read or
written.

528
What’s Inside a Hard Drive ?
• The platters are the actual disks inside the
drive that store the magnetized data. Most
drives have at least two platters, and the larger
the storage capacity of the drive, the more
platters there are. Each platter is magnetized
on each side, so a drive with 2 platters has 4
sides to store data.
• The platters in a drive are separated by disk
spacers and are clamped to a rotating spindle
that turns all the platters in unison. The spindle
motor is built right into the spindle or mounted
directly below it and spins the platters at a
constant set rate ranging from 3,600 to 7,200
RPM.

529
What’s Inside a Hard Drive ?
• The read/write heads read and write data to
the platters. There is typically one head per
platter side, and each head is attached to a
single actuator shaft so that all the heads move
in unison.
• All the heads are attached to a single head
actuator, or actuator arm, that moves the heads
around the platters. Modern hard drives use a
voice coil actuator, which controls the
movement of a coil toward or away from a
permanent magnet based on the amount of
current flowing through it.

530
What’s Inside a Hard Drive ?
• The platters, spindle, spindle motor, head
actuator and the read/write heads are all
contained in a chamber called the head
disk assembly (HDA). Outside of the
HDA is the logic board that controls the
movements of the internal parts and
controls the movement of data into and
out of the drive.

531
Two Types of Hard Disks
• Most hard disks are ATA (originally and still
widely known as “IDE”). Today, ATA drives
are widely used for RAID arrays
• SCSI drives have traditionally been found on
servers and high-performance workstations.
The SCSI advantage is that up to 15 devices
can be attached to the same controller board,
which uses only one slot in the PC. SCSI was
the first drive technology to employ faulttolerant
RAID systems.
• Both ATA/IDE and SCSI hard disks are lowlevel
formatted at the factory, which records
the original sector identification on them.

532
Integrated Drive Electronics
• A type of hardware interface widely used to connect
hard disks, optical disks and tape drives to a PC. IDE
was always the more economical interface, compared
to SCSI. Introduced in the mid-1980s with 20MB of
storage, capacities increased a thousandfold in less
than two decades.
• With IDE, the controller electronics are built into the
drive itself, requiring a simple circuit in the PC for
connection. IDE drives were attached to earlier PCs
using an IDE host adapter card. Subsequently, two
Enhanced IDE (EIDE) sockets were built onto the
motherboard, with each socket connecting two drives
via a 40-pin ribbon cable for CD-ROMs and similar
devices and an 80-wire cable for fast hard disks.

533
Integrated Drive Electronics
• As improvements were made to the IDE/ATA
interface, a new version number was added.
ATA-2 (Fast ATA) defined the faster transfer
rates used in Enhanced IDE (EIDE).
• ATA-3 added interface improvements,
including the ability to report potential
problems.
• Starting with ATA-4, either the word “Ultra”
or the transfer rate was added to the name in
various combinations. For example, at 33
MBytes/sec, terms such as Ultra ATA and
ATA-33 have been used.

534
SCSI
• (Small Computer System Interface) Pronounced
“scuzzy.” SCSI is a hardware interface that allows for
the connection of up to 15 peripheral devices to a
single PCI board called a “SCSI host adapter” that
plugs into the motherboard. SCSI uses a bus structure
and functions like a mini-LAN connecting 16 devices,
but the host adapter counts as one device. SCSI allows
any two devices to communicate at one time (host to
peripheral, peripheral to peripheral).
• Host adapters are also available with two controllers
that support up to 30 peripherals. Introduced in 1986
and originally developed by Shugart Associates ,SCSI
is widely used in servers, mainframes and storage
area networks (SANs).

535
RAID Was Originally SCSI
• Until the late 1990s, SCSI hard
disks were the only ones used
in RAID configurations for
improved performance or fault
tolerance. Since the advent of
IDE RAID controllers, SCSI
and IDE have become more
equalized, although SCSI
continues to be the drive
interface of choice in the
server market.
• The advantage of SCSI is that
several peripherals can be
daisy chained to one host
adapter, using only one slot in
the bus.

536
7.2.3 Removable cartridge

537
PocketZip

538
Jaz

539
LS-120

540
HiFD

541
SyJet

542
7.3 The Sholes (QWERTY) Keyboard
• It is commonly believed that the original layout
of keys on a typewriter was intended to slow
the typist down, but this isn’t strictly true. The
main inventor of the first commercial
typewriter, Christopher Latham Sholes,
obviously wished to make their typewriters as
fast as possible in order to convince people to
use them.
• However, one problem with the first machines
was that the keys jammed when the operator
typed at any real speed, so Sholes invented
what was to become known as the Sholes
keyboard.

543
The Sholes (QWERTY) Keyboard

544
The Sholes (QWERTY) Keyboard
• The term digraph refers to combinations of two letters
that represent a single sound, such as “sh” in “ship,”
where these letters are frequently written or typed one
after the other.
• What Sholes attempted to do was to separate the letters
of as many common digraphs as possible. But in
addition to being a pain to use, the resulting layout also
left something to be desired on the digraph front; for
example, “ed”, “er”, “th”, and “tr” all use keys that
are close to each other. Unfortunately, even after the
jamming problem was overcome by the use of springs,
the monster was loose amongst us — existing users
didn’t want to change and there was no turning back.

545
The Sholes (QWERTY) Keyboard
• The original Sholes keyboard (which is known to us as
the QWERTY keyboard, because of the ordering of the
first six keys in the third row) is interesting for at least
two other reasons: first, there was no key for the
number ‘1’, because the inventors decided that the
users could get by with the letter ‘I’; and second, there
was no shift key, because the first typewriters could
only type upper case letters. (Sholes also craftily
ensured that the word “Typewriter” could be
constructed using only the top row of letters. This was
intended to aid salesmen when they were giving
demonstrations.) For example, instead of the top row
of characters saying QWERTY, keyboards in France
and Germany spell out AZERTY and QWERTZU,
respectively.)

546
The First Shift-Key Typewriter
• The first shift-key typewriter
(in which uppercase and lowercase letters are
made available on the same key) didn’t appear
on the market until 1878, and it was quickly
challenged by another flavor which contained
twice the number of keys, one for every
uppercase and lowercase character.
• For quite some time these two alternatives vied
for the hearts and minds of the typing
fraternity, but the advent of a technique known
as touch-typing favored the shift-key solution,
which thereafter reigned supreme.

547
The Sholes (QWERTY) Keyboard
• the figure above shows the ‘A’, ‘S’, ‘D’, and ‘F’
keys in white to indicate that these are the
home keys for the left hand. Similarly, the
other four keys shown in white are the home
keys for the right hand. The terms home keys
and home row refer to the base position for
your fingers (excluding thumbs, which are used
to hit the space bar) when you’re practicing
touch typing, which means that you type by
touch without looking at the keyboard.

548
The Sholes (QWERTY) Keyboard
• However, Sholes didn’t invent these terms,
because he actually gave very little thought to
the way in which people would use his
invention. The end result was that everyone
was left to their own devices, effectively
meaning that two-fingered typists using the
“hunt-and-peck” method ruled the world. It
was not until 1888 that a law clerk named
Frank E. McGurrin won a highly publicized
typing contest with his self-taught touch-typing
technique, and a new era was born.

549
The Sholes (QWERTY) Keyboard
• Finally, lest you still feel that the
QWERTY keyboard is an unduly harsh
punishment that’s been sent to try us, it’s
worth remembering that the early users
had a much harder time than we do, not
the least that they couldn’t even see what
they were typing!

550
The Dvorak Keyboard
Almost anyone who spends more than a
few seconds working with a QWERTY
keyboard quickly becomes convinced that
they could do a better job of laying out
the keys. Many brave souls have
attempted the task, but few came closer
than efficiency expert August Dvorak in
the 1930s.

551
Analyzing the Model of the QWERTY
• When he turned his attention to the typewriter,
Dvorak spent many tortuous months analyzing the
usage model of the QWERTY keyboard (now there’s
a man who knew how to have a good time). The
results of his investigation were that, although the
majority of users were right-handed, the existing
layout forced the weaker left hand (and the weaker
fingers on both hands) to perform most of the work.
Also, thanks to Sholes’ main goal of physically
separating letters that are commonly typed together,
the typist’s fingers were obliged to move in awkward
patterns and only ended up spending 32% of their
time on the home row.

552
Finding the Keys’s Optimal Placement
• Dvorak took the opposite tack to Sholes, and
attempted to find the optimal placement for the
keys based on letter frequency and human
anatomy. That is, he tried to ensure that letters
which are commonly typed together would be
physically close to each other, and also that the
(usually) stronger right hand would perform
the bulk of the work, while the left hand would
have control of the vowels and the lesser-used
characters. The result of these labors was the
Dvorak Keyboard, which he patented in 1936.

553
The Dvorak Keyboard

554
Improvements
• Note that Dvorak’s keyboard had shift keys, but they are
omitted from the above figure for reasons of clarity. The
results of Dvorak’s innovations were tremendously effective.
Using his layout, the typist’s fingers spend 70% of their time
on the home row and 80% of this time on their home keys.
Thus, as compared to the approximately 120 words that can
be constructed from the home row keys of the QWERTY
keyboard, it is possible to construct more than 3,000 words
on Dvorak’s home row (or 10,000 words if you’re talking to
someone who’s trying to sell you one). Also, Dvorak’s scheme
reduces the motion of the hands by a factor of three, and
improves typing accuracy and speed by approximately 50%,
and 20%, respectively.

555
The Dvorak Keyboard
• Unfortunately, Dvorak didn’t really stand a
chance trying to sell typewriters based on his
new keyboard layout in the 1930s. Apart from
the fact that existing typists didn’t wish to relearn
their trade, America was in the heart of
the depression years, which meant that the last
thing anyone wanted to do was to spend money
on a new typewriter.
• In fact, the Dvorak keyboard might have faded
away forever, except that enthusiasts in Oregon,
USA, formed a club in 1978, and they’ve been
actively promoting Dvorak’s technique ever
since.

556
7.4 Two Types Of Monitors
• The common way of classifying monitors
is in terms of the type of signal they
accept: analog or digital. Nearly all
modern monitors accept analog signals,
which is required by the VGA, SVGA,
8514/A, and other high-resolution color
standards.

557
Analog
• Analog Also spelled analogue, describes a device or
system that represents changing values as
continuously variable physical quantities. A typical
analog device is a clock in which the hands move
continuously around the face. Such a clock is capable
of indicating every possible time of day. In contrast, a
digital clock is capable of representing only a finite
number of times (every tenth of a second, for
example). In general, humans experience the world
analogically. Vision, for example, is an analog
experience because we perceive infinitely smooth
gradations of shapes and colors.

558
Analog
• When used in reference to data storage and
transmission, analog format is that in which
information is transmitted by modulating a
continuous transmission signal, such as amplifying a
signal’s strength or varying its frequency to add or
take away data. Computers, which handle data in
digital form, require modems to turn signals from
digital to analog before transmitting those signals
over communication lines such as telephone lines that
carry only analog signals. The signals are turned back
into digital form (demodulated) at the receiving end
so that the computer can process the data in its digital
format.

559
Digital • Digital describes any system based on discontinuous
data or events. Computers are digital machines
because at their most basic level they can distinguish
between just two values, 0 and 1, or off and on. There
is no simple way to represent all the values in between,
such as 0.25. All data that a computer processes must
be encoded digitally, as a series of zeroes and ones.
• In general, humans experience the world analogically.
Vision, for example, is an analog experience because
we perceive infinitely smooth gradations of shapes and
colors. Although digital representations are
approximations of analog events, they are useful
because they are relatively easy to store and
manipulate electronically.

560
Screen size
• The most important aspect of a monitor is its
screen size. Like televisions, screen sizes are
measured in diagonal inches, the distance from
one corner to the opposite corner diagonally.
• A typical size for small VGA monitors is 14
inches. Monitors that are 16 or more inches
diagonally are often called full-page monitors.
• VGA is abbreviation of video graphics array, a
graphics display system for PCs developed by
IBM and has become one of the de facto
standards for PCs.

561
Resolution
• The resolution of a monitor indicates
how densely packed the pixels are. In
general, the more pixels (often expressed
in dots per inch), the sharper the image.
Most modern monitors can display 1024
by 768 pixels, the SVGA standard. Some
high-end models can display 1280 by
1024, or even 1600 by 1200.

562
Bandwidth
• The amount of data that can be
transmitted in a fixed amount of time.
For digital devices, the bandwidth is
usually expressed in bits per second(bps)
or bytes per second. For analog devices,
the bandwidth is expressed in cycles per
second, or Hertz (Hz).

563
Refresh rate
• The refresh rate for a monitor is measured in
hertz (Hz) and is also called the vertical
frequency, vertical scan rate, frame rate or
vertical refresh rate. The old standard for
monitor refresh rates was 60Hz, but a new
standard developed by VESA sets the refresh
rate at 75Hz for monitors displaying
resolutions of 640×480 or greater. This means
that the monitor redraws the display 75 times
per second. The faster the refresh rate, the less
the monitor flickers.

564
LCD History
• Today, LCDs are everywhere we look, but they
didn’t sprout up overnight. It took a long time
to get from the discovery of liquid crystals to
the multitude of LCD applications we now
enjoy.
• Liquid crystals were first discovered in 1888,
by Austrian botanist Friedrich Reinitzer.
Reinitzer observed that when he melted a
curious cholesterol-like substance, it first
became a cloudy liquid and then cleared up as
its temperature rose. Upon cooling, the liquid
turned blue before finally crystallizing.

565
LCD History
• Eighty years passed before RCA made
the first experimental LCD in 1968. Since
then, LCD manufacturers have steadily
developed ingenious variations and
improvements on the technology, taking
the LCD to amazing levels of technical
complexity. And there is every indication
that we will continue to enjoy new LCD
developments in the future!

566
Passive-matrix
• There are two main types of LCDs used in computers,
passive matrix and active matrix.
• LCDs use a simple grid to supply the charge to a
particular pixel on the display. Creating the grid is
quite a process! It starts with two glass layers called
substrates. One substrate is given columns and the
other is given rows made from a transparent
conductive material. This is usually indium-tin oxide.
The rows or columns are connected to integrated
circuits that control when a charge is sent down a
particular column or row. The liquid crystal material
is sandwiched between the two glass substrates, and a
polarizing film is added to the outer side of each
substrate.

567
Passive-matrix
• To turn on a pixel, the integrated circuit sends a
charge down the correct column of one substrate and
a ground activated on the correct row of the other.
The row and column intersect at the designated pixel,
and that delivers the voltage to untwist the liquid
crystals at that pixel.
• The simplicity of the passive-matrix system is
beautiful, but it has significant drawbacks, notably
slow response time and imprecise voltage control.
• LCDs depend on thin film transistors (TFT). Basically,
TFTs are tiny switching transistors and capacitors.
They are arranged in a matrix on a glass substrate.

568
Active Matrix
• To address a particular pixel, the proper row is
switched on, and then a charge is sent down the
correct column. Since all of the other rows that the
column intersects are turned off, only the capacitor at
the designated pixel receives a charge. The capacitor
is able to hold the charge until the next refresh cycle.
And if we carefully control the amount of voltage
supplied to a crystal, we can make it untwist only
enough to allow some light through.
• By doing this in very exact, very small increments,
LCDs can create a gray scale. Most displays today
offer 256 levels of brightness per pixel. 

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