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2-7 Hardware

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					2.7 Configurability of Hardware and Software
A computer system is made up from various components, some of which are inside the “system unit”, some of
which are outside, and some of which are used to communicate with the user (see also the section on User
Interfaces – 2.10).

Peripherals
Peripherals are hardware devices attached to the computer system that expand the capabilities of the system.
They are attached via an electrical connection (e.g. they are plugged into one of the ports at the back of the
computer), and generally require some software running on the computer to control or “drive” them (see
Drivers).

Examples of peripherals include such things as:

   Printers
   Plotters
   Scanners
   Barcode readers

Storage Devices
Computers need to store information, in order that it can be retrieved, searched, sorted, etc. There are two
categories of storage used by computers:

Primary Storage:

This is the computer’s “main memory”, or RAM (Random Access Memory). RAM is so-called because the
information can be accessed by the processor in any order, unlike the information on, say, a tape, which
always comes in sequence.

When we say that a computer school has 512Mb (mega-bytes) of memory, for example, it is the primary
storage we are talking about. The programs you are currently running, and in most cases the data they are
using (e.g. your current document in Word) are stored in the primary store.

Primary storage is usually very fast – information can be retrieved in nanoseconds – but is normally the most
expensive of type of storage per byte.

Secondary Storage:

The problem with RAM is that it is volatile. This means that is the computer is switched off or the power fails,
then what is currently in the memory is lost (the word volatile means “evaporates quickly”). For this reason we
need another form of storage; secondary storage.

Secondary storage is a permanent, non-volatile form of storage that usually involves a storage device
recording the information onto some sort of physical medium. In the early days of computing, things such as
punched cards, paper tape and magnetic drums were used, but modern examples are things like:

   Hard discs
   Floppy discs
   ZIP Drives
   CD-ROMs
   DAT or other data cartridges

Secondary storage is usually much cheaper per byte than primary storage, which is convenient, because
users often have a much greater need for it. My computer at home, for example, has 1Gb of RAM, and 250Gb
(1Gb is a “giga-byte”, or 1,000Mb) of secondary storage, and these two cost very roughly the same.

Virtual Memory:

As primary storage is usually more expensive than secondary storage, sometimes users don’t have as much
as they require. A professional quality A4-size image, for example, would take up more than 64Mb on its own,
without the operating system and the program used to process it.

What some operating systems can do is operate a system called virtual memory. The memory manager can
use secondary storage to save the information that won’t fit into the primary storage. Because it’s unlikely that
all of the data is used all of the time, the bits being used can be swapped between primary and secondary
storage in a way that is transparent to the program.


Input and Output Devices
These are devices for communicating with the computer system.

Input devices are used to receive input from the user, and include the following:

   Keyboard
   Mouse
   Graphics tablet
   Joystick

Output devices are used to communicate information to the user in a form that he or she can understand, and
include the following:

   Printers
   Plotters
   VDUs (Visual Display Units)

You will almost certainly be familiar with printers. Plotters produce also produce paper output ("hard copy"),
but do so by "drawing" on the paper using pens; they are mainly used to produce technical drawings and plans
(they are quite rare these days, but I’m sure you can find examples on the internet).


Parallel and Serial Transmission

Peripherals are usually connected to a PC via either a serial or parallel connector on the back. The data is
sent by a different method through each. USB stands for Universal Serial Bus, and so they are serial cables
too.

Through a parallel port, data is sent a byte at a time. The parallel connector has eight wires, each of which will
carry one bit of information, plus some other connections for control purposes (e.g. to indicate that the device
is ready to receive, etc.) Printers and some scanners are usually connected through the parallel port.

In a serial connection, there is only one wire for the data (plus others for control), and the information is sent a
bit at a time. This is obviously slower than a parallel connection, but more reliable over longer distances.

Serial communications can be either synchronous or asynchronous (see the 2.9 – Networking Environments).
The serial ports on PCs use the asynchronous method in which there is no clock pulse.


Buffering and Spooling

Peripherals often work at a much slower rate than the computer itself. To avoid everything grinding to a halt
when you print, for example, the operating system or drivers can employ certain techniques.

Buffering and spooling are both methods of using temporary storage for data, between the processor and the
input or output device. A buffer is usually an area of RAM, either in the computer itself, or in the peripheral
device, where data can be stored before being processed, leaving the computer's main processor and
memory to get on with other things.

Spooling works in a similar way except that the data is usually written to secondary storage, e.g. the disc. This
is a common practice when printers are used on a network. When users print out their work, it is written to a
file on the network server if the printer is busy, and printed out when the printer becomes available.
Printers
There are various different types of printer, and which one you would select would depend on your
requirements for:

   Volume of output
   Print quality
   Location
   Colour
   Different sizes of paper, envelopes and transparencies
   Cost – both the initial cost and the cost of consumables
   Noise (i.e. whether you need it to be quiet)

Printers fall into two main categories; impact printers and non-impact printers. As the names suggest, the
former produce characters or graphics by striking the page, and the latter type use alternative methods.

Impact Printers
Dot matrix and daisy-wheel printers and both impact printers. Dot matrix printers have a column of pins in the
print head, which can move to strike the paper individually or in various combinations. They strike the paper
through an inked ribbon and leave a dot. As the head moves across the paper, the trail of dots makes up the
characters or an image. The quality of the image will depend on the number and size of the pins, and
therefore the dots. Early printers had 8 or 9 pins, then came 24 pins and possibly more in modern printers,
although it’s quite an old-fashioned technology.

Daisy wheel printers work like a type-writer; there is a plastic wheel with a character at the end of each "petal".
The wheel rotates to the correct position, and the letter, number or symbol is hammered onto a ribbon over the
paper, thereby making an impression. Daisy wheel printers are limited to only one font (unless you change
the wheel) and are not capable of producing text in different orientations (e.g. landscape), or graphics.

Non-Impact Printers
The two most common types of printer are ink jet (or "Bubble Jet") and laser printers. These are both non-
impact methods. Ink jet printers effectively spray the ink onto the page; this can make the output quite wet if
large areas are covered (e.g. a picture is printed). They can also be quite fussy about the sort of paper used;
if it is too absorbent, then the ink will spread like in blotting paper, and if the paper isn't absorbent enough,
then the ink will set on the surface and take longer to dry, therefore being prone to smudging.

Laser printers don't use ink at all, but a black powder called toner, which is melted onto the paper by a hot
roller. This means that you need to be careful when using transparencies and labels as they can melt (in the
case of labels, the adhesive backing can melt and the labels can come off and stick on the roller). Laser
printers are generally faster, but more expensive (especially if you want colour) than ink jet printers.

Display Devices
The computer's output is normally displayed on a monitor, or Visual Display Unit (VDU). For a PC to do this, it
needs to have a video card installed inside it (although some motherboards have support for video built in).
Other types of computer, such as the Apple Mac or Acorn RISC machine have support built in.

The size of the monitor is usually given in inches, measured diagonally from corner to corner (just as with a
television). Common sizes are 14", 15" and 17", although larger sizes are becoming more popular.

When talking about the quality of the output, there are two attributes we need to consider:

Resolution: the image on the screen is made up of a series of dots called pixels (picture elements); the higher
the number of dots, the better the quality of the image. The resolution describes the number of pixels that can
be displayed on the screen in each direction (horizontal x vertical). The most common resolutions on PCs are:

   640 x 480
   800 x 600
   1024 x 768
   1152 x 864
   1280 x 1024

These can be set in the Display section of Control Panel. To give you a comparison, the resolution of
broadcast television is 625 lines.

Colour Depth: the other thing to consider is how many colours can be displayed. The more colours, the more
realistic photographic images will be. The most common colour depths are:

   16 colours
   256 colours
   "High Colour" (16 bit, or 65,536 colours)
   "True Colour" (24 bit, or over 16 million colours)

Higher resolutions are greater number of colours require more memory. At 256 colours, each pixel requires
one byte of memory. There are 8 bits in a byte, so each pixel in High Colour requires two bytes, and each
pixel in True Colour requires three. Because of this, some video cards may restrict you to fewer colours in
higher resolutions, due to the amount of memory required.

For example, a True Colour display with a resolution of 1280 x 1024 pixels would require memory of 1280 x
1024 x 3 = 3,932,160 bytes, or nearly 4Mb!


Drivers
Different printers have different features, such as the ability to print in different fonts, and different resolutions
(i.e. numbers of dots per inch) when printing graphics. Even where there are common features, such as the
ability to print in bold, the codes that the computer needs to send to switch that feature on and off can vary
from model to model.

What the computer requires is some software called a driver, which can translate the codes generated by the
program into codes suitable for the printer. In Windows, the drivers are installed centrally in the Control Panel
so that each application, e.g. Excel or Word, can use the same drivers. In the days of DOS, each program
often had its own printer driver.

Other hardware devices, such as video cards, modems and scanners also require drivers. These perform the
same function, allowing the computer to control the device. Sometimes the same driver can be used for
similar devices (e.g. you may hear about “Hayes compatible modems”, or “HP LaserJet compatible printers”),
but even in these cases it is often only by using the specific driver for that device that the most advanced
features can be utilised.


WYSIWYG and TrueType Fonts
One of the problems with printing from a word processor or other application is that your text may not appear
on paper as it did on the screen. With modern Graphical User Interfaces, this should be less of a problem, but
with older “character-based” systems (such as DOS, or UNIX), fonts on the screen were not “proportionally
spaced”. Characters could only appear at fixed intervals across the screen, and so it wasn’t possible to insert
fractions of spaces to justify the text. This meant that sometimes you could print out justified text, but not see
what it was going to look like on the screen.

A solution to this problem came with the arrival of WYSIWYG user interfaces. WYSIWYG stands for “What
You See Is What You Get”, and means that your print should appear exactly as it does on the screen. Word
(in Page Layout View, at least) is a WYSIWYG word processor; you get exactly what you see as you are
typing. Other word processors, such as WordPerfect 5.1 offer a halfway house solution, with a character-
based input screen, and a graphical print preview.

Even with GUIs and WYSIWYG word processors, there was still the problem of the fonts being used in the
printer not being exactly the same as the ones used for display on the screen. This could lead to things not
quite lining up, or not appearing as expected. A solution for this came with TrueType fonts.

A TrueType font is used by Windows both for display on the screen and for printing. When you select a
typeface in, say, Word, the TT symbol indicates a TrueType font. If you select one of these, you can be sure
that your document will appear in the same way on paper as it did on the screen.


Limitations of Hardware and Software
Sometimes, due to the limitations of the hardware or software being used, a system will not be able to fully
exploit the features of a peripheral. You will not be able to use certain fonts that a printer may support, for
example, if the word processor you are using will not let you select that font, or if you are printing from, say,
Notepad. Also, there is no point in you being able to scan things in 24-bit colour (i.e. 16 million colours) if your
monitor cannot display that many, or you need to save the image as a GIF (GIFs only support 256 colours; if
you want more colours than that, save your picture as a jpeg).

Some modern printer drivers are applications in their own right, and have minimum hardware requirements
specified. The drivers for my printer at home, for example, will not run on a 386 computer.

				
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