3 Hardware

3 HARDWARE

In this chapter, you will learn about

★ primary storage/memory devices
★ secondary storage (including removable devices)
★ the benefits and drawbacks of embedded systems
★ hardware devices used as input, output and storage
★ the differences between RAM, ROM, SRAM, DRAM, PROM and EPROM
★ the use of RAM, ROM, SRAM and DRAM in a range of devices
★ monitoring and control systems
★ the use of logic gates: NOT, AND, OR, NAND, NOR and XOR
★ the construction and use of truth tables
★ the construction of logic circuits, truth tables and logic expressions
from a variety of logic information.

WHAT YOU SHOULD ALREADY KNOW

Try these five questions before you read this
chapter.
1 What is the difference between memory and
storage?
2 Why is it necessary to have both internal and
external memory/storage devices?
3 Can you recognise the memory/storage
devices on the right?
4 What is the difference between online and
offline storage?
5 What is the difference between data access
time and data transfer rate when using
memory and storage devices? ▲ Figure 3.1 Memory/storage devices

3.1 Computers and their components

Key terms
Memory cache – high speed memory external to processor Dynamic RAM (DRAM) – type of RAM chip that needs
which stores data which the processor will need again. to be constantly refreshed.
Random access memory (RAM) – primary memory unit Static RAM (SRAM) – type of RAM chip that uses
that can be written to and read from. flip-flops and does not need refreshing.
Read-only memory (ROM) – primary memory unit that Refreshed – requirement to charge a component to
can only be read from. retain its electronic state.

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Programmable ROM (PROM) – type of ROM chip that Direct 3D printing – 3D printing technique where print
can be programmed once.
Erasable PROM (EPROM) – type of ROM that can be
programmed more than once using ultraviolet (UV) light.
head moves in the x, y and z directions. Layers of melted
material are built up using nozzles like an inkjet printer.
Digital to analogue converter (DAC) – needed to
3
Hard disk drive (HDD) – type of magnetic storage device convert digital data into electric currents that can drive
that uses spinning disks. motors, actuators and relays, for example.
Latency – the lag in a system; for example, the time to Analogue to digital converter (ADC) – needed to
find a track on a hard disk, which depends on the time convert analogue data (read from sensors, for example)
taken for the disk to rotate around to its read-write head. into a form understood by a computer.

3.1
Fragmented – storage of data in non-consecutive sectors; Organic LED (OLED) – uses movement of electrons
for example, due to editing and deletion of old data. between cathode and anode to produce an on-screen

Computers and their components

Removable hard disk drive – portable hard disk drive image. It generates its own light so no back lighting
that is external to the computer; it can be connected required.
via a USB part when required; often used as a device to Screen resolution – number of pixels in the horizontal
back up files and data. and vertical directions on a television/computer screen.
Solid state drive (SSD) – storage media with no moving Touch screen – screen on which the touch of a finger or
parts that relies on movement of electrons. stylus allows selection or manipulation of a screen image;
Electronically erasable programmable read-only they usually use capacitive or resistive technology.
memory (EEPROM) – read-only (ROM) chip that can Capacitive – type of touch screen technology based
be modified by the user, which can then be erased and on glass layers forming a capacitor, where fingers
written to repeatedly using pulsed voltages. touching the screen cause a change in the electric field.
Flash memory – a type of EEPROM, particularly suited Resistive – type of touch screen technology. When
to use in drives such as SSDs, memory cards and a finger touches the screen, the glass layer touches
memory sticks. the plastic layer, completing the circuit and causing a
Optical storage – CDs, DVDs and Blu-rayTM discs that current to flow at that point.
use laser light to read and write data. Virtual reality headset – apparatus worn on the head
Dual layering – used in DVDs; uses two recording layers. that covers the eyes like a pair of goggles. It gives the
Birefringence – a reading problem with DVDs caused user the ‘feeling of being there’ by immersing them
by refraction of laser light into two beams. totally in the virtual reality experience.
Binder 3D printing – 3D printing method that uses a Sensor – input device that reads physical data from its
two-stage pass; the first stage uses dry powder and the surroundings.
second stage uses a binding agent.

3.1.1 Types of memory and storage

Computers require some form of memory and storage.
Memory is usually referred to as the internal devices which the computer can
access directly. This memory can be the user’s workspace, temporary data or
data that is key to running the computer.
Storage devices allow users to store applications, data and files. The user’s data
is stored permanently and they can change it or read it as they wish. Storage
needs to be larger than internal memory since the user may wish to store large
files (such as music files or photographic images).
Storage devices can also be removable to allow data, for example, to be
transferred between computers. Removable devices allow a user to store
important data in a different building in case of data loss.
However, all of this has become a lot less important with the advent of
technology such as ‘data drop’ (which uses Bluetooth) and cloud storage.
Internal memory includes components such as registers (which are part of the
processor). There is also memory cache (which is external to the processor);
this is used to store data which the processor will probably need to use again.
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 Figure 3.2 summarises the types of memory and storage devices covered in this

3
chapter.

secondary storage

hard disk drive (HDD)

primary memory

RAM solid state drive (SSD)

3 HARDWARE

ROM removable devices:

- DVD/CD/Blu-ray
- flash memory stick
- hard disk drive

▲ Figure 3.2 Memory and storage devices

Primary memory
Primary memory is the part of computer memory which can be accessed directly
from the CPU and, as Figure 3.2 shows, contains the random access memory
(RAM) and read-only memory (ROM) memory chips. Primary memory allows
the processor to access applications and services temporarily stored in memory
locations. The structure of primary memory is shown in Figure 3.3.

Primary memory

RAM ROM

SRAM DRAM PROM EPROM EEPROM

▲ Figure 3.3 Structure of primary memory

All computer systems come with some form of RAM. These memory devices
are not really random, it refers to the fact that any memory location can be
accessed independent of which memory location was last used. Access time to
locate data is much faster in RAM than in secondary devices. RAM can also be
» written to or read from, and the data stored can be changed by the user or
by the computer
» used to store data, files, part of an application or part of the operating
system currently in use
» volatile (memory contents are lost on powering off the computer).

In general, the larger the RAM, the faster the computer will operate. In reality,
RAM never runs out of memory, it continues to operate but just becomes slower
and slower as more data is stored. As RAM becomes ‘full’, the processor has to
continually access the secondary data storage devices to overwrite old data on
RAM with new data. By increasing the RAM size, the number of times this has
to be done is considerably reduced, thus making the computer operate more
quickly.
There are currently two types of RAM technology, dynamic RAM (DRAM) and
static RAM (SRAM).

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 Dynamic RAM (DRAM)
Each DRAM chip consists of a number of transistors and capacitors. Each of
these parts is tiny since a single RAM chip will contain millions of capacitors 3
and transistors.
» Capacitors hold the bits of information (0 or 1).
» Transistors act like switches; they allow the chip control circuitry to read the
capacitor or change the capacitor’s value.
This type of RAM needs to be constantly refreshed (that is, the capacitor

3.1
▲ Figure 3.4 Two pieces needs to be re-charged every 15 microseconds otherwise it would lose its
value). If it is not refreshed, the capacitor’s charge will leak away very quickly,

Computers and their components

of dynamic random access
memory (DRAM) leaving every capacitor with the value 0.
DRAMs have a number of advantages over SRAMs. They:
» are much less expensive to manufacture than SRAMs
» consume less power than SRAMs
» have a higher memory capacity than SRAMs.
Static RAM (SRAM)
A major difference between SRAM and DRAM is that SRAM does not need to be
constantly refreshed.
It makes use of flip flops (see Chapter 15) which hold each bit of memory.
SRAM is much faster than DRAM when it comes to data access (typically, access
time for SRAM is 25 nanoseconds and for DRAM is 60 nanoseconds).
DRAM is the most common type of RAM used in computers, but where absolute
speed is essential, for example in the processor’s memory cache, SRAM is the
preferred technology. Memory cache is a high speed portion of the memory.
It is effective because most programs access the same data or instructions
many times. By keeping as much of this information as possible in SRAM, the
▲ Figure 3.5 Static RAM computer avoids having to access the slower DRAM.
Table 3.1 summarises the differences between DRAM and SRAM.

DRAM SRAM
n consists of a number of transistors and n uses flip-flops to hold each bit of
capacitors memory
n needs to be constantly refreshed n does not need to be constantly
n less expensive to manufacture than SRAM refreshed
n has a higher memory capacity than SRAM n has a faster data access time than DRAM
n main memory is constructed from DRAM n processor memory cache makes use
n consumes more power than SRAM under of SRAM
reasonable levels of access, as it needs n if accessed at a high frequency, power
to be constantly refreshed usage can exceed that of DRAM

▲ Table 3.1 Differences between DRAM and SRAM

Another form of primary memory is the read-only memory (ROM). This is similar
to RAM in that it shares the same random access properties, but it cannot
be written to or changed. As the name suggests, ROM is a read-only memory
device.
ROMs are
» non-volatile (the contents are not lost after powering off the computer)
» permanent memory devices (the contents cannot be changed)
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 » often used to store data which the computer needs to access when powering

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up for the first time for example, the basic input/output system (BIOS).
Table 3.2 summarises the main differences between RAM and ROM.
RAM ROM
n temporary memory device n permanent memory device
n volatile memory n non-volatile memory device
n can be written to and read from n data stored cannot be altered
n used to store data, files, programs, part n sometimes used to store BIOS and other
3 HARDWARE

of OS currently in use data needed at start up

n can be increased in size to improve
operational speed of a computer

▲ Table 3.2 Differences between RAM and ROM

PROM and EPROM

A programmable read-only memory (PROM) is a type of ROM chip that
can be altered once. A PROM is made up of a matrix of fuses. Programming
a PROM requires the use of a PROM writer which uses an electric current to
alter specific cells by ‘burning’ fuses in the matrix. Due to the method of
programming (writing), a PROM can only be written to once. They are often
used in mobile phones and in RFID tags.
An erasable programmable read-only memory (EPROM) is different to a PROM
because they use floating gate transistors and capacitors rather than fuses.
Ultra violet (UV) light is used to program an EPROM through a quartz window.
They are used in applications which are under development, such as the
programming of new games consoles.
Embedded systems
Embedded systems involve installing microprocessors into devices to enable
operations to be controlled in a more efficient way. Devices such as cookers,
refrigerators and central heating systems can now all be activated by a
web-enabled device (such as a mobile phone or tablet). The time a central
heating system switches on or off and the temperature can all be set from an
app on a mobile phone from anywhere in the world.
There are pros and cons of devices being controlled in this manner, as shown in
Table 3.3.

Pros of embedded systems Cons of embedded systems

n small in size and therefore easy to n difficult to upgrade devices to take
fit into devices advantage of new technology
n relatively low cost to make n troubleshooting faults in the device
n usually dedicated to one task, becomes a specialist task
making for simple interfaces n although the interface can appear to be
and often no requirement of an simple, in reality it can be more confusing
operating system (changing the time on a cooker clock can
n consume very little power require several steps, for example)
n very fast reaction to changing input n any device that can be accessed over the
(operate in real time) internet is also open to hackers, viruses,
n with mass production comes and so on
reliability n due to the difficulty in upgrading and fault
finding, devices are often just thrown away
rather than being repaired (wasteful)

▲ Table 3.3 Pros and cons of controlling devices with embedded systems
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 EXTENSION ACTIVITY 3A
Describe how ROM and RAM chips could be used in: 3
a) a microwave oven
b) a refrigerator
c) a remote-controlled model aeroplane (the movement of the aeroplane is
controlled by a hand-held device).

3.1
Secondary storage devices

Computers and their components

Secondary storage includes storage devices that are not directly accessible by
the CPU. They are non-volatile devices which allow data to be stored as long as
required by the user. This type of storage is much larger than primary memory,
but data access time is considerably slower than RAM and ROM. All applications,
the operating system, device drivers and general files (for example, documents,
photos and music) are stored on secondary storage. The following section
discusses the various types of secondary storage that can be found on the
majority of computers. Secondary storage devices fall into three categories:
magnetic, solid state and optical.
Hard disk drives (HDD)
Hard disk drives (HDD) are still one of the most common methods used to
store data on a computer.
Data is stored in a digital format on the magnetic surfaces of the disks
(or platters, as they are frequently called). The hard disk drive will have a
number of platters which can spin at about 7000 times a second. A number
of read-write heads can access all of the surfaces in the disk drive. Normally
each platter will have two surfaces which can be used to store the data.
These read-write heads can move very quickly – typically they can move from
the centre of the disk to the edge of the disk (and back again) 50 times a
second.
Data is stored on the surface in sectors and tracks.
A sector on a given track will contain a fixed number of bytes.
track Unfortunately, hard disk drives have very slow data access when compared
to, for example, RAM. Many applications require the read-write heads to
constantly seek for the correct blocks of data; this means a large number of
head movements. The effects of latency then become very significant. Latency
is defined as the time it takes for a specific block of data on a data track to
sector
rotate around to the read-write head.
Users will sometimes notice the effect of latency when they see messages such
as, ‘Please wait’ or, at its worst, ‘not responding’.
▲ Figure 3.6 Tracks and When a file or data is stored on an HDD, the required number of sectors needed
sectors on a hard disk drive to store the data will be allocated. However, the sectors allocated may not be
adjacent to each other. Through time, the HDD will undergo numerous deletions
and editing, which leads to sectors becoming increasingly fragmented,
resulting in a gradual deterioration of the HDD performance (in other words, it
takes longer and longer to access data). Defragmentation software can improve
on this situation by ‘tidying up’ the disk sectors.
An HDD is a direct access device; however, data in a given sector will be read
sequentially.
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 Removable hard disk drives are essentially HDDs that are external to the

3
computer and can be connected to the computer using one of the USB ports. In
this way, they can be used as back-up devices or as another way of transferring
files between computers.

EXTENSION ACTIVITY 3B
The length of a track on each disk in an HDD disk pack becomes much
shorter towards the centre of the disk. Find out how manufacturers have
3 HARDWARE

overcome this issue with regards to disk data capacity and data access time.

Solid state drives (SSD)

Latency is an issue in HDDs, as discussed earlier. Solid state drives (SSD)
reduce this issue considerably. They have no moving parts and all data is
retrieved at the same rate. They do not rely on magnetic properties. The most
common type of solid state storage devices store data by controlling the
movement of electrons within NAND chips. The data is stored as 0s and 1s in
millions of tiny transistors (at each junction one transistor is called a floating
gate and the other is called a control gate) within the chip. This effectively
produces a non-volatile rewritable memory.
However, a number of solid state storage devices sometimes use electronically
erasable PROM (EEPROM) technology. The main difference is the use of NOR chips
rather than NAND. This makes them faster in operation but devices using EEPROM
are considerably more expensive than those that use NAND technology. EEPROM also
allows data to be read or erased in single bytes at a time. Use of NAND only allows
blocks of data to be read or erased. This makes EEPROM technology more useful in
certain applications where data needs to be accessed or erased in byte-size chunks.
Because of the cost implications, the majority of solid state storage devices
use NAND technology. The two are usually distinguished by the terms flash
memory (use NAND) and EEPROM (use NOR).
So, what are the main benefits of using an SSD rather than an HDD?
Solid state drives
» are more reliable (no moving parts to go wrong)
» are considerably lighter (which makes them suitable for laptops)
» do not have to ‘get up to speed’ before they work properly
» have a lower power consumption
» run much cooler than HDDs (both these points again make them very
suitable for laptop computers)
» are very thin (because they have no moving parts)
» access data considerably faster.
The main drawback of SSD is the still unknown longevity of the technology.
Most solid state storage devices are conservatively rated at only 20 GB write
operations per day over a three year period – this is known as SSD endurance.
For this reason, SSD technology is not commonly used in servers, for example,
where a huge number of write operations take place every day. However, this
issue is being addressed by a number of manufacturers to improve the durability
of these solid state systems and they are rapidly becoming more common in
applications such as servers and cloud storage devices.
Note that it is also not possible to over-write existing data on a flash memory
device; it is necessary to first erase the old data and then write the new data
at the same location.
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Memory sticks/flash memories (also known as pen drives) use solid state

3
technology. They usually connect to the computer through the USB port. Their
main advantage is that they are very small, lightweight devices which make
them suitable for transferring files between computers. They can also be used
as small back-up devices for music or photo files, for example.
Complex or expensive software, such as an expert system, will often use a
memory stick as a dongle. The dongle contains additional files which are
needed to run the software. Without this dongle, the software will not work
properly. It therefore prevents illegal or unauthorised use of the software, and

3.1
also prevents copying of the software since, without the dongle, it is useless.

Computers and their components

Optical media: CDs, DVDs and Blu-ray discs
CDs and DVDS are described as optical storage devices. Laser light is used to
read data from, and write data onto, the surface of a disk.
single spiral track runs
from the centre to outer
part of disk

pits or bumps

▲ Figure 3.7 CDs and DVDs use a single, spiral track

Both CDs and DVDs use a thin layer of metal alloy or light-sensitive organic
dye to store the data. As shown in Figure 3.7, both systems use a single, spiral
track which runs from the centre of the disk to the edge. When a disk spins, the
optical head moves to the point where the laser beam ‘contacts’ the disk surface
and follows the spiral track from the centre outwards. As with an HDD, a CD/DVD
is divided into sectors allowing direct access of data. Also, as in the case of an
HDD, the outer part of the disk runs faster than the inner part of the disk.

EXTENSION ACTIVITY 3C
The outer part of an optical disk runs faster than the inner part of the disk.
Find out how manufacturers have overcome this issue with regards to disk
data capacity and data access time.

The data is stored in ‘pits’ and ‘bumps’ on the spiral track. A red laser is used to
read and write the data. CDs and DVDs can be designated R (write once only) or
RW (can be written to or read from many times).
DVD technology is slightly different to that used in CDs. One of the main
differences is the use of dual layering which considerably increases the
storage capacity. This means that there are two individual recording
layers. Two layers of a standard DVD are joined together with a transparent
(polycarbonate) spacer, and a very thin reflector is sandwiched between the
two layers. Reading and writing of the second layer is done by a red laser
focusing at a fraction of a millimetre difference compared to the first layer.
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 polycarbonate layer first layer

3 polycarbonate layer second layer

laser reads laser reads

layer 1 layer 2

▲ Figure 3.8 Dual layering in a DVD

Standard, single layer DVDs still have a larger storage capacity than CDs because
3 HARDWARE

the ‘pit’ size and track width are both smaller. This means that more data can be
stored on the DVD surface. DVDs use lasers with a wavelength of 650 nanometres;
CDs use lasers with a wavelength of 780 nanometres. The shorter the wavelength
of the laser light, the greater the storage capacity of the medium.
» Blu-ray discs are another example of optical storage media. However, they
are fundamentally different to DVDs in their construction and in the way
they carry out read-write operations.
» Blu-ray uses a blue laser, rather than a red laser, to carry out read and write
operations; the wavelength of blue light is only 405 nanometres (compared
to 650 nm for red light).
» Using blue laser light means that the ‘pits’ and ‘bumps’ can be much smaller;
consequently, a Blu-ray can store up to five times more data than a DVD.
» Blu-ray uses a single 1.1 mm thick polycarbonate disk; DVDs use a sandwich
of two 0.6 mm thick disks.
» Using two sandwiched layers can cause birefringence (light is refracted into
two separate beams causing reading errors); because Blu-ray uses only one
layer, the discs do not suffer from birefringence.
» Blu-ray discs automatically come with a secure encryption system which
helps to prevent piracy and copyright infringement.

Table 3.4 summarises the main differences between CDs, DVDs and Blu-ray.

track pitch
laser wavelength (distance
disk type colour of laser light disk construction between tracks)
CD red 780 nm single 1.2 mm 1.60 µm
polycarbonate layer
DVD red 650 nm two 0.6 mm 0.74 µm
polycarbonate layers
Blu-ray blue 405 nm single 1.1 mm 0.30 µm
polycarbonate layer
nm = 10 −9 metres
µm = 10 −6 metres

▲ Table 3.4 Main differences between CDs, DVDs and Blu-ray

All these optical storage media are used as back-up systems (for photos,
music and multimedia files). This also means that CDs and DVDs can be used
to transfer files between computers. Manufacturers sometimes supply their
software (such as printer drivers) on CDs and DVDs. When the software is
supplied in this way, the disk is usually in a read-only format.
The most common use of DVD and Blu-ray is the supply of movies or games. The
memory capacity of CDs is not big enough to store most movies.
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 EXTENSION ACTIVITY 3D
A recent development is PRAM (parameter RAM) or PCRAM (phase-change 3
RAM) which utilises chalogenide glass. This is glass containing elements
such as sulphur, antimony, selenium, germanium or tellurium. Chalogenide
compounds used in PRAMs/PCRAMs can be changed between the
amorphous (glass-like) state and crystalline state, which changes the optical
and electrical properties allowing the storage of data when used as a film on
the surface of optical media.

3.1
Find out more about this technology and determine whether this could result
in the demise of the current solid state removable devices.

Computers and their components

3.1.2 Input and output devices
This section will consider laser printers, inkjet printers, 3D printers, speakers,
microphones, screens and sensors.
Laser printers
Laser printers use dry powder ink rather than liquid ink and make use of the
properties of static electricity to produce the text and images. Unlike inkjet
printers, for example, laser printers print the whole page in one go. Colour laser
printers use four toner cartridges – blue, cyan, magenta and black. Although
the actual technology is different to monochrome printers, the printing method
is similar, but colour dots are used to build up the text and images.
When a user wishes to print a document using a laser printer, the following
▲ Figure 3.9 A laser printer
sequence of events takes place.

Stage Description of what happens

1 data from the document is sent to a printer driver
2 printer driver ensures that the data is in a format that the chosen printer
can understand
3 check is made by the printer driver to ensure that the chosen printer is
available to print (is it busy? is it off-line? is it out of ink? and so on)
4 data is sent to the printer and stored in a temporary memory known as a
printer buffer
5 printing drum given a positive charge. As this drum rotates, a laser beam
scans across it removing the positive charge in certain areas, leaving
negatively charged areas which exactly match the text/images of the page
to be printed
6 drum is coated with positively charged toner (powdered ink). Since the toner is
positively charged, it only sticks to the negatively charged parts of the drum
7 negatively charged sheet of paper is rolled over the drum
8 toner on the drum sticks to the paper to produce an exact copy of the page
sent to the printer
9 to prevent the paper sticking to the drum, the electric charge on the paper
is removed after one rotation of the drum
10 the paper goes through a fuser (a set of heated rollers), where the heat
melts the ink so that it fixes permanently to the paper
11 a discharge lamp removes all the electric charge from the drum so it is
ready to print the next page

▲ Table 3.5 Sequence to print using a laser printer

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 Inkjet printers

3 Inkjet printers are made up of

» a print head consisting of nozzles that spray droplets of ink onto the paper
to form characters
» an ink cartridge or cartridges; either one cartridge for each colour (blue,
yellow and magenta) and a black cartridge, or one single cartridge
containing all three colours and black (note: some systems use six colours)
» a stepper motor and belt which moves the print head assembly across the
page from side to side
3 HARDWARE

» a paper feed which automatically feeds the printer with pages as they are
▲ Figure 3.10 An inkjet required.
printer
The ink droplets are currently produced using one of two technologies: thermal
bubble or piezoelectric.
Thermal bubble – tiny resistors create localised heat which makes the ink
vaporise. This causes the ink to form a tiny bubble, as the bubble expands
some of the ink is ejected from the print head onto the paper. When the
bubble collapses, a small vacuum is created which allows fresh ink to
be drawn into the print head. This continues until the printing cycle is
completed.
Piezoelectric – a crystal is located at the back of the ink reservoir for each
nozzle. The crystal is given a tiny electric charge which makes it vibrate. This
vibration forces ink to be ejected onto the paper and at the same time more ink
is drawn in for further printing.
When a user wishes to print a document using an inkjet printer, the following
sequence of events takes place. Whatever technology is used, the basic steps in
the printing process are the same.

Stage Description of what happens

1 data from the document is sent to a printer driver
2 printer driver ensures that the data is in a format that the chosen printer
can understand
3 check is made by the printer driver to ensure that the chosen printer is
available to print (is it busy? is it off-line? is it out of ink? and so on)
4 data is sent to the printer and stored in a temporary memory known as a
printer buffer
5 a sheet of paper is fed into the main body of the printer. A sensor detects
whether paper is available in the paper feed tray – if it is out of paper (or
the paper is jammed), an error message is sent back to the computer
6 as the sheet of paper is fed through the printer, the print head moves from
side to side across the paper printing the text or image. The four ink colours
are sprayed in their exact amounts to produce the desired final colour
7 at the end of each full pass of the print head, the paper is advanced very
slightly to allow the next line to be printed. This continues until the whole
page has been printed
8 if there is more data in the printer buffer, then the whole process from stage
5 is repeated until the buffer is empty
9 once the printer buffer is empty, the printer sends an interrupt to the processor
in the computer, which is a request for more data to be sent to the printer. The
process continues until the whole of the document has been printed

▲ Table 3.6 Sequence to print using a laser printer

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 3D printers

3.1
Computers and their components
▲ Figure 3.11 A 3D printer

3D printers are used to produce working, solid objects. They are primarily based
on inkjet and laser printer technology. The solid object is built up layer by layer
using materials such as powdered resin, powdered metal, paper or ceramic.
The artificial bone framework in Figure 3.12 was made from many layers (100 µm
thick) of powered metal using a technology known as binder 3D printing.
Various types of 3D printers exist; they range from the size of a microwave
▲ Figure 3.12 Artificial oven up to the size of a small car.
bone framework made 3D printers use additive manufacturing (the object is built up layer by layer);
using an industrial
3D printer
this is in contrast to the more traditional method of subtractive manufacturing
(removal of material to make the object). For example, making a statue using
a 3D printer would involve building it up layer by layer using powdered stone
until the final object was formed. The subtractive method would involve
carving the statue out of solid stone (removing the stone not required) until
the final item was produced. Similarly, CNC machining removes metal to form
an object; 3D printing would produce the same item by building up the object
from layers of powdered metal.
Direct 3D printing uses inkjet technology; a print head can move left to right
as in a normal printer. However, the print head can also move up and down to
build up the layers of an object.
Binder 3D printing is similar to direct 3D printing. However, this method uses
two passes for each of the layers; the first pass sprays dry powder and then on
the second pass a binder (a type of glue) is sprayed to form a solid layer.
Newer technologies use lasers and UV light to harden liquid polymers; this
further increases the diversity of products which can be made.

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 Speakers and microphones

3 Speakers
Digitised sound stored in a file on a computer can be converted into sound as
follows:
» The digital data is first passed through a digital to analogue converter (DAC)
where it is converted into an electric current.
» This is then passed through an amplifier (since the current generated
by the DAC will be small) to create a current large enough to drive a
3 HARDWARE

loudspeaker.
» This electric current is then fed to a loudspeaker where it is converted into
sound.

The following schematic shows how this is done.

▲ Figure 3.13 Digital to analogue conversion

As Figure 3.13 shows, if the sound is stored in a computer file, it must first
pass through a digital to analogue converter (DAC) to convert the digital
data into an electric current which can be used to drive the loudspeaker.
Figure 3.14 shows how a loudspeaker can convert electric signals into sound
waves.

plastic or
paper cone

sound waves permanent

magnet

coil of wire
wrapped
sound waves
around an
produced
iron core

electric current fed to wire

▲ Figure 3.14 Diagram showing how a loudspeaker works

» When an electric current flows through a coil of wire that is wrapped around
an iron core, the core becomes a temporary electromagnet; a permanent
magnet is also positioned very close to this electromagnet.
» As the electric current through the coil of wire varies, the induced magnetic
field in the iron core also varies. This causes the iron core to be attracted
towards the permanent magnet and as the current varies this will cause the
iron core to vibrate.
» Since the iron core is attached to a cone (made from paper or thin synthetic
material), this causes the cone to vibrate, producing sound.
The rate at which the DAC can translate the digital output into analogue
voltages is known as the sampling rate. If the DAC is a 16-bit device, then it
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can accept numbers between +32 767 (216 – 1) and –32 768 (216); the digital

3
value containing all zeros is ignored.
Microphones
Microphones are either built into the computer or are external devices
connected through the USB port or through wireless connectivity.
Figure 3.15 shows how a microphone can convert sound waves into an electric
current. The current produced can either be stored as sound (on, for example, a
CD), amplified and sent to a loudspeaker, or sent to a computer for storage.

3.1
coil wrapped around

Computers and their components

cone a permanent magnet

sound waves
output from
the microphone

diaphragm

▲ Figure 3.15 Diagram of how a microphone works

» When sound is created, it causes the air to vibrate.

» When a diaphragm in the microphone picks up the air vibrations, the
diaphragm also begins to vibrate.
» A copper coil is wrapped around a permanent magnet and the coil is
connected to the diaphragm using a cone. As the diaphragm vibrates, the
cone moves in and out causing the copper coil to move backwards and
forwards.
» This forwards and backwards motion causes the magnetic field around the
permanent magnet to be disturbed, inducing an electric current.
» The electric current is then either amplified or sent to a recording device.
The electric current is analogue in nature.

The electric current output from the microphone can also be sent to a computer
where a sound card converts the current into a digital signal which can then be
stored in the computer. The following diagram shows what happens when the
word ‘hut’ is picked up by a microphone and is converted into digital values:
1000 0001
0001 1110
1000 1110
0001 1100
1100 1100
1101 1110

sound wave for ‘HUT’ digital value after conversion

▲ Figure 3.16 Analogue to digital conversion

Look at Figure 3.16. The word ‘hut’ (in the form of a sound wave) has been
picked up by a microphone; this is then converted using an analogue to digital
converter (ADC) into digital values which can then be stored in a computer or
manipulated as required using appropriate software.

Screens
Screens are used to show the output from a computer. Modern screens use an LCD,
backlit with LEDs or the newer organic light emitting diode (OLED) technology.
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 Figure 3.17 shows a simplified form of how OLED technology works.

3 negative charges
glass or plastic top layer

metallic cathode (negative charge)

emissive layer
conductive layer
positive charges
glass anode (positive charge)
glass or plastic bottom layer
3 HARDWARE

▲ Figure 3.17 Simplified form of how OLED technology works

OLEDs use organic materials (made up of carbon compounds) to create flexible

semiconductors. Organic films are sandwiched between two charged electrodes
(one is a metallic cathode and the other a glass anode). When an electric field
is applied to the electrodes, they give off light. This means that no form of
back lighting is required. This allows for very thin screens. It also means that
there is no longer a need to use LCD technology, since OLED is a self-contained
system.
Screen displays are based on the pixel (the smallest picture element) concept
where each screen pixel is made up of three sub-pixels, which are red, green
and blue. By varying the intensity of the three sub-pixels, it is possible to
generate millions of colours. The greater the number of pixels on a screen,
the greater is the screen resolution (the number of pixels which can be
viewed horizontally and vertically on screen; for example, 1680 × 1080
pixels). LCD and OLED screens use this type of pixel matrix to make up the
picture.

The ‘purple’ pixel is made up of a combination of

three sub-pixels, which are red, green and blue, in the
required intensity, to ‘fool’ the eye into seeing a
purple dot on the screen. The whole screen is filled
with thousands of these tiny pixels.

▲ Figure 3.18 The pixel matrix

Touch screens (which act as both input and output devices) also make use
of LCD and OLED technology. They are particularly used in mobile phones and
tablets.
We shall now consider LCD capacitive and resistive touch screen technologies.
Capacitive
» Made up of many layers of glass that act like a capacitor creating electric
fields between the glass plates in layers.
» When the top glass layer is touched, the electric current changes and the
coordinates where the screen was touched are determined by an on board
microprocessor.

Benefits
» Medium cost technology.
» Screen visibility is good even in strong sunlight.
» Permits multi-touch capability.
» Screen is very durable; it takes a major impact to break the glass.
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Drawbacks
» Only allows use of bare fingers as the form of input; although the latest
screens permit the use of a special stylus to be used. 3
Resistive
» Makes use of an upper layer of polyester (a form of plastic) and a bottom
layer of glass.
» When the top polyester layer is touched, the top layer and bottom layer
complete a circuit.

3.1
» Signals are then sent out, which are interpreted by a microprocessor
and the calculations determine the coordinates of where the screen was

Computers and their components

touched.
Benefits
» Relatively inexpensive technology.
» Possible to use bare fingers, gloved fingers or stylus to carry out an input
operation.
Drawbacks
» Screen visibility is poor in strong sunlight.
» Does not permit multi-touch capability.
» Screen durability is only fair; it is vulnerable to scratches and the screen
wears out through time.

Virtual headsets
Virtual reality has now been around for many years and has many applications.
For example, it is possible to ‘walk around’ inside dangerous areas – such as a
nuclear power plant – without actually being there.
It allows engineers to plan modifications or repairs to a plant in complete
safety and to try out different scenarios first before implementing them. One
of the devices used is a virtual reality headset which gives the engineer the
feeling of being there. We will now describe how these devices work.
» Video is sent from a computer to the headset (either using an HDMI cable or
a smartphone fitted into the headset).
» Two feeds are sent to an LCD/OLED display (sometimes two screens are
used, one for the left side of the image and one for the right side of the
image); lenses placed between the eyes and the screen allow for focusing
and reshaping of the image/video for each eye, thus giving a 3D effect and
adding to the realism.
» Most headsets use 110° field of view which is enough to give a pseudo 360°
surround image/video.
» A frame rate of 60 to 120 images per second is used to give a true/realistic
image.
» As the user moves their head (up and down or left to right), a series of
sensors and/or LEDs measure this movement, which allows the image/video
on the screen to react to the user’s head movements (sensors are usually
gyroscopic or accelerometers; LEDs are used in conjunction with mini
cameras to further monitor head movements).
» Headsets also use binaural sound (surround sound) so that the speaker
output appears to come from behind, from the side or from a distance,
giving very realistic 3D sound.

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 » Some headsets also use infrared sensors to monitor eye movement (in

3
addition to head movement), which allows the depth of field on the screen
to be more realistic; an example of this is to make objects in the foreground
appear fuzzy when the user’s eyes indicate they are looking into the
distance (and vice versa).

Sensors
Sensors are input devices which read or measure physical properties, such as
temperature, pressure, acidity, and so on.
3 HARDWARE

Real data is analogue in nature – this means it is constantly changing and

does not have a discrete value. Analogue data usually requires some form of
interpretation, for example, the temperature shown on a mercury thermometer
requires the user to look at the height of the mercury to work out the
temperature. The temperature, therefore, can have an infinite number of values
depending on the precision of how the height of the mercury is measured.
Equally, an analogue clock requires the user to look at the hands on the clock
face. The area swept out by the hands allows the number of hours and minutes
to be interpreted. There are many other examples.
Computers cannot make any sense of these physical quantities and the data
needs to be converted into a digital format. This is usually achieved by an
analogue to digital converter (ADC). This device converts physical values into
discrete digital values.

ADC 1 0 0 1 1 1 0 0 ...

analogue data digital data

▲ Figure 3.19 Converting analogue data into digital data

When a computer is used to control devices, such as a motor or a valve,

it is often necessary to use a digital to analogue converter (DAC), since
these devices need analogue data to operate in many cases. Frequently,
an actuator is used in these control applications. Although these are
technically output devices, they are mentioned here since they are an
integral part of the control system. An actuator is an electromechanical
device such as a relay, solenoid or motor. Note that a solenoid is an example
of a digital actuator as part of the device is connected to a computer which
opens and closes a circuit as required. When energized, the solenoid may
operate a plunger or armature to control, for example, a fuel injection
system. Other actuators, such as motors and valves, may require a DAC so
that they receive an electric current rather than a simple digital signal
direct from the computer.
Notice the importance of (positive) feedback, which is where the output
from the system can affect the next input. This is due to the fact that sensor
readings may cause the microprocessor to alter a valve or a motor, for example,
which will then change the next reading taken by the sensor. So the output
from the microprocessor will impact on the next input received as it attempts
to bring the system within the desired parameters.

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Table 3.7 shows a number of common sensors and examples of their

3
applications.

Sensor Example applications

temperature n control a central heating system
n control/monitor a chemical process
n control/monitor temperature in a greenhouse
moisture/humidity n control/monitor moisture/humidity levels in soil/air in a
greenhouse

3.1
n monitor dampness levels in an industrial application (for
example, monitor moisture in a paint spray booth in a car

Computers and their components

factory)
light n switch street lighting on at night and off during the day
n monitor/control light levels in a greenhouse
n switch on car headlights when it gets dark
infrared/motion n turn on windscreen wipers on a car when it rains
n detect an intruder in a burglar alarm system
n count people entering or leaving a building
pressure n detect intruders in a burglar alarm system
n check weight (such as the weight of a vehicle)
n monitor/control a process where gas pressure is important
acoustic/sound n pick up noise levels (such as footsteps or breaking glass) in a
burglar alarm system
n detect noise of liquids dripping from a pipe
gas (such as O2 n monitor pollution levels in a river or air
or CO2) n measure O2 and CO2 levels in a greenhouse
n check for CO2 or NO2 leaks in a power station
pH n monitor/control acidity/alkalinity levels in soil
n monitor pollution in rivers
magnetic field n detect changes in in cell phones, CD players, and so on
n used in anti-lock braking systems in motor vehicles
▲ Table 3.7 Common sensors and examples of applications

Sensors are used in both monitoring and control applications. There is a subtle
difference between how these two methods work. The flowchart (Figure 3.21
overleaf) shows a simplification of the process.

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 sensors send signals to the

3
microprocessor or computer

the signals are converted to

digital (if necessary) using an
analogue to digital converter
(ADC)

the microprocessor or computer

3 HARDWARE

analyses the data received

by checking it against stored
values

if new data is outside the if the new data is outside the

acceptable range, a warning acceptable range, the
message is sent to a screen microprocessor or computer
or an alarm is activated sends signals to control valves,
motors, and so on

the microprocessor or computer the output from the system

has no effect on what is affects the next set of inputs
being monitored – it is simply from the sensors
‘watching’ the process
feedback loop

monitoring system control system

▲ Figure 3.20 Sensors for monitoring and controlling systems

Table 3.8 shows some examples of monitoring and control applications of

sensors.

Examples of monitoring Examples of control

n monitoring a patient in a hospital n turning street lights on at night and
for vital signs such as heart rate, turning them off again during daylight
temperature, and so on n controlling the temperature in a central
n checking for intruders in a burglar alarm heating/air conditioning system
system n controlling the traffic lights at a road
n checking the temperature levels in a car junction
engine n operating anti-lock brakes on a car
n monitoring pollution levels in a river when necessary
n controlling the environment in a
greenhouse

▲ Table 3.8 Examples of monitoring and control applications of sensors

One of the most common uses of sensors in modern times is in the monitoring
and control of a number of functions in motor vehicles and aeroplanes. Look at
Figure 3.21 showing a typical modern car and its many sensors used to control
or monitor several functions.

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 3

3.1
Computers and their components
▲ Figure 3.21 Sensors on a typical modern car

Below is an in-depth look at just one of the sensor systems labelled on

Figure 3.21.
Anti-lock braking systems (on cars)
Anti-lock braking systems (ABS) on cars use magnetic field sensors to stop the
wheels locking up on the car if the brakes have been applied too sharply.
» When one of the car wheels rotates too slowly (it is locking up), a magnetic
field sensor sends data to a microprocessor.
» The microprocessor checks the rotation speed of the other three wheels.
» If they are different (rotating faster), the microprocessor sends a signal to
the braking system and the braking pressure to the affected wheel is reduced.
» The wheel’s rotational speed is then increased to match the other wheels.
» The checking of the rotational speed using these magnetic field sensors is
done several times a second and the braking pressure to all the wheels can
be constantly changing to prevent any of the wheels locking up under heavy
braking.
» This is felt as a ‘judder’ on the brake pedal as the braking system is constantly
switched off and on to equalise the rotational speed of all four wheels.
» If one of the wheels is rotating too quickly, braking pressure is increased to
that wheel until it matches the other three.

ACTIVITY 3A
1 a) i) Describe three differences between RAM and ROM.
ii) Compare the relative advantages and disadvantages of SRAM and
DRAM.
Include examples of where each type of memory would be used in
a computer.

87
 b) Secondary storage can be magnetic, optical or solid state.

3 Describe two features of each type of storage which differentiates it

from the other two types.
2 a) Explain the main differences in operation of a laser printer compared
with an inkjet printer.
b) i) Name one application of a laser printer and one application of an
inkjet printer.
ii) For each of your named applications in part b) i), give a reason why
the chosen printer is the most suitable.
3 HARDWARE

3 An art gallery took several photographs of a valuable, fragile painting.

The images were sent to a computer where they were processed by a 3D
printing application. A 3D printout of the painting was produced showing
the texture of the oil paint, canvas and any flaws in the painting.
Give reasons why the art gallery would wish to make this 3D replica.
4 The following diagram shows a schematic of a microprocessor-controlled
street lighting system.

sensor

street ADC microprocessor

light

DAC

The microprocessor is used to control the operation of the street lamp.

The lamp is fitted with a light sensor which constantly sends data to the
microprocessor. The data value from the sensor changes according to
whether it is sunny, cloudy, raining, night time, and so on.
Describe how the microprocessor would be used to automatically switch
on the light at night and switch it off again when it becomes light. Include
a feature to stop the light constantly flickering on and off when it becomes
overcast or cars go past with full headlights at night.

EXTENSION ACTIVITY 3E
1 Look at this simplified diagram of a keyboard; 2 a) Describe how these types of pointing devices
the letter H has been pressed. Explain: work.
a) how pressing the letter H has been i) Mechanical mouse
recognised by the computer ii) Optical mouse
b) how the computer manages the very slow b) Connectivity between mouse and computer
process of inputting data from a keyboard. can be through USB cable or wireless.
Explain these two types of connectivity.

G J letter H has been pressed and

now makes contact with bottom
H conductive layer

conductive layers
letter H
interpreted
insulating layer by computer

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