Chapter 3 - Hardware

3.1 Computers and their components

Primary memory →

Random Access Memory (RAM) →

• Volatile memory that clears when powered off
• Can be read to or written from a number of times
• Memory size larger than ROM
• Stores running programs, part of OS currently in use, data in use

Read-Only Memory (ROM) →

• Non-volatile - retains memory contents when powered off
• Read from but not written to
• Holds kernel of OS, BIOS, start-up instructions, boot program
• Programmable ROM (PROM) - programmed once
• Erasable programmable ROM (EPROM) -
  • programmed more than once using UV light
  • must be entirely erased to reprogram and removed to reprogram
• Electronically erasable programmable ROM (EEPROM) -
  • programmed more than once using pulsed voltages
  • don’t need to entirely erase or remove to reprogram

Storage devices →

Magnetic hard-disk drives →

• Operation:
  • One or more platters of aluminium or glass, the surface of which is a ferrous oxide capable of being magnetised
  • Platters mounted on central spindle and rotated at high speeds
  • Each surface has a read/write head mounted on an arm. Electronic circuits control movement of the arm and heads
  • Concentric tracks divided into sectors. One track in one sector is the basic unit of storage called the block
  • Data is encoded as magnetic pattern on the black by positive or negative polarisation of magnetic particles
  • To write, a variation of current in the head varies magnetic field
  • To read, a variation in magnetic field varies current in the head
• How data is read:
  • Application program executes statement to read file and passes request to OS
  • OS begins to spin the disk if not already spinning
  • OS looks up track and sector where file starts in relevant directory
  • Head moves to correct track. It reads the sectors and writes data to disk buffer
  • Once read, the hard disk generates an interrupt
  • Disk buffer transfers contents to program’s data memory
• Benefits:
  • Inexpensive per unit storage
  • Larger capacity than flash drive

Solid-state media →

• NAND based flash memory with no moving parts
• Grids and columns of rows with two transistors at each intersection, the floating gate and the control gate
• Memory cells store voltages representing a 0 or 1
• Controls movement of electrons to read and write
• Can’t overwrite existing data - must erase and then write
• Semiconductor technology

Pros	Cons
More robust and durable; less susceptible to damage Fast access speeds No moving parts and noise Lighter weight Lower power consumption → extended battery life → cooler devices	More expensive per unit storage Low longevity for read/write functions

Optical media →

• CD-ROM: Disc spins and laser beam reflected from surface. The reflective surface has pits and lands. There is a phase difference caused by the travel of the laser from pits and lands. This is recognised by a photodiode detector and stored as a 0 o 1. Single spiral path from centre to periphery and infrared laser
• DVD-RW: Single spiral track. Only allows read or write to occur separately. Must completely erase to rewrite. Performance degrades and is unreliable after 1000 cycles. Single sided at 4.7Gb per side. Different speeds and constant linear velocity
• DVD-RAM: Concentric tracks on a rotating disk, storing data with lasers. Uses phase changing recording - varying laser intensities are used to cause targeted areas to change between amorphous and crystalline. Read and write can occur simultaneously and can have as many cycles as possible. Uses sectors. Double or single sided - 4.7GB per side. Constant speed and constant angular velocity
• Blu-ray: Shorter wavelength blue laser to store more data

Output devices →

Screen display →

• LCD: liquid crystal cells sandwiched between polarises, backlit by LEDs. individual cells create the pixel matrx, and each has a red, green, blue sub-pixel. each pixel affects transmission of light to cause onscreen display
• OLED: organic films of flexible semiconductors between 2 charged electrodes - a metallic cathode and glass anode. gives off light when voltage is applied. thinner screens as no back-lighting

Laser printer →

• Data from a document sent to the printer driver, ensuring data is in correct format. Stored in buffer while a check ensures printer is available to print
• Drum given electric charge and starts to revolve step by step
• At each step, a laser beam is directed by a mirror and lens assembly to positions across drum width. It is left on to discharge or turned off to keep charge. This repeats until a full page electrostatic image is made
• Drum is coated with charged toner sticking to discharged parts
• A charged piece of paper rolled over charge. Pattern on drum transferred due to electrostatic attraction
• Paper discharged and passed through heated rollers to fuse toner particles and seal image
• Drum discharged and excess toner collected

Use of RAM:

• Currently running parts of software
• Data being printed
• Buffer
• Progress
• Data about printer

Use of ROM:

• Boot-up (BIOS)
• Start-up
• OS
• Printer fonts

3D printer →

• Produces solid, working objects
• Uses a nozzle to squirt material and create a physical layer to match the design. head moves up, down, left, right to build layers
• Additive manufacture - object is built up layer by layer
• At the end, it is cured to ensure layers are welded and produce solid working object
• Binder printing uses dry powder and binder glue to weld

Speaker →

• Output of sound. computer sound card produces a digital signal, converted to analogue by DAC and fed to speaker
• Components:
  • Diaphragm
  • Voice coil of wire
  • Permanent magnet
  • Suspension
• Operation:
  • Takes electrical signal and translates to physical vibrations
  • Current in the coil creates an EM field
  • Changes in audio signal cause current to change
  • Direction of current determines EM polarity
  • Electromagnet repelled or attracted causing coil to vibrate and diaphragm to vibrate
  • Vibrations transmitted to air as sound waves
  • Amount of movement determines amplitude and frequency of sound waves

Input devices →

Touch Screens →

Type	Principle	Pros	Cons
Resistive	Flexible upper surface and two electrically conductive layers with a gap. The screen is not rigid, so when it is touched, it moves to touch the bottom layer. This creates a voltage divider and completes the circuit in horizontal and vertical directions. Point of contact determined. Coordinate calculated	Inexpensive Can use stylus, gloves, fingers	Poor visibility in sun No multi touch Not very durable - Scratches and screen wears over time
Capacitive	Rigid surface above conductive layer. Electrical charge conducted between them. The glass layer acts as a capacitor with electric fields. Finger touching changes the electric field, detected by sensors and point of contact determined. Coordinate calculated	Medium cost Good visibility in sun Multi-touch Durable	Only fingers - no gloves or stylus

Microphones →

• Used for input of sound
• Diaphragm made of flexible material which vibrates with incoming sound. Connected to coil wrapped around permanent magnet with cone
• As the diaphragm vibrates, the cone moves in and out so the coil moves back and forth. Coil moving past magnet disturbs the magnetic field and induces a current
• Current/electrical signal converted to digital signal by ADC before processing with sound card

Virtual reality headset →

• Head mounted device with VR experience for weather. video games, simulations, trainers
• Stereoscopic head-mounted display with separate images for each eye, stereo sound, head motion monitoring trackers (gyroscopes, accelerometers, magnetometers). Eye tracking sensors and IR sensors to monitor depth and a gaming controller. Binaural sound allows it to appear 3D
• The 2 eyepieces are fed paired images that appear 3d when looked at together. Collected with a special photography technique or a 3D graphics package. Wearer controls environment using a controller or by moving head

Embedded and general systems →

• Embedded system: microprocessor based system that performs a dedicated function as part of a larger system
  • Low power, small size, basic OS, low cost, slow CPU
  • Only runs a few commands with low-level language
  • Can’t install new software
  • Reliable → stores programs in ROM
• General system:
  • Fast, can install new software, stores programs on hard drive, less reliable

Monitoring and control systems →

• Control: output can change the value of the next input through action. alters the processing
• Monitoring: computer/microprocessor doesn’t alter process; simply report values and inform users
• Sensor: hardware device measuring a property and sending data to a computer/microprocessor. temperature, IR, motion, pressure
• Actuator: device that receives a signal from a computer for movement/action in a control system

• Sensors detect input
• Sensors convert signals to digital with ADC and send to microprocessor
• Microprocessor compares to stored values in memory
• If outside range:
  • Monitoring: Output warning message/alarm
  • Control: Microprocessor/computer sends actuator a signal to deal with problem
• Process repeats in a loop

System basics →

• Input devices feed data to computer/microprocessor
• Processing done by following instructions
• Microprocessor delivers output with actuator or output device
• Involves primary and secondary storage

Feedback is used in a control system to ensure it operates in given criteria by enabling the system to affect subsequent inputs and automatically adjust conditions. Ensures output is similar to required value.

3.2 Logic gates →

Electronic logic gates take 1 or more inputs and produce a single output.

• NOT - reverse input
• AND - only 1 if both are 1
• NAND - reverses and. only 0 if both are 1
• OR - 1 if either input is 1
• NOR - reverses or. only 1 if both are 0
• XOR - if inputs different, output is 1, else output is 0 (only one 1 for a 1 output)