Overview of this chapter
|3.1 Storage Devices||a) Registors|
-> See chapter 5
|b) Secondary Storage||2|
|c) Offline Storage|
|3.2 Mouse and Keyboards||…||3|
|3.4 Printer and Scanners||…||5|
3.1 Storage Devices
Classification of storage devices:
|a) Processer Component||Register||<See chapter 5>|
|b) Primary Storage*||Random-Access Memory (RAM)||DRAM|
|Read-Only Memory (ROM)||ROM|
|c) Secondary Storage||Magnetical Media||HDD|
* Another way to categorize Primary storage is “Cache memory” and “Main memory”. Cache memory stores data most likely to be used again by the computer.
Going down the table from registers to secondary storages:
- Access time: Longer
- Capacity: Higher
- Storage size: Bigger
- Cost: Lower
The fastest components have limited capacity, are expensive, and they are volatile. Therefore a combination of all sections of memories are used. It’s a compromise.
3.1.b Primary Storage
Consists of two types: RAM and ROM.
Primary memory have the characteristics of random access / direct access, meaning memory can be access at any location, not restricted by previous access location.
Random-access Memory: Volatile memory, can be read/write for any number of times.
|Used In||Cache Memory||Main Memory|
|Energy||More energy||Less energy|
Requires regular recharging
Read-only Memory: Non-volatile memory, can not be written to, but can be read from for any number of times.
- PROM: Programmable ROM
- EPROM: Erasable Programmable ROM
- EEPROM: Electronically Erasable Programable ROM
3.1.c Secondary Storage
i) Magnetic Media – HDD
Direct-access, read-write storage.
Structure of the Hard Disc Drive:
- The alternative states on the discs are interpreted by 1/0.
- All discs spin in union.
- Read / Write Head
- Each disk has a Read / write head for each side.
- The head never touch the surface of the plate.
- An actuator move the Read / write head.
- All actuators are synchronized
- Data is stored in concentric tracks.
- The track is divided into bits, then formatted into sectors, consisting a defined number of bytes. The sector is the smallest unit of storage.
- Data has to be read sequentially in sectors.
ii) Optical Media – Discs
|Read / Write||Read Only||Read & Write|
Writing laser melts the surface.
The surface reverts into the two states,
Depending on the intensity of the laser.
Other previous models: CD(Compact Disk), DVD (Digital Versatile Disc)
- A single spiral track from the center to periphery.
- By formatting the track, direct-access can be done.
- Storage capability
- How close different binary digits can get
- Wavelength of light
- Blue-ray discs
iii) Solid-State Media
Based on flash memory (A form of EEPROM), where the programmable part is for normal use.
Uses transistors, which has no moving parts. NAND gates are usually used.
Read / Write operation had to be conducted on entire block of memory as a whole. e.g. Before writing any data on the block, the whole block has to be removed first, then re-write the new data into the block.
Next page: 3.2 Mouse and Keyboard
3.2 Mouse and Keyboards
A mouse is a pointing device. It has two functions: Click and Controlling Cruiser. Note that a mouse can only know its relative position of moving.
|Mechanical Mouse||Optical Mouse|
|Structure||* Rubber Wheel|
* 2 Roller sets, perpendicular to each other.
* Light beam and detector
|* Light Beam and Detector (Detector more like a camera)|
|How it works||1. The rubber wheel turns while the mouse moves.|
2. This turns one or both rollers.
3. The detector detects the transmission of light through the holes on discs.
4. Information of movement is sent to computer.
|1. A light beam falls onto the object surface.|
2. Light is reflected back to the sensor.
3. The sensor takes successive image of the object surface.
4. By comparing the images in an image processing software, the movement can be deduced.
5. Information is sent to the computer.
Structure of Keyboards:
A key matrix is laid under the keys. Pressing a key will cause a contact in specific intersection, resulting a closed circuit.
A microprocessor and ROM chip is inside the keyboard. It continuously checks the current – when the microprocessor detects a closed circuit, it can identify the specific intersection.
It then converts the result into character code, which is then transmitted to the processor.
Next page: 3.3 Screens
3.3.a Traditional Screens
Principle: Screen shows bit maps.
Every pixel consists of three sub-pixels: Red, Green and Blue. Controlling the amount of light from each pixel creates a full range of color.
|Cathode Ray Technology|
|Structure||For detail, see more in |
* A1 Physics Practical: Cathode Ray Oscilloscope.
* And A1 Physics Theory: Electric Field.
|For detail, see more in A2 Physics: Polarization of light.|
|How it works||There are no individual pixels. |
Pixels dots are created by controlling the direction of the electron beam fired.
When the beam is directed to a spot on the screen, the charge causes phosphor on screen to radiate. This emits light.
|The pixels do not emit light. Light comes from the backlight, from LED tubes.|
The alignment of liquid crystal is affected when a voltage is applied. This changes the polarization of light when it pass through.
This affects the final display.
Organic Light-emitting diode (OLED) screens is.a new technology. OLED can act as a light source, so there is no need for a back lighting.
3.3.b Touch Screens
Touch screens are both input and output device. There are two types of them: Capacitive and Resistive.
(The syllabus is much easier than the IGCSE!)
|Resistive Touch Screen||Capacitive Touch Screen|
|Structure||* Two layers, separated by thin space.|
* Screen is rigid.
|* Conductive layer beneath the screen.|
* Rigid surface.
|How it works||When the two layers meet, a voltage divider is produced, (Horizontally and vertically).||When touched, an electrical state (capacitance) change in circuit component happens beneath the surface.|
Next page: 3.4 Printers and Scanners
3.4 Printers and Scanners
3.4.a 2D Printers
There are three types of 2D printers: Laser printer, Inkjet printer and Graphics Plotter.
|Step||Operation on |
|1||Given electric charge (+).||Given (-) electric charge.|
The drum starts to spin.
|2||A laser beam is directed by a mirror and lens, moving across the drum (from left to right).|
For each location, if:
* Laser off: Charge remains on drum
* Laser on: Charge removed from the drum.
|[Repeat step 1-2, until a full page of electrostatic image is created.]|
|3||Drum is coated with charged (-) toner.|
The toner only sticks on the discharged parts of the drum.
|4||The drum rolls over the paper.|
The ink is transferred to the paper.
|5||The paper is discharged.||The drum is discharged.|
|6||The paper pass through heated rollers to fuse the toner particles onto the paper.|
|[Page print complete. Start form step 1 again.]|
Smaller in size, cheeper to buy, but quality is poorer.
- The printhead moves across the sheet, deposing ink.
- Release: Either by thermal bubble or piezoelectric.
- Ink: Cyan, Magenta, Yellow, and black. (NOT RGB!)
- By a combination of different ink, any color could be formed.
- The paper moves forward a fraction.
- Repeat step 1-2.
Two factors denominate the accuracy:
- Mechanical Precision
- e.g. How far to move the paper for every sweep of inched
- Nozzle accuracy
- i.e. How many nozzles are in the printhead (>Thousands)
Uses a pen to write on a large sheet of paper. Allows the drawing to created directly from vector files – instead of being bitmapped.
The paper is constrained by sprockets, which moves the paper. The pen can move up and down at any time to draw / not draw.
- The design is splitter into layers.
- For every layer, the nozzle squirt material onto the previous layer.
- Some way is ensured that the layers are welded together.
- Steps 1 and 2 are repeated.
For traditional 2-D scanners:
- The paper is fixed.
- A light source moves from one end of sheet to another.
- The reflected light passes through mirror and lenses to a charged-coupled device. (CCD)
- The CCD has an array of photo-sensitive cells. It creates an electrical response proportional to the light intensity.
- An analogue-to-digitial converter is required to create the bitmap image.
Next page: 3.5 Sound
- The diaphragm vibrates from the incoming sound.
- The vibration is converted to electrical signal by:
- Condenser Microphone: Uses capacitance change.
- Piezoelectric: Use the piezoelectric crystal’s vibration.
- The analog signal is converted to digital by an analogue-to-digital convertor.
- The digital signal is converted to analogue by digital-to-analogue convertor.
- The analogue signal (current) flows through a coil suspended in magnetic field. When the current changes, the coil carries the diaphragm vibrates back and forth, generating sound.