The storage and processing of data are explained in terms of characters in the computer. The characters are represented electronically with a two-state binary system of numbers. A binary means two. The binary system is a fundamental principle of digital computers. Everything that goes into a computer is converted into binary digits, 0 and 1

Bit

A bit stands for BInary digT, and is a single digit in a binary number (1 or 0). That is each 0 or 1 in the binary system is bit. Physically, a bit is a cell in memory, a magnetic spot on disk or tape, and a pulse of high or low voltage through a circuit in the computer.

A storage unit in the computer is groups of bits called characters, bytes, or words. The most common storage unit is the byte, which is made up of eight bits and typically represents a single alphanumeric character - number, letter, and special character.

What are the methods to code characters with 0s and 1s in the computer? There are many coding schemes. Among them, ASCII and EBCDIC are two of the most popular binary coding schemes. Both use eight bits to form each byte.

• ASCII: ASCII, pronounced "as-key, " stands for American Standard Code for Information Interchange. This is the most widely used binary code for data communications and microcomputers. ASCII is a 7-bit code, giving 128 possible character combinations. An extra bit is used to hold a parity bit.

• EBCDIC: EBCDIC, pronounced "eb-see- dick, " stands for Extended Binary Coded Decimal Interchange Code. EBCDIC is a binary code set for representing data and an 8-bit code that allows 256 possible characters combination. It was developed by IBM and is used on all IBM computers. EBCDIC is also almost an industry standard for minicomputers and mainframe computers.

The capacity of memory (primary storage) and a storage device is expressed in numbers of bytes, which start with 1 and double: 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, etc. For example, when a microcomputer system is listed with an "8 MB main memory," the computer has primary storage capacity of around 8 million bytes. The capacity of disk, tape and memory is measured in bytes, whereas that of the CPU and memory chips is measured in bits.

• Kilobyte (KB): About 1000 bytes (precisely, 1024 bytes). A kilobyte is abbreviated KB or K-byte.

• Megabyte (MB): About 1 million bytes. A megabyte is abbreviated MB or M-byte.

• Gigabyte (GB): About 1 billion bytes. A gigabyte is abbreviated GB or G-byte. Today's high capacity microcomputer has around 1 GB hard disk space.

• Terabyte (TB): About 1 trillion bytes. A terabyte is abbreviated TB or T-byte. This is a measure used with mainframe computers and supercomputers.

A parity bit is an extra bit added to a byte for error detection purposes in data transmission. Parity checking is accomplished by using a parity bit that is an extra bit automatically added to a byte for purposes of testing accuracy.
There are even-parity systems and odd- parity systems. In an even-parity system, the parity bit is set to either 0 or 1 to ensure that the number of 1s is even. Odd-parity systems perform the opposite.

The main part, processing unit and devices, of a microcomputer is a system unit. A system unit includes a board called a motherboard that holds a microprocessor chip (or a CPU), memory chips, and expansion slots. Electronic circuitry is printed on the board and it connects between two main parts of a microcomputer, the microprocessor and primary storage and other parts. The system unit is housed within the system cabinet. A system unit includes the following parts:

A Motherboard or system board is the main printed, flat circuit board in an electronic device such as microcomputers. The board contains expansion slots (sockets) that accept additional boards (expansion Cards). In a microcomputer, the motherboard contains the microprocessor, the primary storage chips (or main memory cards), the buses, and all the chips used for controlling the peripherals.

A microprocessor is a processor whose elements are miniaturized into one or a few integrated circuits contained in a single silicon microchip. It executes instructions. In a microcomputer, the central processing unit (CPU) is held on a single microprocessor. In order to function as a processor, it requires a system clock, primary storage, and power supply.
Several important lines of microcomputers use some families of microprocessor chips. Intel and Motorola are the major companies that produce important microprocessors for IBM compatible and Macintosh computers.

Microprocessor Capacity

The capacity of a microprocessor chip is represented in word sizes. A word size is the number of bits (e.g., 8, 16, or 32 bits) that a computer (CPU) can process at a time.
If word has more bits, the computer (CPU) are more powerful and faster. For example, a 16-bit-word computer can access 2 bytes (1 byte = 8 bits) at a time, while a 32-bit-word computer can access 4 bytes at a time. Therefore, the 32-bit computer is faster than the 16-bit computer.

CISC and RISC Chips

• CISC Chips: CISC stands for Complex Instruction Set Computer. It is pronounced "sisk." CISC is a computer architecture that has large sets of instructions. CISC machines have several hundred instructions. Intel's Pentium-chip uses CISC design.

• RISC Chip: RISC stands for Reduced Instruction Set Computer. RISC is a computer that executes a few number of instructions. In RISC, most programs generally use only a few instructions. This architecture has a small number of instructions built into the circuits and if those basic instructions are made to execute faster, then RISC computers increase performance. Although RISC machines are only around 30% faster than their CISC machines, RISC chips are less expensive to produce. That is, RISC is less expensive per MIPS.

• Controversy: The proponents of RISC claim that its lower cost and speed are an extreme advantage, while the opponents claim its improvements are not great. The opponents believe that new machine languages and greater performance improvements are going to come. Besides, the reduced instructions make software (e.g., compilers) generate more code to do. Thus, the choice is not obvious.

Central Processing Unit (CPU)

The central processing unit (CPU) is the computing part of the computer that interprets and executes program instructions. It is also known as the processor. In a microcomputer, the CPU is contained on a single microprocessor chip within the system unit. The CPU has two parts: the control unit and the arithmetic-logic unit.

Additional storage units called registers within control unit and ALU help make processing more efficient.

• Control Unit: A control unit is the circuitry that locates, retrieves, interprets and executes each instruction in the central processing unit. The control unit directs electronic signals between primary storage and the ALU, and between the CPU and input/output devices.

• Atithmetic-Logic Unit (ALU): ALU is a high-speed circuit part in the CPU. The arithmetic-logic unit (ALU) performs arithmetic (math) operations, logic (comparison) operations and related operations. The ALU retrieves alphanumeric data from memory and then does actual calculating and comparing. It sends the results of the operation back to memory again.

Memory Chips

A memory chip is a chip that holds programs and data either temporarily or permanently. The major categories of memory chips are RAMs and ROMs.

RAM Chips

RAM stands for random-access memory. Random- access memory holds the data or instructions that the CPU is presently processing. The type of primary storage is RAM. That is, a collection of RAM chips builds primary storage.
Whenever a CPU writes data or instructions to RAM, it wipes out the previous contents of RAM, and when a CPU read data or instructions from RAM, it keeps their contents.

ROM Chips

ROM stands for read-only memory. A ROM chip is a memory chip that stores instructions and data permanently. Its contents are placed into the ROM chip at the time of manufacture and cannot be modified by the user. A CPU can read and retrieve the instructions and data from the ROM chip, but it cannot change the contents in ROM.
ROM chips usually contain special instructions for computer operations such as ROM BIOS. The variations on the ROM chip are the following:

• PROM (Programmable Read-Only Memory): A permanent storage device that becomes a read-only memory after it is written once by the customer rather than by the chip manufacturer. For example, a software producer can write instructions onto the PROM using special equipment.

• EPROM (Erasable Programmable Read-Only Memory): EPROM is a reusable PROM-chip that can be erased by a special ultraviolet light. EPROM holds its content until erased and new instructions can be written on it.

• EEPROM (Electrically Erasable Programmable Read-Only Memory): EEPROM-chip can be erased, either within a computer or externally, by electric power. The process usually requires more voltage than the common +5 volts used in logic circuits.

Primary Storage (Memory)

Primary storage (internal storage, main memory or memory)is the computer's working storage space that holds data, instructions for processing, and processed data (information) waiting to be sent to secondary storage. Physically, primary storage is a collection of RAM chips.
The contents are held in primary storage only temporarily. Capacity varies with different computers. Data or instructions are stored in primary storage locations called addresses.

The clock is a device that generates periodic, accurately spaced signals used for several purposes such as regulation of the operations of a processor or generation of interrupts. The clock circuit uses the fixed vibrations generated from a quartz crystal to deliver a steady stream of pulses to the processor. The system clock controls the speed of all the operations within a computer.
The clock speed is the internal speed of a computer. The clock speed is expressed in megahertzes (MHz). 33 MHz means 33 million cycles per second. A computer processor's speed is faster if it has higher clock speed. For example, a 100-Mhz processor is four times as fast internally as the same processor running at 25MHz.

Open/Closed architectures

• Open Architecture: This architecture is a system whose specifications are made public to encourage third-party vendors to develop add-on products for it. Most microcomputers adopt open architecture. They allow users to expand their systems using optional expansion boards.

• Closed Architecture: This is a system whose technical specifications are not made public. With a machine that has closed architecture, users cannot easily add new peripherals.

Expansion Slots

Expansion slots are receptacles inside a system unit that printed circuit boards (expansion boards) are plugged into. Computer buyers need to look at the number of expansion slots when they buy a computer, because the number of expansion slots decides future expansion. In microcomputers, the expansion slots are directly connected to the bus.

Expansion Boards

Expansion boards are also called expansion cards, controller cards, plug-in boards, adapter cards, or interface cards. Expansion boards are printed circuit boards that have many electronic components including chips. They are plugged into expansion slots.
Expansion boards are connected to peripherals through ports located on the edge of expansion boards. Expansion boards include memory expansion cards (e.g., SIMM), I/O controller cards (e.g., SCSI Card), video display card, sound cards, communications cards, etc.

A port is an external connecting socket on the outside the computer. This is a pathway into and out of the computer. A port lets users plug in outside peripherals, such as monitors, scanners and printers.

Serial Ports

Serial ports are external I/O connectors used to attach modems, scanners or other serial interface devices to the computer. The typical serial ports use a 9-pin DB-9 or a 25-pin DB-25 connector. Serial ports transmit bits one after another on a single communications line. Serial lines frequently are used to link equipment that is not located close by.

Parallel Ports

Parallel ports are external I/O connectors on a computer used to hook up printers or other parallel interface devices. The parallel port uses a DB-25 connector. This port transmits several bits simultaneously. Parallel lines move information faster than serial lines do.

A bus is a data pathway between several hardware components inside or outside a computer. It not only connects the parts of the CPU to each other, but also links the CPU with other important hardware. The other important hardware includes memory, a disk control unit, a terminal control unit, a printer control unit, and a communications control unit. The capacity of a bus is expressed as bits. A larger capacity bus is faster in data transfer. For example, a 32-bit bus is faster than an 8-bit bus.

Three Main Bus Architectures

• ISA (Industry Standard Architecture): ISA is pronounced i- suh. This is the original PC bus architecture. It includes the 8-bit (PC, XT) and 16-bit (AT) buses in IBM personal computer series and compatibles. Now, it refers specially to the 16-bit AT bus.

• MCA (Micro Channel Architecture): A 32-bit bus used in IBM P/S 2 series and other IBM models. This architecture allows multiprocessing that allows several processors to work simultaneously. Micro channel architecture is not compatible with PC bus architecture.

• EISA (Extended Industry Standard Architecture): EISA is pronounced eesa. This is a bus standard for PCs that extends the AT bus (the ISA bus) architecture to a 32-bit bus. This architecture also allows more than one CPU to share the bus. The purpose of EISA is to extend and amend the old ISA standard, so that all existing AT expansion boards can work with an EISA slot.

Local Buses

The performance of a microcomputer is often restrained by the relatively slow video cards and other peripherals, which cannot keep up with today's fast CPUs. A local bus reduces the performance gap between the high-speed microprocessors and slower hard disks, video boards and other peripherals.
There are two local-bus systems available today. Each bus hopes to boost microcomputer performance for I/O-intensive tasks. They are a VL-Bus and a PCI local bus.

• VL-Bus (VESA Local Bus): VL-Bus specification was introduced by the VESA (Video Electronics Standards Association). VL-Bus added peripheral components and connectors to the existing motherboard's 486 local bus and was available first. Performance of the VL-Bus architecture declines sharply when supporting more than two devices, and the specification is currently limited to a 32-bit data path and 33-MHz operation. This design is vanishing.

• PCI (Peripheral Component Interconnect): A PCI chip set adds a 64-bit-wide bus between the microprocessor and peripherals to offer a 64-bit data path. This chip supports speeds of 66-MHz. PCI can transfer data either 32- or 64-bits at a time. This architecture is developed by Intel, Compaq, DEC, IBM and NCR. PCI technology incorporates a managing layer to route and manage data for efficient handling of high-speed data transfers between the microprocessor and peripherals.
  Its design goals are to produce a low-cost, high-performance interface and support future generations of peripherals. PCI provides excellent compatibility, higher throughput and automatic configuration of peripheral cards. PCI also has features such as expandability and plug-and-play flexibility.

• Comparison: Both technologies employ a microprocessor's local bus instead of the system input/output bus to rapidly exchange data between the processor and peripherals. The VESA design reached the market first and is less expensive than PCI, but PCI is technically superior. A VL-Bus usually supports only two or three local-bus peripherals, while PCI can support up to 10 local buses. PCI uses fewer bus lines than VL-Bus. This enables PCI to eventually cost less to manufacture. PCI is now dominating the market.

Secondary storage means external storage. This storage device includes a magnetic disk, optical disk, magnetic tape and others. The most widely used external storage is as follows:

Logical Data Elements

If there is no systematic way to store and retrieve data, it is too difficult to get any information from an information system. Therefore, data resource should be organized in some logical manner. Data are logically organized into characters, fields, records, files, and database.

• Character: A character is the most elementary logical data element, whereas bit and byte are basic physical storage elements. A character consists of a single alphabetic letter, numeric digit, or special symbol. The character is equivalent to a byte.

• Field: The next higher level of data is the field. A field consists of a grouping of related characters. For example, a set of characters in a customer's name makes a name field. A field represents an attribute of an entity.

• Record: A record is a collection of related fields. The record represents a collection of attributes that describe an entity.

• File: A set of related records makes a file. For example, a customer file consists of many records of customers.

• Database: A database is an integrated collection of logically related files. A database combines several records previously stored in separated files.

Floppy disks are removable, direct access storage media inserted into disk drives. Floppy disks are flat, circular pieces of Mylar plastic that rotate within a jacket. These are also called flexible disks, floppies, Diskettes or simply disks.
Data and instructions are stored as forms of bits and bytes using the ASCII or EBCDIC data coding schemes. They are stored as electromagnetic charges on a disk surface. The two most common disks are 5 1/4-inch (5.25") and 3 1/2-inch (3.5") in their diameter. More efficient size, high storage capacity and sturdier design of a 3.5"-disk make microcomputers now use the 3.5" disks more.

Tracks/Sectors

• Track: The disk surface is divided into several concentric circles called tracks. The thinner the tracks, the more storage capacity of the disk. Data are recorded as tiny spots on these tracks. These tracks are closed concentric circles, not a single spiral. Each track has the same number of bits although the outer tracks are longer than the inner ones.

• Sector: The circular tracks are further divided into wedge-shaped sections known as sectors. The fields of data within a particular record are organized according to tracks and sectors on a disk. - graph

Hard / Soft Sectored Disks and Formatting

There are two kinds of sectors for a disk. A hard sectored disk is one and a soft sectored disk is the other. The hard sectored disk identifies sectors with some physical marks on the disk, whereas the soft sectored disk identifies sectors with sector identification contained in the recording tracks.
A format program places the sector identification on the disk. When a user buys a disk manufactured without tracks and sectors in place (this kind of disk does not have "Formatted" label on it), he or she must put the tracks and sectors using a format program.

Capacity

• DS/DD (2S/2D) Disk: DS/DD means Double Sided Double Density. This is a floppy disk recorded on both of its sides (DS) and has twice the capacity of the prior format (DD). For the IBM and IBM compatible PCs, DD/DS disk usually has 40 tracks on each side and each side contains 9 sectors of 512 bytes each.
  DS/DD: 360KB
  DS/DD: 720 KB (PCs), 800 KB for Mac formats.

• DS/HD Disk: DS/HD means Double Sided High Density. An HD disk has increased storage capacity usually with more tracks per square inch. For IBM and IBM compatible PCs, an HD disk is usually formatted to have 80 tracks on each side. Each side then contains 18 sectors of 512 bytes each - double capacity of a DD disk.
  DS/HD: 1.2MB
  DS/HD: 1.44 MB (PCs), 1.44 MB for Mac formats.

Floppy Drives

A floppy drive grabs a disk at its center and spins it inside its plastic jacket. The floppy drive obtains stored data and instructions from a floppy disk and stores them onto the disk. The drive is made up of a box with a slot into which a user inserts a disk. The slot has a drive gate. This drive rotates the disk with a motor inside the drive. Electronic read/write heads "read" data from the disk and "write" data to it while the disk rotates.
A microcomputer usually has internal floppy drives inside the computer cabinet, but it sometimes has external floppy drive, a separate component outside the cabinet.

Working Process of Floppy Drive

• Inserting disk: Inserting a disk to let it work.
  
• Closing gate: After inserting, the drive gate should be closed. This places the disk around a spindle and holds it.
  
• Revolving: Spinning the disk inside the jacket.
  
• Reading/ Writing: Read- write heads on an access arm transfer data signals from disk to computer or computer to disk.
  
• Seek: Seek operation is to let the access arm move the read-write head to the requested track on the disk.
  
• Search: Search operation is to let the drive rotate the disk to the proper position.

Hard Disks (Hard Drives)

A hard disk is a magnetic disk made of metal and covered with a magnetic recording surface. Hard disks come in removable and fixed varieties that hold from several hundreds of megabytes to several gigabytes. They are tightly sealed to prevent any foreign matter from getting inside which causes head crash.

Interface Type

• IDE(Integrated Drive Electronics): An IDE interface has a disk drive that contains its own controller electronics. The IDE interface is also called an AT and XT interface. IDE-ready motherboards have a 40-pin socket that connects directly to an IDE drive eliminating the use of an expansion slot.

• SCSI (Small Computer System Interface): SCSI is pronounced scuzzy. This is an 8-bit-bus peripheral interface for up to seven peripherals. The SCSI bus allows any two devices to communicate at one time (host to peripheral, peripheral to peripheral). SCSI provides high-speed (4MB/sec.), parallel data transfer and multiple peripheral connections while taking only one expansion slot.

Access Time

• Access time: This is an average time taken to complete the transfer of data after the request instruction has been enacted. Today's fast hard drives have access times under 10 milliseconds (ms). Access time is made up of the following four times.

• Seek Time: This is the time taken to move an access arm to a certain track on a disk after the computer requests data. Most of the access time is made up of seek time.

• Head Switching Time: The time taken for changing from one read/write head to another to read from or write on another part on a disk.

• Search Time: It is also called rotational delay time. This is a time required for the read/write head to locate particular position on a track.

• Data Transfer Time: This is the time for data to be transferred from the disk to primary storage or vice versa.
  

Different Forms of Hard Disks

• Internal Hard Disk: Internal hard disk is made up of several metallic platters, a motor, an access arm and read-write heads sealed inside a container. An internal hard disk is looked like a part of a system unit inside a computer cabinet. There are two sizes of drives (5.25" and 3.5" in a diameter). 3.5" hard disks are faster because the access arm travels shorter distances across the diameter of the disk. Internal hard disks have advantages over flexible disks. They are high capacity and speed. The disadvantage of internal hard disks or hardcards is that they have only a fixed amount of storage and cannot be easily removed.

• External Hard-Disk Drives: This is a drive that is not built into the system cabinet of microcomputers. External hard disk drives are treated as peripherals. Using external hard disk drives, we can expand the hard disk capacity when all available drive bays are occupied.

• Hard Disk Cartridges: A cartridge is a removable storage module, so a hard disk cartridge contains disks in the module. They can be removed from a dock easily and can give fast access to large data. An internal or external dock is available.
  In internal hard disks and external hard disk drives, the storage capacity is fixed, but in the hard disk cartridge, the capacity limitation of storage does not exist. That is, a user may add more cartridges any time.

• Removable Drives with Cartridges: Today's advanced technology allows a new form of removable storage. The technology combines the function of the hard disk drive and the convenience of the hard disk cartridge. This form consists of a removable drive and several cartridges.
  The removable drive looks like an external floppy disk drive. And the cartridge resembles a floppy disk and allows users to add 100 MB or 1 GB at a time. It allows an SCSI connection as well as a parallel port connection. Its low cost is a big advantage.

More advanced technology created a new disk storage forms. Optical disk is a disk written and read by laser beam. This optical disk has a great impact on today's storage technology. Optical disk does not spin, does not need to move access arms and read/write heads, because a laser beam can be moved electronically. The capacity of the storage is considerably greater than their magnetic disk counterparts, and optical disk storage may eventually replace all magnetic tape and disk storage.
Then how do they work? To write data, a laser beam burns tiny cavities into the surface of a disk to mark bits for data. To read the data, a laser beam scans these areas. There are three forms of optical disks available:

• CD-ROM: CD-ROM (compact disk read only memory) is an optical disk storage that contains text, graphics and hi-fi stereo sound. CD-ROM is a 4.75-inch optical disk storage that can store around 650 MB of data. CD-ROM disk is almost the same as the music CD, but uses different forms of track for data. A CD- ROM drive can read music CD, but a CD player cannot read CD-ROM. CD-ROM is a read-only disk that cannot be written on or erased by the user. In CD- ROM standard, data (text or pictures) cannot be viewed with audio play simultaneously. CD-ROM XA standard can do.

• WORM: A WORM (write once, read many) disk is an optical disk that written on just once by the user's environment and then cannot be overwritten. A WORM disk is ideal for use as archive because it can be read many times, but the data cannot be erased. The storage capacity of WORM disk ranges from 400 MB to 6.4 GB.

• Erasable Optical Disks: This is an optical disk that can be erased and written on repeatedly. An erasable optical disk has a great deal of data capacity. It can store up to 4.6 GB. An erasable optical disk functions like a magnetic disk and has huge capacity, so it will replace the magnetic disk in the future.

A magnetic tape is a tape coated with a magnetic material on which data can be stored. This is a sequential storage device that is usually used for a backup purpose. A magnetic tape is slower than direct access storage such as disk, because it is sequential access storage. The biggest advantage of magnetic tape is the cost. It is much less expensive than magnetic disk and optical disk. An advanced tape backup technology (DAT) is available these days.

A cache is a specially designed buffer storage used to improve computer performance by reducing access time. It holds instructions and data that are likely to be needed for next operation by the processor. The cache copies frequently accessed data and instructions from primary storage (main memory) or secondary storage (disks).

• Disk Cache: A disk cache is in a reserved segment of primary memory or in an extra memory on the disk controller card. It contains a large block of frequently accessed data copied from a disk. The data in a disk cache can be used to fulfill the following data requests from a processor in a high speed. The disk cache lets the processor avoid a slow disk access.

• Memory Cache: A memory cache is a high-speed memory storage between memory and the CPU. It is smaller and much faster than main memory (primary storage). The memory cache copies blocks of instructions and data from the main memory so that execution and data updating are performed in the higher-speed memory bank.

An input device is a peripheral device that converts symbols that people understand into bits that computers can process. An input device includes a keyboard, a terminal, a touch screen, a mouse, a scanner, etc.

Keyboard Entry

In keyboard entry, a user types characters, numerics and special symbols using a keyboard. The input usually appears on a monitor. A keyboard entry might be the most common way to input data.

Keyboards

A keyboard is a device used to encode data by key depression, which enters information into a system. The keyboard converts alphabets and numbers, and other special symbols into electrical signals that processor can understand and process. These signals are sent to the computer's CPU. There are three different layouts.

• QWERTY: This is a standard keyboard layout. QWERTY indicates the arrangement of the upper left corner six letters in the first row of the alphabetic keys.

• AZERTY: This is a keyboard layout that is similar to the QWERTY layout and some European countries use this keyboard. This layout is slightly modified from the QWERTY keyboard.

• Dvorak: This is another keyboard layout modified greatly from a standard layout. The keyboard is devised to increase typing speed by placing frequently used keys more naturally. In the past, mechanical jams were a problem in typing. Thus, the standard keyboard layout was designed to limit typing speed. Interesting, huh?

Terminals

A terminal is an INPUT/OUTPUT DEVICE that usually includes a keyboard for input, a video display for output, and a communications link to send and receive information. There are three different types:

• Dumb Terminal: This is an input/output terminal that does not have a capability of processing. It only enters and receives data without processing.

• Smart Terminal: This terminal has some processing capability. It has a small memory. It performs some editing of data before sending them to a main computer.

• Intelligent Terminal: This is a terminal that has a full processing capability. The terminal has a processing unit, primary storage. It may or may not have local storage. Recently, most intelligent terminals have local disk. An intelligent terminal is actually a microcomputer with communications capability.

Direct input is a data entry form that does not use a keyboard to input data. Today, more data and instructions are entered a CPU of microcomputers directly using direct input devices used to provide a more natural user interface. These entry devices reduce users' typing errors. While direct input is an advanced fourth generation data entry form, voice input (speech input) is the mode of next generation input technology. There are many forms of direct input devices:

• Mouse
  
• Touch screen
  
• Light Pen
  
• Graphics Tablet
  
• Scanner
  
• Bar-code readers
  
• Magnetic Entry
  
• Voice-Input Devices

Mouse

An object used as a pointing and drawing device. The mouse usually has a ball and buttons and is connected to the system unit through serial port. As a mouse is rolled across the flat desktop in any direction, it locates the pointer correspondingly on the screen. Then it issues commands using the selection buttons on the mouse. Many portable microcomputers such as lap-tops use trackballs instead of mice.

Touch Screen

A touch screen is a monitor screen that allows users to interact with a computer system by touching an area of the display screen. The screen is covered with a clear plastic layer that has a matrix of cells. It uses invisible beams of infrared light. A user touches a graphic button that displays option on the screen. Touch screens are easy to use. Thus, many kiosks use touch screens as input forms.

Light Pen

A light pen is a light-sensitive pen-like device used by pointing it at the display surface. A user brings the light pen to the desired point on the screen and presses a button, causing it to identify the current location. It is used to select options from a menu or to draw images.

Digitizer Tablet

A digitizer tablet is also called a graphics tablet or just a digitizer. The digitizer is a drawing tablet used to sketch new images or trace old drawing or photograph. The user uses a pen-like device called a cursor to draw images. Designers and architects usually use digitizers. Light pen and digitizer technologies are used for pen-based computing.

Scanner

A scanner is a device that reads spatial pattern such as images, graphics and texts, and then generates digital signals of that pattern. Converted digital data may be processed by a computer, stored in a disk, printed by a printer or displayed on a monitor. Scanners are commonly used to capture graphic images that can then be placed in a page or on any document.

Scanners usually include optical character recognition (OCR) software so that scanners can read and capture texts directly through optical scanning.

Bar Code Readers

A bar code is a specialized code represented by sets of parallel bars of varying thickness and separation. This is used for fast identification of items with an optimal scanner. The optical scanner is called a bar code reader. The bar code reader is a photoelectric scanner that read the bar code.

Magnetic Data Entry

There are two technologies in magnetic data entry. A magnetic ink character recognition (MICR) technology reads iron oxide ink preprinted or encoded on checks, deposit slips or on documents. An MICR reader electronically captures data, by first magnetizing the magnetic ink characters and then sensing the signal.
Another form of magnetic data entry is the magnetic stripe technology that makes computers read credit cards. The dark magnetic stripe on the back of credit cards is the iron oxide coating. A magnetic stripe reader reads this magnetic stripe.

Voice Input Devices

Voice input devices are also called speech- recognition devices or voice-recognition systems. This device uses a voice recognition technology that converts a user's speech into a digital code. Spoken words are first digitized and then matched against a dictionary of patterns previously stored in the computer.
Speaker-dependent systems should be trained by taking actual user's word sample before using, but speaker-independent systems can recognize only limited vocabularies. The advantage of the voice input systems is that they enable users to keep their hands free for other tasks.

An output device is any peripheral device that converts machine-readable information into people-readable form such as a monitor, printer, plotter and voice output device.

A computer display is also called a display screen or video display terminal (VDT). A monitor is a screen used to display the output of a computer. Images are represented on monitors by individual dots called pixels. A pixel is the smallest unit on the screen that can be turned on and off or made different shades. The density of the dots determines the clarity of the images, the resolution.

• Screen resolution: This is the degree of sharpness of a displayed character or image. The screen resolution is usually expressed as the number of columns by the number rows. A 1024x768 resolution means that it has 1024 dots in a line and 768 lines. A smaller screen looks sharper on the same resolution. Another measure of display resolution is a dot pitch.

• Interlaced/Non-interlaced: An interlaced technique refreshes the lines of the screen by exposing all odd lines first then all even lines next. A non-interlaced technology that is developed later refreshes all the lines on the screen form top to bottom. The non- interlaced method gives more stable video display than interlaced method. It also requires twice as much signal information as interlaced technology.

There are two forms of display: cathode-ray tubes (CRTs) and flat-panel display.

Cathode Ray Tubes (CRT)

A CRT is a vacuum tube used as a display screen for a computer output device. Although the CRT means only a tube, it usually refers to all monitors. IBM and IBM compatible microcomputers operate two modes unlike Macintosh based entirely on graphics mode. They are a text mode and a graphics mode. Application programs switch computers into appropriate display mode.

Monochrome Monitors

A monochrome monitor has two colors, one for foreground and the other for background. The colors can be white, amber or green on a dark (black) background. The monochrome monitors display both text and graphics modes.

Color Monitors

A color monitor is a display peripheral that displays more than two colors. Color monitors have been developed through the following paths.

• CGA: This stands for Color Graphics Adapter. It is a circuit board introduced by IBM and the first graphics standard for the IBM PC. With a CGA monitor, it is harder to read than with a monochrome monitor, because the CGA (320 X 200) has much fewer pixels than the monochrome monitor (640 X 350). It supports 4 colors.

• EGA: It stands for Enhanced Graphics Adapter. EGA is a video display standard that has a resolution of 640 by 350 pixels and supports 16 colors. EGA supports previous display modes and requires a new monitor.

• VGA: VGA stands for Video Graphics Array. This is a video display standard that provides medium to high resolution. In a text mode, the resolution of this board is 720 by 400 pixels. It supports 16 colors with a higher resolution of 640 by 480 pixels and 256 colors with 320 X 200 pixels.

• Super VGA: This is a very high resolution standard that displays up to 65,536 colors. Super VGA can support a 16.8 million colors at 800 by 600 pixels and 256 colors at 1024 by 768 pixels. A high-priced super VGA allows 1280 by 1024 pixels. Larger monitors (17" or 21" and larger) with a high resolution of 1600 by 1280 pixels are available. VESA (Video Electronics Standards Association) has set a standard for super VGA.

Flat Panel Displays

Portable computers such as a lap top use flat panel displays, because they are more compact and consume less power than CRTs. Portable computers use several kinds of flat panel screens:

Liquid-Crystal Displays (LCDs)

A display technology that creates characters by means of reflected light and is commonly used in digital watches and laptop computers. LCDs replaced LEDs (light emitting diodes) because LCDs use less power. LCDs are difficult to read in a strong light, because they do not emit their own light. Portable computers wanted to have brighter and easier to read displays. Backlit LCDs are used for the purpose now.

• Backlit LCDs: This is a type of LCD display having its own light source provided from the back of the screen. The backlit makes the background brighter and clear, as a result the texts and images appear sharper. However, this still is much less clear than CRTs. Thus, better technology is needed.

• Active Matrix LCDs: This is an LCD display technique in which every dot on the screen has a transistor to control it more accurately. This uses a transistor for each monochrome or each red, green and blue dot. It provides better contrast, speeds up screen refresh and reduces motion smearing.

Electroluminescent (EL) Displays

A flat panel display technology that actively emits light at each pixel when it is electronic charged. This provides a sharp, clear image and wide viewing angle. The EL display type of flat panel is better than LCD.

Gas Plasma Displays

This is also called a gas panel or a plasma panel and is another flat screen technology. A plasma panel contains a grid of electrodes in a flat, gas filled panel. The image can persist for a long time without refreshing in this panel. The disadvantages of the gas plasma displays are that they must use AC power and cannot show sharp contrast.

A printer is an output device that produces a hard copy of data. The resolution of printer output is expressed as DPI. Printers can be classified into different types in several ways. First, the printers can be divided into three categories by the way they print.

• Serial Printers: Also called a character printer. Print a single character at a time. They are usually inexpensive and slow.

• Line Printers: Print a line at a time. They are expensive and very fast. Line printers use a band, a chain, etc.

• Page Printers: Also called a laser printer. Print a page at a time. They usually use a laser to produce page images. Quality is best. This is a little bit expensive, but the price of the personal laser printer is decreasing. The price range of the personal laser printer is around $400, today.

Second, printers can be classified into two forms according to the use of a hammer.

• Impact Printers: Hammer hits ribbons, papers or print head. Dot-matrix and daisy-wheel printers are the example. Noisy.

• Nonimpact Printers: They do not have the hammer and do not hit. An example is an ink-jet and laser printer.

Another classification can be made by the way they form characters.

• Bit-Mapped Printers: Images are formed from groups of dots and can be placed anywhere on the page. They have many printing options and good printing quality. They use PostScript as a standard language for instructing a microcomputer.

• Character-based Printers: Printer print characters into the lines and columns of a page. These printers use predefined set of characters and are restricted in position of characters.

Microcomputers use five kinds of printers. They are daisy wheel printers, chain printers, dot-matrix printers, ink-jet printers, and laser printers.

Daisy-Wheel Printer

Daisy-Wheel is a printer mechanism that uses any kind of hub (wheel) having a set of spokes at the margin of the hub. The wheel can be removed to use a different character set. The end of each spoke is a raised image of a type character. When the wheel is turned and the required character is aligned to the print hammer, the character is then struck into a ribbon and onto a paper with the hammer.
Daisy-Wheel Printer prints typewriter-like very high quality characters. However, they are slower and less reliable than dot-matrix printers. Microcomputer users seldom use this printer, because the better dot-matrix printers and inexpensive laser printers are available today.

Chain Printer

A chain printer uses a printing mechanism that uses character typefaces linked together in a chain. The chain spins horizontally around a set of hammers aligned with each position. When the required character is in front of the selected print position, hammer in that position hits the paper into the ribbon against the character in the chain.
This printer is not commonly found around microcomputers, because it is a very expensive, high-speed machine designed originally for mainframes and minicomputers. Chain printers are very reliable and can speed up to 3000 lines per minute.

Dot-Matrix Printer

Dot-matrix printers are printers that write characters and form graphic images using one or two columns of tiny dots on a print head. The dot hammer moving serially across the paper strikes an inked-ribbon and creates images on paper.
Dot matrix printers are popular printers used with microcomputers, because the printers are highly reliable and inexpensive. They are used for tasks where a high-quality image is not essential. Many users, however, move from dot printers to laser printers, because the price of laser printers is falling down. Several kinds of dot matrix printers are available with print heads that have 7, 9, 18, or 24 pins.

Ink-Jet Printer

Ink-jet is a printer mechanism that sprays one or more color of ink at high speed onto the paper and produces high-quality printing. This printer also produces color printing as well as high-quality image. That is, ink-jet printers can be used for variety of color printing at a relatively low cost. Ink-jet printing has two methods: Continuous stream method and drop-on- demand method.

Laser Printer

A laser printer is a printer that uses the electrophotograpic method used in a copy machine. The printer uses a laser beam light source to create images on a photographic drum. Then the images on the drum are treated with a magnetically charged toner and then are transferred onto a paper. A heat source is usually applied to make the images adhere.
In 1984, Hewlett-Packard introduced the first desktop laser printer, called the LaserJet. The laser printer revolutionized personal computer printing and has spawned desktop publishing. The laser printer produces high-resolution letters and graphics quality images, so it is adopted in applications requiring high-quality output. Although a high-priced color laser printer is also available in the market, a less expensive, desktop gray scale laser printer is widely used. Recently, the laser printer is gaining its market share dramatically, mainly because the lowered price and the quality.

A plotter is a special-purpose output device that draws images with ink pens. That is, the plotter is a graphics printer for making sophisticated graphs, charts, maps, and three-dimensional graphics as well as high-quality colored documents. It can also produce larger size of documents.
Plotters require data in a vector graphics format that can produce images with a series of lines. There are two main types of plotters:

• Drum Plotter: This is a plotter that has a drum. A paper wraps the drum that rotates to produce plots. Pens in a drum plotter move across the paper while the drum is turning. A drum plotter is usually used to produce smaller drawings.

• Flatbed Plotter: This is a plotter that has a bed. This is also called a table plotter. The plotter draws graphics on the paper placed on the bed. There are several size of beds. This plotter is usually used for producing large drawings.

This device produces a human speech like sound, but actually is prerecorded vocalized sounds. Voice output is used in the telephone information system, where the requested number is reported using a voice output system.
For example, when a student enrolls courses using a telephone registration system, he or she hears voice output upon your request. Voice output is becoming common in voice messaging systems.

Another form of the output device is a modem. A modem is short for "MOdulator-DEModulator." Modulation is the process of converting from digital to analog. Demodulation is the process of converting from analog to digital.
The modem enables digital microcomputers to send output through analog telephone lines. Both voice and data can be carried over through the modem. The modem is not only an output device but also an input device that receives data and voice through a communication channel.