Input is data that goes into the computer and output is data that comes out of the computer. There are several types of devices used for sending information from a person to a computer as well as from the computer to the person. Below are examples of both.
Generally, we think of input/output as devices which allow us to communicate with the computer. But, the PC has a lot more I/O to direct inside of it. Millions of bits of data are travelling through the circuitry in the motherboard. Input/Output controllers, which work like street lights, make sure that all the data transfer inside the computer doesn't cause a crash. The bus acts like an intricate set of highways for the data to travel on. It transports data among the processor and other components. The bus also includes many microchips, and slots in which we plug expansion circuit boards, called adapter or expansion cards. The main types of bus circuitry are the ISA (Industry Standard Architecture), MCA (Microchannel Architecture), and the EISA (Extended Industry Standard Architecture). The ISA bus by IBM could move data 8 bits at a time which was relatively slow. But in IBM's PS/2 computer in 1987, they gave it the MCA bus which could handle 32 bits of data at a time. Compaq and six other IBM competitors invented the EISA bus which could also move 32 bits at a time, but it was also compatible with ISA cards.
In 1992, manufactures averted their attention to increasing bus speed, rather than increasing the amount of bits of data at a time. EISA and MCA were still operating at 8.22 and 10 megahertz respectively. So the local bus was invented. As the name implies, the local bus gave the processor direct access to expansions slots, giving the slots "local" access. This way, the local bus communicates with the processor at almost the same speed as the processor's speed. There are two versions of the local bus. One is VESA (Video Electronics Standards Association), created by an alliance of PC vendors, which accelerates video displays with working speeds up to 50 Mhz. Intel and other big PC companies developed PCI (Peripheral Component Interconnect) local bus. PCI allows speeds only up to 33 Mhz, but it is a more comprehensive design that is the first to incorporate Plug and Play. Even though it has a slow speed, it can move 132 megabytes a second compared to to VESA's 107MB/sec.
Serial ports have a simple underlying concept. They have one line to send data, another line to receive data, and a few others regulating the way the data is sent over the other two lines. The most common uses for serial ports are the mouse and the modem. This is because the serial port is an inefficient way to transfer data. It can only send one bit at a time, greatly reducing the transfer rate. Mice transmit very little data so speed is not crucial, making the serial port a good idea. Most phone lines can only carry one signal at a time, so serial ports are perfect for modems. The serial port is sometimes referred to as the RS-232 port. RS-232 is the Electronics Industries Association's designation for how the different connectors in a serial port are to be used. But 9-pin and 25-pin connectors are in the market, so the RS-232 standard still has to be decided.
Parallel Ports are much faster than serial ports. They can send several bits of data over eight wires simultaneously. This means that a parallel port can send a byte in the same amount of time it takes a serial port to send a bit. There is a disadvantage to parallel ports, though. All of the voltages in the lines can create a cross talk, where the voltages leak from one line to the to other. The longer the cable there is, the more the cross talk. So parallel connections are limited to 10 feet. Printers are devices that need to be connected to the computer through a parallel connection. This is because they need large amounts of data for printing graphics and scalable fonts. Parallel ports are also used in transporting files between PC's. Portable computers, which often lack expansions slots, can parallel connections to external drives and sound generators. But built in circuitry and PC Cards are taking over many tasks the laptop needs the parallel ports for.
PC Cards are the new marvel of computer technology. They are barely the size of a credit card and can be anything from a modem or network adapter to a hard disk! Almost all portables contain PC Cards because of their miniature size and weight. But if you have a desktop computer with a PC Card slot, you can share the card between the two. If you want to take your files on a trip with you, just take the card out of the PC and slip it into the laptop.
PC Cards were originally called PCMCIA Cards, which stood for Personal Computer Memory Card Interface Association.
When the PC Card is manufactured, most of its circuitry may be installed in the form of RAM, in which case it is designed for use as a solid state drive. The circuit may function as a modem, network adapter, or other peripheral. Some PC Cards may contain actual spinning hard drives.
The PC Card has non-volatile memory, which doesn't lose its memory when the power is turned off. In its memory, the PC Card stores information about its configuration so that jumper and DIP switches need not to be changed to set it up. The information determines how the laptop will access the card, and the data is coordinated with a device driver that's run on the laptop during boot-up. This will allow the laptop to have access to the card. Device drivers are software extensions to the operating system which lets DOS or Windows know how to communicate with different pieces of hardware. Memory and input/output memory registers, which hold the data the card is working with, are individually mapped to addresses in a memory window used by the laptop. Memory addresses let the PC know where to store or find information in RAM. By changing the addresses mapped to the window, the card's driver may use the window to link to different memory locations in RAM.
On the laptop motherboard, there is a controller chip which links the signals coming from the card to the laptop's circuit board and RAM.
Through the window of memory locations, the card's driver lets the operating
system indirectly access the card. Any data the laptop writes to the memory
window goes to the appropriate memory location in the card. Data from the card
is placed in the window for the laptop to use.
From microphones or other equipment, a sound card receives sound in its native format. This is a continuous analog signal of a sound wave that contains constantly changing frequencies and volumes. These signals go to an Analog to Digital Convertor (ADC), which changes the analog signal to binary 1s and 0s. A ROM chip contains the information on how to handle the digital signal. Newer computers have EPROM (erasable, programmable read only memory). This allows it to be updated as newer versions are made. The ADC sends the binary information to a Digital Signal Processor (DSP), and this processor relieves the main processor by working with the sounds itself. The DSP gets instructions from the ROM chip, and its typical job is to compress the data to take up less space. The compressed data is sent to the main processor, and it is then sent to the hard drive to be stored.
To play a recorded sound, the CPU retrieves the file containing the digital format of the sound from a hard drive or a CD-ROM, and it sends it to the DSP. The DSP decompresses the sound and sends it to the Digital to Audio Convertor (DAC), which changes the digital sound to analog sound. The analog current is sent to the speakers, where it is amplified and is heard as a series of vibrations.
A camera and microphone capture the video and sound in analog form and send them to a video-capture adapter board. The board only captures about 15 frames per second to reduce the data to be processed. On the adapter card, an Analog to Digital Convertor (ADC) converts the wavy analog signals to binary language. A compression/decompression software reduces the amount of data needed to re-create the video signals. For example, MPEG (Motion Pictures Expert Group) compression, which can display movies full screen, only records key frames. It predicts what the missing frames look like by comparing the changes in the key frames. Microsoft Video for Windows looks for repetitious information and reduces it. Instead of saving each pixel of a large expanse of a single color, it saves the color and the directions of where to use it.
The monitor adapter card receives digital signals from the operating system environment or application software. The signals go through a chip on the card called a digital-to-analog convertor (DAC), which actually contains three DACs; one for each of the primary colors: red, green, and blue. The DAC compares the digital values given by the PC to a table which contains the matching voltage levels for each of the primary colors. The three combined colors form a pixel on the screen. The table for a VGA monitor contains values for up to 262,144 possible colors, of which 256 values can be stored in the VGA adapter's memory at one time. Super VGA have more memory, and thus have more colors and better resolution.
The adapter sends signals to three electron guns at the back of the monitor's cathode ray tube (CRT). The electron guns shoot streams of electrons for each primary color, and the intensity of each stream is controlled by the adapter's signals.
The adapter sends signals to a mechanism in the neck of the CRT, called a magnetic deflection yoke. The yoke focuses and aims the electron beams using electromagnetic fields. The signals sent to the yoke help determine the monitor's refresh rate (the rate of how quickly the screen's image is redrawn), and its resolution (the number of pixels horizontally and vertically).
The beams pass through a metal plate called a shadow mask, which is a metal plate with minute holes. The purpose of it is to keep the electron beams precisely aligned. The CRT's dot pitch is how close the holes are to each other. The closer the holes, the smaller the dot pitch, and the sharper the image.
The inside of the screen is coated with three different types of phosphor; one for red, one for green, and one for blue. Phosphors are materials which glow when hit by electrons. The higher the intensity of a beam of electrons, the more light the phosphor emits. To create different colors, the intensity of each beam of electrons is varied. After a beam leaves a phosphor dot, it glows briefly. It must be reactivated by repeated scans of electron beams.
There are two ways a monitor can be scanned. One way is called raster scanning, where every line is scanned from top to bottom, one by one. The second way is called interlacing. This is where every other line is scanned first, and the missed lines are scanned second.
A modem is a device that connects your computer to a phone line. They can be either internal or external. First, communications software sends a voltage along pin 20 of the serial port that the modem is connected to. This voltage is called a Data Terminal Ready signal (DTR). The DTR tells the modem that the computer is turned on and is ready to use the modem to transmit data. At the same time, the PC detects a voltage from the modem on pin 6. This is the Data Set Ready signal (DSR). The DSR lets the computer know that the modem is ready to receive data or instructions. In order for anything to happen, both signals must be present.
Once the two signals are present, the communications software sends a command over line 2 (the Transmit Data line) to the modem. This tells the modem to open a connection with the phone line (this is also known as going off hook). The communications software then sends the modem tones or pulses needed to dial a specific phone number. The modem acknowledges the command by replying to the computer on line 3 (the Receive Data line).
When the remote modem at the other end of the call answers, the local modem sends a hailing tone to let the remote modem know that it is being called by another modem. The remote modem then responds with a higher pitched tone. When communications have been established, the local modem sends the computer a Carrier Detect (CD) signal on line 8. This signal informs the communications software that the modem is receiving a carrier signal, which is a steady tone of a certain frequency and which later will be modulated to transmit data.
Next, the modems handshake. This process consists of the two modems exchanging information about how the will send data to each other. The modems agree on such information as the transfer speed. When the communications software wants to send data, it sends voltage to line 4 on the serial port. This Request to Send (RTS) signal asks if the modem is free to receive data from the computer. If the modem is receiving remote data it wants to pass on to the computer while the computer is busy doing somethings else, the computer will suspend the RTS signal to tell the modem to stop sending it data until the computer finishes its other work and restarts the RTS signal.
The modem is always sending the computer a Clear to Send (CRT) signal on serial port line 5, unless it is too busy handling other data. The computer responds by sending the data to be transmitted on line 2. The modem sends received data to the computer through line 3. If the modem can't transmit data as fast as the computer sends data to it, the modem drops the CTS signal until it catches up with the computer.
The remote modem hears incoming data as a series of tones with different frequencies. It demodulates these tones back into digital signals and sends them to the receiving computer. Both computers can send signals back and forth at the same time because the use of a standard system of tones allows modems on either end to distinguish between incoming and outgoing signals.
When the local computer breaks the communication, it sends the modem a command to hang up. When the remote system breaks the communication, the local modem stops sending the computer the Carrier Detect signal so the communications software knows that the communications have been broken.
In a decade or so, film cameras will probably be outdated and part of the past. In its place will be the digital camera. Just as color film consists of three different layers which react to three different colors, the PC's monitor fires three different colors at the screen. The best digital cameras have about 1.5 million pixels, each one being capable of detecting light. After the pictures have been taken, the images can be manipulated through software such as Hijaak 95 and Adobe Photoshop.
When the shutter of a digital camera opens, light passes through the lens the same way as it passes through a film camera. The image is focused on a chip called a charge coupled device (CCD). The CCD contains with many types of transistors that create electrical currents which are proportionate to the type of light striking them. The transistors make up the pixels of the pictures. If the camera detects color, many transistors contribute to one pixel. The transistors generate a continuous analog electrical signal which is converted into a digital format through the analog-to-digital convertor (ADC). The digital format contains the information in 1s and 0s. The digital information then goes to the digital signal processor (DSP) which has been programmed to manipulate the images. It adjusts the contrast and detail and compresses the pictures to take up less space. The image is then temporarily stored in the camera's RAM from which it is transferred to the computer through a serial or SCSI cable. The camera may also store it to a PC Card or a mini-floppy drive. The floppy or card can be transferred to the PC where it is copied to the hard drive.
The most common, useful, and original input device is the keyboard. It is a fairly flat, rectangular piece of plastic with buttons on it. There is a button for every letter of the alphabet, every digit in our base ten number system, a space key, enter key, and several special purpose keys to make navigating your way through the computer easier. Some of these special keys are called the function keys labeled F1,F2,F3...F12. What the function keys do depends on the program the computer is currently running. Each program defines each function key a certain command. On the right hand side of the keyboard' is the numeric key pad.
All keyboards have a typematic action, or are extremely touch sensitive. If a key is held down for more than half a second, the key will repeat approximately eight times.
When a key is pressed, it causes a change in current flowing through the circuits associated with that key cap. A microprocessor built into the keyboard constantly checks circuits leading to the key caps. When it detects an increase or decrease in current by a key that has been pressed, it can tell when the key has been pressed and when it was released. Depending on which key was pushed, the processor develops a scan code. Each character has two scan codes, one for when it was pressed, and one for when it was released. The processor stores the number in the keyboard's memory, and it loads the number in the port connection where the BIOS can read it. After the BIOS reads the scan code, it tells the keyboard that it can delete it from the keyboard's memory. Then, the BIOS chip translates the appropriate scan code into the appropriate ASCII code, which is used by the PC, that stands for a character or into a special key code for a function or movement key. The ASCII or special key code is stored in the BIOS's own memory buffer, where it waits to be retrieved by the operating system.
Mice make controlling complex programs much easier than a keyboard. Icons and mice give programs more flexibility and give faster control.
The mouse contains a ball which rotates when it is pushed. The ball turns two slotted wheels mounted at right angles. Each wheel has two light emitting diodes on one side and two photodiodes on the other side. When the wheel turns, light intermittently shines through the holes and produces an electrical signal in the photodiode. The frequency of signals from each wheel is computed by the computer, and the result is a cursor moving on the screen.
A scanner is used to load images into the computer that aren't already files. It works the same way as a photocopier, but the output is into the computer instead of another piece of paper. First, a light source illuminates the piece of paper to be scanned. The light bounces off the paper different amounts depending on how light the area was--blank or white spaces reflect more light than inked or colored areas do. Then, the scan head, moved by a motor, captures the light bounced off each individual area of the page. Each area is only about 1/90,000 inches squared!
From the head, the image travels among a series of mirrors that continually pivot to stay aligned with a lens. This lens focuses the beams of light onto light-sensitive diodes that translate the amount of light into an electrical current. The more light there is in an area, the more electric current it produces.
After being converted to electric current, an analog-to-digital converter(ADC) stores each reading of voltage as a digital pixel which represents a black or white area along a line that contains 300 pixels to the inch. Some scanners are more sophisticated and can translate the voltages into shades of gray. In the case of color scanners, the head makes 3 passes-one with each of the primary colors of light (red, green, and blue) before converting the image to electrical current.
Finally, the digital information is sent to software in the PC, where the data is stored in a format with which a graphics program or an optical character-recognition program can work.
A joystick is a device that serves about the same purpose as a mouse. This purpose is to move an object (pointer, etc.) around the screen. However, while the mouse is used for more general things such as getting around in graphic operating systems, the joystick is mainly used for games. It consists of a stick-like appendage which can be moved up, down, left, right, and anywhere in between. The input to the computer is based on a constant reading of the position of the stick relative to the center of the joystick. So it must be held in a direction for the correct time interval in order to move the pointer the correct amount.
In addition to the stick, joysticks contain buttons. There are normally 1, 2, or 4 buttons on a joystick, but any number isn't unheard of. Since joysticks are all different from each other, there is normally only 1 button which has a predefined function. All other buttons are normally programmable.
