The work of a computer can be characterized by an input-process-output model in which a program receives input from an input device, performs some processing on the input, and produces output to an output device. Users employ a variety of input devices to interact with the computer, but most user interfaces today are based upon a keyboard and a mouse pointing input device.
A keyboard consists of a number of switches and a keyboard controller. The keyboard controller is built into the keyboard itself. When a key is pushed, a signal called a scan code is sent to the controller. A different scan code is sent when the key is released. The use of two scan codes allows keys to be used in combination. The controller is able to tell whether a key is being held down while another key is struck, or to determine when a key causes a repeated action. Keyboard scan codes are sent to the computer via a serial port. New keyboards have been designed for ergonomic reasons.
A mouse is a small device that a computer user pushes across a flat surface, points to a place on a display screen, then clicks on icons and menus. The mouse first became a widely used computer tool when Apple Computer made it a standard part of the Macintosh. Today, the mouse is an integral part of the graphical user interface (GUI) of any personal computer.
Types of Input Devices
Hundreds of devices can be used as computer input devices, ranging from general-purpose input devices to special-purpose devices used to input specific types of data.
A digital camera records and stores photographic images in digital form that can be fed to a computer for viewing and printing. First, the impressions are recorded or stored in the camera. The picture can then be downloaded to a computer by removable disk or by parallel port connection.
A light pen uses a photodetector in the tip of the pen, which can be moved around the screen to move a corresponding cursor.
Magnetic Stripe Readers and Magnetic Tape.
Magnetic stripe readers are used to read alphanumeric data from a magnetized stripe on a card, such as a credit card. Magnetic tape can be easily rewritten and stored indefinitely. There are two basic tape mechanisms: reel-to-reel and cartridge. Generally, reel-to-reel tape drives are used with large mainframe computers . Smaller computers use tape cartridges. Regardless of type, the tape is removable from the tape drive for offline storage. When the tape is in the tape drive ready for operation, it is said to be mounted. Tape heads store bits across tracks in units called frames, with the most common recording densities being 630 frames per centimeter (1,600 frames per inch) and 2,460 frames per centimeter (6,250 frames) per inch.
Punching paper tape for storage and data input is an old technique dating back to Sir Charles Wheatstone, who used it in 1857 for the telegraph. Small sprocket holes appear along the length of the tape to feed the tape mechanically. Data are recorded on the paper tape by punching holes in a row across its width. Each row represents one character, and the pattern of holes punched indicates the particular character. Although paper tape is inexpensive, the difficulty of correcting errors and the tape's slow speed have led to its disappearance as a computer input device.
Punched cards, popularly known as IBM cards, were the dominant input device prior to the introduction of personal computers. Punched cards are produced by a keypunch machine offline from the computer, and then read into it with a high-speed card reader. Punched cards use a Hollerith code, named after its inventor, Herman Hollerith (1860–1929). Each card has eighty columns with one character per column; therefore each punched card holds eighty characters or exactly one row of text or characters—a unit record. Punched card readers can operate at speeds from 100 cards per minute to 2,000 cards per minute.
Page and hand-held scanners convert black/white and color images into digital data. A bar code scanner uses a laser scanner to read alphanumeric bar-coded labels. Two types of optical data scanners can be used to scan documents: optical mark recognition (OMR) and optical character recognition (OCR). OMR is used in standardized test scoring and surveys where marks are placed in designated boxes. OCR readers use reflected light to convert typed or handwritten documents into digital data. In magnetic ink character recognition (MICR), data are placed on the bottom of a form using special magnetic ink so that they can be scanned by computers.
Voice Recognition Devices.
Voice recognition devices use microphones and special software to convert a human voice into language that can then be put into digital form. These systems require training the computer to recognize a limited vocabulary of standard words for each user. Voice input devices may be the input device of the future, but progress has been slow.
Past and Future
Until the advent of stored program computers in the 1950s, punched card machines were the state-of-the-art. Dating back to the U.S. Census of 1890, Hollerith developed a punched card reader that could repeatedly tabulate and sort. The key event that signaled the end of the punched card era was the launch in 1959 of the IBM 1401 computer, which had magnetic tape and disk storage. The keyboard has dominated interactive personal computing and voice input devices have not proven to be reliable. Future hands-free wireless infrared input devices may include a wand with a walk-around button, a ring on the index finger, and a reflective dot that sticks to the user's forehead and signals data by eye movement.
see also Computer System Interfaces; Games; Reading Tools.
William J. Yurcik
White, Ron. How Computers Work, 6th ed. Indianapolis, IN: Que, 2002.