Computer Input/Output, I/O Devices by Duane Bristow The Helping Hand BBS (606) 387-4002 An I/O device is anything connected to a computer CPU which allows the CPU to communicate with the outside world. The most basic I/O devices are the typewriter style keyboard by which a human can give the CPU commands and the monitor (TV screen) by which the computer can display information to be read by a human. Another very basic output device is the printer on which the computer can print information on paper. It must be understood though that a computer can operate just fine without either of these devices. For example, a computer could be set up to control a home furnace taking its input from a thermometer and its output being electrical signals to turn the furnace on or off. This would perform the same function as the familiar wall thermostat. The only difference between the thermostat and the computer would be that the computer operates electrically by use of an ALU while the thermostat is basically a mechanical device. In general I/O devices can be catagorized as those that are basically devices for storage of data such as floppy and hard disk drives and CDROM drives, and those that allow communications such as keyboards, printers, monitors, modems, scanners, mouses, LANs, and sound boards. Floppy disk drives use either 5 1/4 inch diskettes or 3 1/2 inch diskettes. They may be either double density or high density. Capacities in terms of bytes are as follows: 5 1/4 inch double density - 360K 5 1/4 inch high density - 1.2 megabytes 3 1/2 inch double density - 720K 3 1/2 inch high density - 1.44 megabytes A byte is enough space to store one alpha-numeric character. Thus there are about 3000 bytes in one typewritten page if the page contains 50 lines of 60 characters per line. 50X60=3000. For technical reasons one kilobyte, 1K, is actually 1024 bytes, not 1000 bytes as one would assume. Hard disk drives in general allow access to information with about 10 times the speed of floppy disks, are less susceptible to physical damage, and can store as much information as great numbers of floppy disks. Common sizes of hard disk drives are 20 megabytes, (million bytes), 40 megabytes, 80 megabytes, 120 megabytes, and upward. Like floppy disk drives, hard disk drives are usually physically mounted inside the computer case but could be an external device. Types of hard disk drives include MFM, IDE, and SCSI. The main differences between these types have to do with maximum number of drives and storage capability available to one computer and with the way they are connected to the computer by an interface daughter board. CDROM drives are capable of reading a music type CD specially formatted for computer use. Since they read by means of a laser they are more accurate at higher data densities. This means that several hundred megabytes of information can be available on one optical disk although reading is slower than on a hard disk. Also a major difference is that the CPU can write or store information on a floppy or hard disk but can only read information from an optical disk. It is possible to put the entire contents of a 24 volume encyclopedia on one optical disk and to make unlimited amounts of information available to the CPU simply by changing the disk in the CDROM drive. These are especially valuable for storage of graphical and sound data because this type of data requires tremendous amounts of storage space in contrast to text type data. For example a picture containing 640 pixels by 480 pixels with 256 colors, a common resolution, would require 640X480 bytes to store or 307K. This is enough to store about 100 typewritten pages. At 500 words per page, this means that a picture is worth 50,000 words. Floppy and hard disk drives are both input and output devices because the CPU can either read data from them or write data to them. A CDROM drive, on the other hand, is an input device only because it cannot be written to by the CPU. A keyboard is an input device by which data can be sent to the CPU by pressing keys, each key sending a different code to the computer and having various codes for the same key according to which of the three status keys, SHIFT, CTRL, or ALT are pressed at the same time. The standard keyboard has 101 keys. A printer is an output device by which a computer can transfer information to paper. Common printer technologies include dot matrix in which the character is made of individual ink dots, laser in which output is like a photocopy machine, and bubble jet which shoots ink onto the paper by heated ink bubbles. Most printers print in black and white although color printers are available. Printers vary mainly in cost, quality, speed, and noise. 24 pin dot matrix printers are most common but laser printers are becoming cheaper and therefore more prevalent. Printers generally cost from $150 to $1500 with a good business printer being available in the $600 to $900 range. Print quality is best with a laser printer, but very acceptable with a good quality dot matrix. Speed varies from 60 to 400 characters per second for dot matrix and ink jet printers and from 4 to 8 pages per minute for laser printers. Laser and ink jet printers are quietest but good quality dot matrix printers have acceptable noise levels. A monitor is an output device very similiar to a television screen, although higher resolution. Monitors are either monochrome (2 color) or color and if color the quality and resolution varies as follows: Type Pixels Colors CGA 320X200 4 EGA 640X350 16 VGA 320X200 256 SVGA 640X480 256 & higher The best VGA and SVGA monitors have a dot pitch of .28 to .31 with higher numbers meaning lower quality. Modems are input/output devices which allow a computer to transfer information over phone lines so that data can be exchanged between any two computer systems connected by phone. Modems should follow Hayes Standards for accepting control codes from the computer. Most are 2400 baud which is a speed designation meaning capable of transferring 2400 bits per second but many 9600 baud modems are now on the market. A bit is 1/8 of a byte of information so a 2400 baud modem operating at maximum efficiency should be able to transfer 300 bytes of data per second. Actual transfer rates are more like 220 to 240 bytes per second, however. A scanner is an input device which is capable of reading the image off a piece of paper and transferring the information into a computer where a program can store and interpret it. This information is in the form of a graphic image or picture and not in the form of text. Even if the paper contains text the information transferred is not text but a picture of text. OCR, Optical Character Recognition, software is necessary to read this image and convert it to actual text data and this software varies widely in its ability to do so. Scanners range from monochrome (2 color) hand scanners 4 inches wide selling for about $200 to flatbed scanners and color scanners capable of scanning a whole page at once and costing several thousand dollars. A mouse is an input device which sends a signal to the computer giving the position in X and Y coordinates of a ball which is being rolled by the operator over the surface of a table. It is also capable of signaling when either of two or three buttons on it have been pressed. Mouses (mice?) are especially useful for computer applications which involve much pointing such as playing some games and drawing pictures. Other similiar devices include trackballs, joysticks, pens, and light pens. With a trackball the ball is rolled in one place with the thumb rather than rolled over the table. Joysticks usually include two upright sticks which are moved with the hand and a button to press on either stick. A pen is a mouse like device shaped more like a writing pen. A light pen is made to point at the surface of the monitor and transmits these coordinates to the computer. Many other similiar devices are now in development. A sound board is an output device which is able to drive speakers from data received from the computer. It can also act as an input device in accepting sound signals from a microphone and converting them to the digital data needed by the computer. The most common sound boards are the Adlib music board and the Sound Blaster which includes not only music capability but the ability to record and playback any sound picked up from a microphone or other sound input device. A LAN (Local Area Network) board allows the computer with appropriate software to connect to a network of other computers to share data and various I/O devices. All devices use a controller board which connects to the computer either by means of the bus or by a special port (connection) to interpret the data being passed back and forth. Some computers have standard controller circuits such as that for a parallel printer and perhaps a monitor built into the motherboard.