Peripheral devices include all the input and output devices used with a computer system. When these devices are under control of the CPU, they are said to be on line. When they perform their function independently, not under direct control of the CPU, they are said to be off line. The following peripheral devices are used commonly for input and output. Those that perform only input are marked (I), those that perform only output are marked (0), and those that perform both input and output are marked (I/0).

Optical Character Reader (I)

An optical character reader reads printed data (characters) and translates it to machine code.

Keyboard (I)

The keyboard is used by a computer operator to communicate with a computer system.

Mouse (I)

A small hand-held device used by a computer operator for positioning the cursor, making freehand sketches, or selecting items from menus on a screen. When the mouse is rolled across the surface of the desk, the cursor moves a corresponding distance on the screen.

Scanner (I)

A means of converting hand-drawn art or photographs into digital form.

CD-ROM Drive (I)

CD-ROM (compact disk-read only memory) technology is similar to audio disks, except it includes routines for detection and correction of data errors. A CD-ROM drive may be internally or externally installed. It holds a vast amount of information that makes it valuable to store digital images. Once recorded, information on CD-ROM cannot be erased or changed, but it can be read many times. The expression Write Once, Read Many (WORM) describes this type of technology.

Magnetic-Tape Unit (UO)

The magnetic-tape unit moves magnetic tape across read-write heads that actually read and write the information. Data is recorded sequentially in the form of magnetic spots along the entire length of the tape.

Magnetic-Disk Drive Unit (UO)

The magnetic disk drive unit is a storage device that reads and writes information on the magnetized surfaces of rotating disks. The disks are made of thin metal, coated on each side so data can be recorded in the form of magnetized spots. As the disks spin around like a music record, characters can be stored on them or retrieved from them in a random (direct) manner. Accessing data directly has several advantages over accessing data stored sequentially. It provides fast immediate access to specific data. You can direct the disk drive to read at any point. Magnetic disk drives come in two types: hard drives and floppy drives. Magnetic disk drives are in computer systems and electronic cameras. Floppy disks come in several sizes, ranging from 3 to 8 inches in diameter; the 3 1/2-inch diskette is most common.

Optical Disk Drive (UO)

A drive used with a form of data storage in which a laser records data on a disk that can be read with a lower-power laser pickup. Three types of optical disks are used: Read Only (RO), Write Once, Read Many (WORM), and erasable. Two types of erasable disks are used: Thermo Magneto Optical (TMO) and Phase Change (PC).

Printer (0)

The printer is a widely used output device that expresses characters, graphics, drawings, or photographs on hardcopy (paper or film). A large range of printers is available. Printers are discussed in more detail later in this chapter.

Monitor (0)

The monitor is similar to a television screen. It allows you to see the program or data.

INPUT/OUTPUT CHANNELS

Input/output (I/0) channels provide a means of communication between the CPU and peripheral devices. This is accomplished by electrical cables that carry both data and control information between the computer and the peripheral devices.

Signals are transmitted and received through a cable that connects the CPU to an on-line device. This cable provides a path (channel) for the signal to travel. Signals for both monitoring and data are transmitted by way of I/0 channels. These I/0 channels may be used specifically for data input, data output, or data input and output. On desktop computers, the I/0 channels are referred to as communication ports.

Types of Channels

Channels, both input and output, may be either simplex or duplex. A simplex channel is only capable of communication in one direction. When a peripheral device, such as a mouse, is connected to a simplex circuit, it is only capable of transmitting data. Simplex circuits are seldom used because a return path is generally required for acknowledgment, control information, or error message.

Duplex channels provide two paths for the transmission of data. One path is for sending, and one is for receiving data.

Data Transmission Methods

Data may be transmitted over a channel in one of two ways, either in the serial or the parallel mode.

SERIAL MODE.-In serial mode (transmission), three wires are required: one to transmit data, one to receive data, and one for a ground. The data is sent or received in the form of bits, one alter another in a series, as shown in figure 3-2 This type of transmission is desirable whenever a computer system is linked to outside peripherals over a long distance, such as remote terminals.

PARALLEL MODE. - In parallel mode (transmission), the data bits are all sent or received simultaneously. Parallel transmission requires nine or ten wires to connect the computer to the peripheral device (fig3-2). Seven or eight lines are required for data bits, one or two lines for "handshake" purposes, and one line for a ground. The handshaking signals communicate information back and forth between the peripheral devices and the computer. This information lets the peripheral know when the computer is ready to accept another character and vice versa. This type of transmission is useful for fast data transfer. The principal drawback is the computer must not be located relatively close to the peripheral device.