PERIPHERAL DEVICES

Peripheral devices include all the I/O devices used with a computer system. When these devices are under control of the CPU, we say they are online. When they perform their functions independently, not under direct control of the CPU, we say they are offline. The peripheral devices described in the following paragraphs are the ones we commonly use: magnetic ink character readers, scanners, bar-code readers, key-to-online data entry terminals, magnetic tape units, magnetic disk drive units, floppy disk drive units, cathode-ray tube (CRT) terminals, printers, and plotters.

Magnetic Ink Character Readers

Magnetic ink character readers read magnetically inscribed information (like on a bank check) and translate it to machine code.

Scanners

Scanners read text, drawings, and photographs and convert them into electronic representations of the images.

Bar-Code Readers

Bar-code readers scan data encoded in special bar codes using reflective light and translate them to machine code. Examples are the code on the front of this manual and the codes on supermarket and department store products.

Key-to-Online Data Entry Terminals

Using a key-to-online data entry terminal, a person enters input data into the computer directly from the point of origin. This eliminates the need for human intervention between source recording and the ultimate processing by the computer.

Magnetic Tape Units

Magnetic tape units move magnetic tape across read/write heads that read and write the information. Characters are recorded (written) on the tape in the form of magnetized spots along the entire length of the tape. On tape, data is stored in a sequential manner. In sequential processing, the CPU must begin searching at the beginning of the tape and check each record until the desired data is found.

Magnetic Disk Drive Units

Computer Consoles

Magnetic disk drive units are storage devices that read and write information on the magnetized surfaces of rotating disks. The disks are made of thin metal, coated on each side so that data can be recorded in the form of magnetized spots. As the disks spin, characters can be stored on them or retrieved from them in a random (direct) manner. This direct accessing of data has a big advantage over the sequential accessing of data. You can direct the disk drive to begin reading at any point.

Floppy Disk Drive Units

Floppy disk drive units consist of a diskette drive (the mechanism within which the diskette rotates) and a controller containing the electronic circuitry that feeds signals into and from the diskette. Diskettes are thin, flexible platters (floppy disks) coated with magnetic material so characters can be recorded on the surface in the form of magnetized spots. Just like hard disks, as the disk spins, characters can be stored on them or retrieved from them in a random (direct) manner. Computer consoles usually include at least a display and a keyboard. They may also include a printer. The main console is located near the CPU and is used by the computer operator to control the operations of the computer system. Other consoles maybe used at the peripheral devices to specify what media to mount, what forms to use in the printer, what alignment is required, and so on. They may also be used by the peripheral equipment operator to start and stop jobs, bring up other peripheral devices, requeue jobs, and so on. Computer consoles are not normally used for keying data from source documents for production jobs.