Text 1. Input-output environment

Data and instructions must enter the data processing system, and information must leave it. These operations are perfonned by input and output (I/O) units that link the computer to its external environment.

The I/O environment may be human-related or human-in­dependent. A remote banking terminal is an example of a hu­man-related input environment, and a printer is an example of a device that produces output in a human-readable format. An example of a human-independent input environment is a de­vice that measures traffic flow. A reel of magnetic tape upon which the collected data are stored in binary format is an ex­ample of a human-independent output. ;

Input-Output Interfaces. Data enter input units in forms that depend upon the particular device used. For example, data are entered from a keyboard in a manner similar to typing, and this differs from the way that data are entered by a bar-code scan­ner. However, regardless of thejorms in which they receivetheir inputs, all input devices must provide a computer with data that are transformed into the binary codes that the primary memo­ry of the computer is designed to accept. This transformation is accomplished by units called I/O interfaces. Input interfaces are designed to match the unique physical or electrical character­istics of input devices to the requirements of the computer sys­tem. Similarly, when output is available, output interfaces must be designed to reverse the process and to adapt the output to the external environment. These I/O interfaces are also called channels or input-output processors (IOP).

The major differences between devices are the media that they use and the speed with which they are able to transfer data to or from primary storage. .

Input-Output Device Speed. Input-output devices can be clas­sified as high-speed, medium-speed, and low-speed. The devic-'es are grouped according to their speed. It should be noted that the high-speed devices are entirely electronic in their operation or magnetic media that can be moved at high speed. Those high­speed devices are both input arid output devices and are used as secondary storage. The low-speed devices are those with com­plex mechanical motion or operate at the speed of a human operator. The medium-speed devices are those that fall be­tween — they tend to have mechanical moving parts which are more complex than the high-speed devices but not as complex as the low-speed. -fa •

High-speed devices: magnetic disk; magnetic tape.

Medium-speed devices: card readers; line printers; page print­ers; computer output microfilms; magnetic diskette* optical character readers; optical mark readers; visual displays.