Input-Output Environment
Data
and instructions must enter the data processing system, and information must
leave it. These operations are performed 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 the forms in which they receive their 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
devices 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 and 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.
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.
Low-speed devices: bar-code
readers; character printers; digitizers; keyboard input devices; plotters;
voice recognition and response units. |