most cases, the processing speed far exceeds the
ability of input devices to supply information. One
common limitation of most input devices is that each
involves some mechanical operation. For example,
the movement of a tape drive or card feeder. Because
a mechanical movement of some part of these devices
cannot take place fast enough to match electronic
speeds with the computer, these input devices limit
the speed of operation of the associated computer.
This is particularly evident in cases where successive
operations are dependent upon the receipt of new data
from the input medium.
Several methods of speeding up mechanical
operation have been devised, all of which are
designed to move a smaller mass a shorter distance
and with greater driving force. Many of these designs
have been directed toward increasing the drive speed
of magnetic tapes.
For example, present-day tape
drives can pass up to 400 inches of tape per second of
a tape reading head.
Card readers can read up to
2,000 cards per minute.
Present-day disk systems
operate at speeds up to 3,600 RPM.
The comparative rates of data for these systems
are as follows:
. Card systems2,700 characters per second
. Tape systems350,000 characters per second
. Disk systems15,000,000 characters per
Another method of entering data into a computer,
which we have not previously mentioned, is to link
two or more computers together and program them to
communicate with each other. This is perhaps the
fastest method of entering or extracting data from a
computer. An example of this method is the data link.
Output information is also made available in three
. Displayed information, such as codes or
symbols presented on a monitor screen, that is used by
the operator to answer questions or make decisions.
. Control signals, which is information that
operates a control device, such as a lever, aileron, or
. Recordings, which is information stored in a
machine language or human language on tapes or
Devices that store or read output information
include magnetic tapes, punched cards, punched
paper tapes, monitors, electric typewriters, and
INTEGRATED CIRCUIT (IC)
Learning Objective: Identify integrated
circuit classifications and various types of
Modern digital computers are made with ICs. ICs
provide the most economical and practical method of
implementing the circuits required to carry out the
functions of a digital computer.
ICs fall into four basic levels of complexity.
These are small scale integration (SSI), medium scale
integration (MSI), large scale integration (LSI), and
very large scale integration (VLSI). The designations
define the size and complexity of individual ICs.
Small Scale Integration (SSI)
SSI circuits contain very little circuitry, generally
fewer than 10 circuits no more complex than a typical
logic gate. Typical SSI circuits include multiple logic
gates, flip-flops, and simple combinational logic
Medium Scale Integration (MSI)
MSI circuits are more complex and sophisticated
than SSI circuits. Most MSI circuits contain 12 or
more circuits equivalent in complexity to a typical
logic gate. MSI circuits are functional in nature in
that the y perform a specific logic operation with no
further interconnection. Typical MSI circuits include
counters, shift registers, arithmetic-logic circuits,
decoders, multiplexer, and other combinational and
sequential logic circuits. MSI circuits are highly
beneficial because they significantly cut design time.
They also reduce the number of ICs in design,
minimize circuit wiring, and reduce size and power
Most digital equipment can be