SERIAL DATA TRANSMISSION
The simplest, most economical, and easiest to use
method of transferring digital information from one
point to another is serial data transmission. In a serial
system, the digital data is sent one bit at a time. This
means only one pair of transmission wires is required.
The serial transmission of data is far slower than
parallel transmission. In most computer systems, the
low speed is not a disadvantage. Data rates
achievable in serial data systems are sufficiently high
to make them very practical.
Serial data transmission is preferred because it is
inexpensive. It is especially beneficial in transferring
data over long distances. For long distances, you can
see that multiple parallel lines are far more expensive
than a single cable.
Low-speed serial data transmission also offers
another benefit. The data rate is slow enough to
permit the transmission of data over telephone lines.
In this case the digital data is converted into a pair of
audio tones representing binary 1s and 0s. These can
be transmitted economically for long distances over
standard telephone lines. In addition, low cost tape
recorders can be used to record the serial data. This
provides a low cost means of mass data storage and
retrieval. Standard audio cassette recorders are
widely used in this application.
Serial data transmission also permits transmission
of data by radio.
A radio communications path
represents only a single interconnecting link similar
to a transmission line pair. Therefore, for data to be
transmitted by radio, it must be in serial format.
Serial digital data is used to modulate a radio carrier
in various ways.
In digital computer systems, you will find that
both serial and parallel data transmission methods are
used. Where high speeds and short distances are
required, the parallel method is used. The serial
method is used where low cost, simplicity, low speed,
and long distances are necessary.
INPUT/OUTPUT (I/O) DEVICES
I/O devices are similar in operation but perform
It is through the use of these
devices that the computer can communicate with
devices external to the computer itself (peripheral
The I/O section of a computer provides the
necessary lines of communication and generates such
signals necessary for the computer to communicate
with, and when necessary, control the operation of the
I/O devices. The I/O section, once it has been
initiated by the control section, usually operates
independently of the control section, except when it
must time-share memory with the control section.
Input data may be in any one of three forms:
. Manual inputs from a man-machine interface
(MMI), such as a keyboard or console
. Analog and/or digital inputs from instruments
l Inputs from a source on or in which data has
been previously stored in a form intelligible to the
Computers can process hundreds of thousands of
computer words per second. Thus, a study of the first
method (manual input) reflects the inability of
human-operated keyboards or keypunches to supply
data at a speed that matches the speed of digital
A high average speed for keyboard
operation is two or three characters per second. This
actually translates to a data input rate of less than one
computer word per second due to coding time. Since
the computer can read several thousand times this
amount of information per second, it is clear that
manual inputs should be minimized to make more
efficient use of computer time.
Instruments used as input sensors are capable of
supplying several thousand samples regarding
pressure, temperature, speed, and other measurements
per second. This is equivalent to 10,000 to 20,000
bits or binary digits per second. Digital computers
that use these devices must be equipped with
analog-to-digital (A/D) converters (assuming the
input is in an analog format) to convert physical
change to specific increments.
Input data that has been previously recorded on
punched cards, perforated tapes, magnetic tapes, or
magnetic drums or disks in a form understood by the
program may also be entered into the computer. This
method is much faster than entering data manually
from a keyboard. The most commonly used input
devices in this category are magnetic tape readers and
paper tape (perforated tape) readers.
One of the main features of computers is their
ability to process large amounts of data quickly. In