Specifications

www.quatech.com
10
Data Communication Explained
Data communication is fundamentally a simple operation. Point
A sends information to Point B and Point B receives it. A slightly
more complex, and more practical, system allows Point A to send
information to and receive information from Point B, and vice
versa. It is what lies between points A and B that has been the
substance of data communication system development since
before the personal computer, or any computer for that matter,
was ever invented.
For example, consider several simple examples of data
communication systems that have nothing to do with computers:
Paul Revere used a very basic system, whereby one light in the
tower signified the British were approaching by land, and two
indicated they were approaching by sea. During a game of blind-
man's-bluff, the subject hears a sound when he comes within a
certain distance of an object, and nothing when he is out of
range. To solve the problem of knowing when dishes in a
dishwasher are clean or dirty, a family might decide to place a
black magnet on the dishwasher when dirty dishes are put in,
and then change it to a white magnet when the cleaning cycle is
started, and return the black magnet after it is emptied. The one
thing all these examples have in common is that they all use
two-state systems of communication. A two-state system is one
which uses only two possible values to transmit information--
the lamp is on or it is off, there is a sound or there is silence, the
magnet is black or white, etc. The way these values are combined
allows complex messages to be transmitted using very simple
tools.
Consider Morse Code, a two-state data communication system
that functions very similarly to today's computerized data
communication systems. Developed by Samuel E.B. Morse in
the 19th century, Morse code uses electrical current to transmit
a series of dashes and dots that represent letters of the alphabet,
numbers, a comma and a period. A basic Morse Code transaction
works as follows: A "message" is given to an operator who
translates that message into dots and dashes (Point A), then the
transmitting operator uses the telegraph key to send an electrical
signal to the receiving operator at the desired location to indicate
that a message is about to come through. The receiving operator
(point B) sends back an acknowledgment that he is ready, and
the transmitting operator then sends the message which the
receiving operator takes down. When the message is completely
transmitted, the transmitting operator signals to the receiving
operator that he is done, and the transmission line is closed. The
receiving operator then translates the code back into the original
message, and delivers it to the designated recipient.
Clearly, in a system of this type, accuracy is extremely important.
As only two characters--dot or dash--are used to create a code
for an entire language system, the transmitting and receiving
operators must be extremely accurate. (Indeed, it makes a big
difference whether the message says "Give one million dollars to
Ted" or "Give one million dollars to Ned"--an easy mistake to
make using Morse code, as the letter T is "-" and the letter N is
"-.") This system can only work if both sides of the data
communication system know the code and can encrypt and
decode messages. It is also essential that the transmitter not
send faster than the receiver can take-down the information.
Even using expert operators, static on the line could obscure
the signals making a dash sound like a dot and thereby corrupting
the message. Thus, it becomes obvious that the most important
aspect of designing a data communication system is ensuring
not only that Point B can receive and understand the data
transmitted by Point A, but also that the data remain uncorrupted
during transmission.
These are the very same concerns faced by computerized data
communication system designers. Indeed Morse code is often
though of as the forerunner of the computer's binary
communication system. The binary system uses the numbers 0
and 1 as the symbols for transmission of data. Using positional
notation, any value is represented by a weighted series of 1s
and 0s. Thus the decimal number "33" would be represented
by "100001" (1 x 2
5
+ 0 x 2
4
+ 0 x 2
3
+ 0 x 2
2
+ 0 x 2
1
+ 1 x 2
0
).
The above is a simple example of binary coding, but the same
system is used by computers to transmit complex text messages,
complex graphics, and streaming video. In order to accurately
transmit many different types of data, numerous interfaces and
protocols have been created. When selecting the appropriate
equipment for your data communication application, it is
important to examine both your application and the
communication peripherals that must be incorporated into the
system. First, determine whether your application is best suited
for a wired system using cables or whether you need a wireless
solution. Also determine whether you need a portable system
using a laptop computer, or a desktop PC or Server. After deciding
upon your host computer, determine the ports available, and
the type of expansion boards this computer can best
accommodate. Then, decide which of those options will best
meet the speed and versatility demanded by your application.
Once you've selected an interface, you can then begin to look at
the type of data communication adapter you will need. You
must also consider the distance you will need between the
Data Communication Introduction