3-17
Codes
Two of the codes the Navy uses are found in manual telegraphy and in teletypewriter operation. One
is very easy to understand while the other is more complex. Let's look at these two types and how they
work.
MANUAL TELEGRAPHY.—In manual telegraphy, the most widely used code is the Morse code.
In this code, two distinctive signal elements are employed-the dot and the dash. The difference between a
dot and a dash is its duration, a dash being three times as long as a dot. Each character is made up of a
number of dots and/or dashes. The dot and dash elements making up any character are separated from
each other by a time interval equal to the duration of one dot. The time interval between the characters for
each word is equal to the duration of three dots. The interval between words is equal to seven dots. (A
signal-man uses the Morse code to send visual flashing-light messages. The radioman uses the Morse
code to send messages electrically.)
TELETYPEWRITER MESSAGE TRANSMISSION.—In teletypewriter operation, the code
group for each character is of uniform length. Since the Morse code is an uneven length code, it cannot be
used in teletypewriter operation without additional code converters.
The FIVE-UNIT (five-level) CODE has been the most commonly used in modern printing
telegraphy and is universally used in teletypewriter operation. This is also known as the Baudot code. The
mechanical sending device in the teletypewriter divides the sending time for each character into five short
code elements (impulses) of equal duration. The five-unit code is an example of what is called an even
length or constant length code (one in which the number of signal elements for a character is the same for
every character and the duration of each element is constant). In the five-unit code, each character
consists of a combination of five signal elements; each element may be either a mark or a space. A total
of thirty-two combinations of signal elements are possible with this arrangement.
The thirty-two possible combinations available from the five-unit code are insufficient to handle the
alphabet and numbers since twenty-six combinations are required for the letters of the English alphabet
alone. This leaves only six combinations for numerals, symbols, or nonprinting functions. This number of
combinations is obviously inadequate; therefore, two of the thirty-two combinations are used as shift
signals. The shift signals are often referred to as case-shift signals (one case is a letter shift, and the other
a figure shift.) These two shift signals permit the remaining code combination to be used as letter-shift
signals for letters and as figure-shift signals for numerals, function signs, and so forth. When a letter shift
is transmitted, it sets the receiving instrument in a condition to recognize any letter signal combination. It
will recognize letter combinations until a figure shift is received. Then the receiving instrument sets itself
in a condition to recognize any figure signal combination received. The interpretation of a signal
combination is determined by the previous shift signal. This plan enables 30 of the 32 available
combinations to have two meanings.
Q14. There are not enough combinations of the five-unit code to handle the alphabet, symbols and so
forth. What is used to increase the number of available code combinations?
Modes of Operation
The two basic modes of teletypewriter operation are ASYNCHRONOUS (start-stop) and
SYNCHRONOUS. The most common mode used in teletypewriter operation is the start-stop mode.
Synchronous operation is used more in high-speed data systems. Let's examine their differences.
ASYNCHRONOUS.—In the start-stop mode of operation, the receiving device is allowed to run for
only one character. It is then stopped to await the reception of a start signal indicating the next character is
