character. There is a penalty for this advantage.
Character length is increased to include the start
(space) and stop (mark) signals. These start and stop
signals are part of the five unit code (BAUDOT
CODE) shown in figure 2-14.
Synchronous systems have an advantage over
asynchronous systems. The start and stop elements are
not used. This allows more room for information
transmission. Time is not wasted on start and stop
units. Additionally, this mode has a higher capacity to
accept distorted signals because it does not depend on
the start and stop signals for timing.
The terms used in referring to tty modulation rates
or signaling speeds are baud, words per minute (wpm),
and bits per second (bps). Baud is the only term that is
technically accurate. The others are either ap-
proximations or require explanation.
By definition, the word baud is a unit of
modulation rate. To find the modulation rate of a signal
in bauds, divide 1 by the time interval of the shortest
unit in the signal. For example, 22 milliseconds (.022
see) is the time interval of the shortest unit in the five-
unit code at 60 wpm. To find the number of bauds equal
to 60 wpm, divide 1 by .022. Rounding off the results
provides the figure 45.5, which is the baud equivalent
of 60 wpm. You can see that increasing the wpm means
the unit time interval has decreased.
The most common baudot data speeds range from
45.45 to 100 bauds, or about 60 to 133 wpm. Higher
speeds are obtained using the American National
Standard Code for Information Exchange (ASCII).
This is a seven-unit digital code used for the
transmission of teleprinter information. ASCII is used
primarily with computer systems, but it is also used in
some teletypewriter applications. The Defense
Communications System standard speed for teletype
operation is 100 wpm or 75 baud.
Words per minute is used only when speaking in
general terms about an approximation of speed. At a
speed of 100 wpm, 100 five-letter words with a space
between them can be transmitted in a 60-second
period. But, you can also get this wpm rate by varying
the modulation rate or the length of individual
characters. Because of this, the baud method of
reference rather than wpm is used.
In binary signals, the term bit is equivalent to one
place in a binary number. Because of the influence of
computer language, modulation rate is sometimes
expressed as bits per second (bps). When it is expressly
understood that each unit in the baudot character is the
same length, the modulation rate expressed in bps is
the same as the modulation rate expressed in baud.
The two most common methods of creating the
mark and space conditions are neutral and polar
operation. In neutral operation, current flow represents
the mark and no current flow represents the space. In
polar operation, current impulses of one polarity
represent the mark and impulses of the opposite polarity
represent the space.
Neutral circuits use the presence or absence of
current flow to convey information. These circuits use
high level (60/20 milliamperes) as the line current
value. (Low level operation uses (±6 vdc at 20 micro-
amps and can be balanced or unbalanced). A neutral
teletypewriter circuit consists of a transmitting device,
a current source, a variable resistor to control current, a
receiving device, and a transmission line.
Polar operation differs from neutral operation. In a
polar system information is always present in either a
positive or negative condition. The circuit composition
is the same, but polar operation requires an additional
current source. This current source is usually a solid-
state dc power supply that provides variable current to
the teletypewriters. The extra current source normally
provides current from the positive side for marks and
current from the negative side for spaces.
There are some advantages to using polar circuits
rather than neutral circuits. In a polar circuit, it is
almost impossible to distort a signal by high reactance,
low-line currents, or random patching of circuits or
equipment. A big advantage for the ET is that a com-
plete loss of current (a zero reading on the milliamme-
Figure 2-14.Mark and space signals in the teletypewriter-