Lesson 2/Learning Event 2
g. Mode of Operation.
Two modes of operation are generally used in
military DC telegraph communication systems, namely half-duplex and full-
duplex. Modes of operation in circuits to be interfaced must be similar.
Circuits cannot function properly if the technical controller attempts to
interface a half-duplex circuit with a full-duplex circuit.
h. Type of Code. The type of telegraph code must be acceptable by the
receiving device.
The normal code used for teletypewriter operation is
the Baudot code, in which each letter begins with a unit-length start
pulse and ends with a longer stop pulse, with five intelligence pulses
between.
Other types of codes are used for data systems.
The standard
data code in government uses eight equal-length pulses. A teletypewriter
designed to receive the 7.42-unit Baudot code cannot operate on the 8-unit
data code.
i. Speed of Transmission.
Speed characteristics of the sending and
receiving teletypewriter devices must be identical.
For example, the
circuit will not pass traffic if the technical controller tries to connect
a transmitter-distributor operating at 100 words per minute (wpm) to a
printer operating at 60 wpm.
Further, motor speeds must have close
tolerances before communication can be achieved between two-speed devices.
Relays and regenerative repeaters can isolate Circuits, making patching
simpler, but they cannot compensate for speed differences.
Learning Event 2: TELEGRAPH TEST SIGNALS
Five general types of test signals are used in teletypewriter test
and operation.
They include test message, repeated letter, reversals,
miscellaneous, and repeated space-bar signals.
a. Test Message. The message is normally the QUICK BROWN FOX JUMPED
OVER THE LAZY DOG'S BACK 1234567890, usually called the "fox" message.
This combination of words contains all possible operation combinations of
a teletypewriter keyboard. Only upper case (capital) letters are used in
teletypewriter message transmission.
b. Repeater Letter. Some tests required the transmission of a given
letter combination to take advantage of the arrangement in transitions
(mark-to-space and space-to-mark).
c. Reversals. Reversals are continuous streams of alternate marks and
spaces.
Within the pulse stream, all pulses have identical length.
If
reversals are not available, a stream
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