Lesson 2/Learning Event 1
(1) Neutral.
In a neutral circuit, reversed direction of current
forces the relay line winding to hold the armature on the space contact
continuously because the magnetic field created by the line winding aids
the magnetic field created by the bias winding; the teletypewriter printer
therefore runs open constantly.
However, if the printer uses direct-
magnet operation (no relay), the direction of current is unimportant
because the armature of the magnet assembly will be pulled to its pole
piece on each current pulse regardless of the direction of current. Some
of the newer printers, however, have selector magnets designed to combine
the functions of a polar relay with those of a selector magnet. This type
of unit suffers the same restrictions as to direction of current as does
the polar relay.
(2) Polar. In a polar circuit, the bias winding of the polar
relay is
not needed because current reverses itself when going from mark
to space
and vice versa.
When current is opposite to normal in a polar
circuit,
marking current pulses will appear as spacing current pulses,
and vice
versa. The result is unreadable copy.
(3) Turnover.
When the direction of current is opposite to normal,
turnover exists.
Turnover can be corrected by inverting the polarity of
battery or reversing the jack wiring (tip to ring, and ring to tip). Some
types of terminal equipment and test sets have switching arrangements to
invert polarity of the received signal.
e. Characteristics of Circuits.
When a circuit uses a full-metallic
wire pair, both lines are normally isolated from ground. If either line
is grounded, fuses may be blown because of excessive current.
When a
full-metallic loop is to be interfaced with a single-wire ground-return
circuit, an isolating relay must be used.
Furthermore, there must be
agreement as to the current source (battery). If two sources of battery
are used, current may increase to twice the normal values when battery
polarities aid each other, or current may decrease to near zero when
battery polarities are in opposition.
f. Classification
of
Circuit.
Circuits
must
bear
the
same
classification before they can be interfaced.
A neutral circuit can be
interfaced only with a neutral circuit, and a polar circuit with a polar
circuit.
Furthermore, both terminating devices must be arranged for the
same type of signal.
However, interfacing of a neutral with a polar
circuit can be achieved by using a suitable isolating relay.
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