The Simplexed Teletypewriter Circuit.
a. Superimposing a teletypewriter circuit on an existing telephone circuit saves wire. The circuit that results is
called a simplexed teletypewriter circuit.
b. A simplexed teletypewriter circuit is derived from a telephone circuit by using the pair of parallel wires of the
telephone set as one conductor of the teletypewriter circuit and providing a ground return for the second conductor (fig.
87). A repeating coil is installed at each end of the telephone circuit and the teletypewriters are connected between the
midpoint of the line winding of each repeating coil and ground. Direct current from the teletypewriter at the west station
divides and flows in opposite directions through the line winding of the west repeating coil, over both lines in parallel,
through the line winding of the east repeating coil, and through the east teletypewriter to ground.
c. If the resistance of each wire in a pair is identical--that is, the lines are balanced--the operation of the
telephone and the telegraph will not interfere with each other. This is because the teletypewriter currents in the line
windings of each repeating coil are opposing. They will induce no net voltage in the primary windings of the repeating
coils. Also, the center taps of the line windings of the repeater coils are used to ac current from telephone operation will
not affect teletypewriter operation.
The Phantom Teletypewriter Circuit.
a. You can have two telephone circuits and a full-metallic teletypewriter circuit operating at the same time
(figure 88). This is called a phantom teletypewriter circuit. The pair of wires of line side circuit #1 is one conductor for
the teletypewriter circuit and the pair of wires of line side circuit #2 is the other conductor.
b. The operation of the circuits will not interfere with each other as long as the lines are balanced.