Other Teletypewriter Terms.
a. Working with teletypewriter equipment, you'll often hear the terms, "running closed" and "running open."
(1) "Running closed" means that the machine is receiving a steady mark signal. When it is running closed, the
teletypewriter does not perform any mechanical operations.
(2) "Running open" means that the machine is receiving a steady space signal. You can see the teletypewriter
performing mechanical operations, but it does not print out any messages.
b. A term less frequently used is "lock up
condition." It means you are receiving a steady mark
when you know you should be receiving an input signal.
For example, you have a lock up condition when the
teletypewriter starts typing a message from a distant
terminal and then it suddenly stops typing before the
FIGURE 78. Polar signals for the
message is completed. This is usually caused by a
defect of other equipment in the circuit.
Kinds of Pulses in DC Teletypewriter Circuits.
Either neutral or polar pulses may be used to represent the Baudot code in dc teletypewriter circuits.
a. Neutral pulses are characterized by current flow for a mark, and no current flow for a space, as shown in
b. Polar signals are characterized by current flow in one direction for a mark, and current flow in the opposite
direction for a space. Pulses for the letter A would look like those of figure 78. In this figure, the direction of the current
is represented by positive and negative pulses. In polar transmission there's always current in the circuit; it flows either in
one direction or the other.