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show the signalling both for single current Aj and for double current A 2 . The
following description will relate to single current signalling. (All diagrams
have a vertical time axis and A denotes start polarity and Z stop polarity.)
Outgoing
Traffic
In the free line condition a current of about 5 tnA flows through the subscriber's
loop. On initiation of a call (depression of a button on subscriber's
control unit which reduces the impedance on the subscriber's loop) the current
rises to about 40 mA. The subscriber's line relay then operates. The terminal
exchange ARB is through-connected. The time for the through-connection is
about 400 ms. The superior transit centre is then called and at the same time
the polarity of the subscriber's line is reversed for starting of the teleprinter
motor. A printed proceed-to-select signal GA is sent after receipt of a proceed-to-select
signal from a register in the transit centre, but a time of 2
seconds must have elapsed after starting of the motor to allow it to come up
to speed.
The subscriber then sends the B subscriber's number preceeded by figureshift
(keyboard selection). A call-connected signal from the B subscriber is
received in the form of his answer-back preceded by a time signal indicating
hours and minutes.
After completion of the telex message the connection is cleared. The signalling
diagrams shows, first, the procedure when the call is cleared by the A subscriber.
Under the dotted line is shown the form of the signals when clearing
takes place from the B side with the special signal mentioned earlier, and
which in this case is used to prevent printing of characters on the A subscriber's
teleprinter.
Incoming
Traffic
On an incoming call the polarity of the subscriber's line is reversed and
the current is raised to 40 mA.
The B subscriber receives a time signal, after which his answer-back is
tripped. The signal diagram in fig. 9 shows, first, clearing from the A side
and, under the dotted line, from the B side.
Fig. 9
Signalling on called subscriber's line
etc.
Terminal exchange
Called subscriber's equipment consisting
of teleprinter and control unit
Subscriber's line which may be either
a single current line or
double current line
Forward signalling path,i.e. fromexchange
to subscriber
Backward signalling path, i.e. in opposite
direction
Time signal, hours and minutes, i.e. 08.30
Answer-back trip signal
Called subscriber's answer-back
Called subscriber's number
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