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Keyboard Selection
In order to avoid receiving equipment both for teleprinter and dial signals
in registers, among other reasons, it is advisable that the subscriber's selection
information should be sent from the teleprinter keyboard. As mentioned above,
the transmission of the selection information should be quicker than with dial
selection, owing both to the telex subscriber's familiarity with the use of keyboards
and to the fact that this procedure enables the subscriber to use his
automatic tape transmitter also for sending the selection information. (This is
convenient for the subscriber especially on international calls, which may involve
a large number of digits.)
Fig. 6
Form and meaning of printed service signals.
Signal
A-subscriber
reads
1 Proceed-to-select GA
2 Time announcement e.g. 09.11
3 Busy 09.11
occ
4 No-circuit 09.11
NC
5 Connection not admitted 09.11
NA
6 Not a working line 09.11
NP
7 Office closed, e.g. for holidays 09.11
ABS
8 Out-of-order 09.11
DER
With keyboard selection it may happen that, between two digits, the telex
operator keys another character than a digit. Instead of, in this case, sending
NP (see below) to the A subscriber, which may cause confusion as the latter
is presumably not aware of the error he has committed, other characters than
digits are accepted but not registered by the register and the connection is
established in the normal way when the necessary digits have been received.
Before the subscriber starts numerical transmission he sends a "prepare for
digits" signal which consists of Combination No. 30 (figure-shift) and which
"opens" the register.
The possibility both of keyboard and dial selection was achieved through
the fact that the register can be equipped with a special decadic receiver and
through the use of classification (see below) for informing the register whether
the subscriber uses keyboard or dial selection.
Printed Proeeed-to-Select Signal
With the use of keyboard selection the teleprinter motor must start at an
early stage. The sound of the starting of the motor might be used as a proceed-to-select
signal. On the other hand silent electronic teleprinters may come
into use within the not too distant future. It is also necessary that the teleprinter
motor runs up to speed before the subscriber starts to send selection
information. It is therefore advisable to give the subscriber a clear indication
that he can start, which is done in the form of a teleprinter signal GA (go
ahead), as the subscriber's attention is directed to the teleprinter after the
motor has started, not to the control unit.
Printed Service Signals
Primed service signals in accordance with CCITT recommendations could
be introduced as the teleprinter motor starts at an early stage in the setting up
of the connection. Such signals give the A subscriber more detailed information
concerning the reason for failure of a connection than a simple busy-andcongestion
signal. Unnecessary renewed attempts at connection can thereby
be avoided.
The service signals used are as follows:
OCC
NC
NA
ABS
DER
NP
Subscriber engaged
No circuits, e.g. all trunks busy
Connection not admitted
Subscriber absent or office closed
Out of order
Not a working line
Each service signal is preceeded by a time announcement, e.g. 09.11, as
shown in fig. 6.
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