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The tones are generated by a transistor unit placed on one of the exchange
relay sets and have a frequency of 425 c/s. The length of the tones and
ringing signals is controlled by relays in the exchange.
Switching Procedure
The trunking diagram of the P.A.B.X. is shown in fig. 12.
Internal Calls
When the extension raises his handset, his line is identified by ID, which
seizes the marker M. The marker connects the extension to a free register
REG, after which marker and identifier release. REG extends dial tone to the
extension, who can then dial the wanted 2-digit number.
On receipt of the called number, REG seizes the marker M. The calling
and called lines are now connected to a free connecting circuit SNR. M and
REG release simultaneously with the transmission of the first ringing signal.
Intermittent ringing continues concurrently with the transmission of ringing
tone to the caller. When the called party raises his handset, connection is
established. If the called extension is engaged, the caller hears busy tone. If
the caller has priority, he can enter an engaged circuit by dialling one digit.
A faint busy tone is sent to the conversing parties as a sign that a third party
is on the line.
As soon as either of the extensions replaces his handset, his line is cleared.
Outgoing Calls to Public Exchange
When an extension raises his handset and receives dial tone from REG,
he can dial the prefix of the public exchange. This prefix may be either 0 or
1. On receipt of the prefix REG connects the extension to a free FDR-C
and then releases. The extension now receives dial tone from the public exchange
and can dial the wanted subscriber's number.
Incoming Calls from Public Exchange
Incoming calls are registered on an exchange line relay set t'DR-C, ana a
call lamp on the operator's console OPI lights. The call is answered by the
operator pressing the key in which the lamp is lit. Connection with the
operator is now established. The operator puts the call through to the wanted
extension by pressing the extension's line key. On the console there are three
lamps, one green, one yellow and one red, which indicate whether the extension
is free, engaged or restricted. If either of the first two lamps are alight,
the operator can leave the connection by depressing the X button. If the
extension is engaged, the call can be placed on an automatic camp-on circuit.
If the red lamp lights, the operator has no possibility of putting the call
through to the wanted extension.
Enquiry Calls
Enquiry calls can be made by an extension during a conversation on an
exchange line. There are three types of enquiry calls.
An enquiry to the operator is initiated by pressing the earth button on the
telephone set and, on receipt of dial tone, dialling 9. The operator observes the