British Telecommunications (BT) Senator, System Manual

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CPE/MS 526 Issue 2 (12/88)
Part 8 HOW IT WORKS
The system employs nonnal signalling to the exchange (either public
exchange or PBX). Each SENA TOR telephone has a keypad which can be
either loop-disconnect pulsing (LD), or multi-frequency ac signalling (MF)
to suit the type of exchange working. There is no transmission bridge
connected into the circuit at the CCU on exchange line calls. The extension
is directly connected through to the exchange on exchange line calls.
The telephones are connected to the CCU by 4-wire connections which are
arranged as speech and data pairs. Signals from the keypad and speech
signals in the usual fonn of 'varying current' use the speech pair. All other
signals use the data pair, and are in the form of coded -
3V pulses
superimposed on a standing 50V supply from the CCU power unit.
Briefly, the microprocessor in the CCU continuously monitors the state of
incoming exchange lines and the data pairs from the SENATOR telephones.
The microprocessors in the telephones monitor the incoming data pairs from
the CCU and the state of the gravity switch springs and the 'function'
buttons. Any change in the state of these is detected, causing an interchange
of information within the system, which is used to process the call.
The mains driven power unit within the CCU supplies:
All power required by the CCU.
Power to the microprocessor, LEDs and tone caller at each telephone,
this power being fed to the telephone over the data pair.
Power for the speech circuit and keypad at each telephone on
intercom calls, this power being fed to the telephone over the speech
pair.
The power feed at each telephone contains a diode to protect the
microprocessor circuitry against damage in the event of a reversal of the data
pair. However under these circumstances a complete failure will result
as
there is no 'constant polarity' circuit provided.
The power circuit in the telephone contains a series regulator to control the
voltage and a high impedance filter to block data signals from the power
supply. All power supplies are generated by constant current circuits to
prevent fluctuations in the de supply caused by the operation of the LEDs or
tone caller being detected as false data signals.
The system is designed to provide for up to 5
+
10 size installation. Although
the system sizes may differ the microprocessors in all CCUs are programmed
on the assumption that all systems have
10 extensions. Where there are fewer
than 10, 'padding routines' are included in the programme to replace the
'missing' extensions, so keeping the operating cycle of the microprocessor
the same in all systems. The 'padding routines' are added or taken away
automatically within the microprocessor as extensions are recovered or
provided.
The CCU generates two tones, one of 350 Hz and one of 440 Hz which are
used to provide all the tones required by the system. The CCU also controls
the signals which are used to light the LEDs and operate the tone callers.
8.1