Note: Descriptions are shown in the official language in which they were submitted.
502~L
Signalling in PBX systems
This invention concerns si~nalling in PBX systems where a
plurality of telephone sets are connected to a central device
in a radial configuration (star-arrangement). In small
private branch exchanges (PBXs) ~here two-way alternate
signalling ta~es place between the individual telephone
sets and a central control device common to themr this
signalling takes place via a signalling wire-pair which,
together with the speech wire-pair, forms the four-wire
subscriber line. The telephone sets receive mainly connection
data for visual display elements fitted to them; the display
elements may for example be in the form of LED's and/or a
display-indicator. By means of the display-indicator, a
particular subscriber can for example be informed about
the call number selected or about accumulated charges. The
engaged condition of the internal or the existing external
circuits can be indicated by lighting up of the respective
LED's.
The data for all display elements on a telephone set can be
gathered together in one data block and transmitted as
binary digital signals. This can be effected by digital
signals whose binary values are defined by different
amplitudes of the signalling line voltage. The ~-bit, for
example, can correspond to the quiescent value of the
signalling voltage, while the l-bit can represent an active
signal in the form of a voltage reduction. Control of the
assigned voltage states is by means of a sending unit, which
in turn is controlled by a oontrol uhitr wihicb can for ex~mple be a
processor, contained in tih~ central d~vice. The-signalling wire-pair
is also used to transmit information wnich comes frcm the telephone sets
and in par,icular si~s the ac,uation oi in.dividual]ieys. ~esides
5~)~4
the number selection keys, other ke~s can be provided which
allow specific operating procedures or by the actuation of
which it is possible to sign~l the type of connection
intended. It is therefore possible to distinguish whether
an internal connection to another telephone set of the private
branch exchange is to be established, or an external
connection to a telephone set of the public network or to a
telephone set of another private automatic branch exchange.
The binary values of the data coming from the telephone sets
can be characterized by different signalling line current
values. In the case of two-way alternate signalling the
data for each direction can be combined in a data block.
The data blocks are transmitted continually and not only
when there is a change in the data which they contain. It
is also possible to have hit-by-bit interleaving of the data
to be assigned to both directions within a data block.
Two-way alternate signalling requires accurate timing of the
sending functions. To achieve this, the clock pulse used as
a basis for sending the data bits in the telephone set must
coincide in time with the clock pulse which determines the
sending of the data bits in the central device. Since it is
a prerequisite when signalling for the voltage or the current
on the signalling line to be varied, it is necessax~ to
ensure that the permissible noise voltage is not exceeded by
the sending pulses thereby generated. This also applies
when the four-wire subscri~er lines have a very short line
leng~h.
It is an object of the invention to sa~isfy the requirements
o~ synchroni7ation and suppression of noise by simple means.
.
S0;24
According to this invention there is provided apparatus
ror use in a small private branch exchange system
in which a pluralty of telephone sets are connected in a
star-arrange~ent to the central device via respective
associated four-wire subscriber lines each comprising a
speech wire-palr and a signalling wire-pair which latter
wire-pair provides two-way alternate digital data
.ransmission between the associated telephone set and the
central device, the transmission in one direction being
provided by variation of signalling line voltage and in the other
direction by signalling line current variation, the
transmission from the central device to the telephone sets
employing a higher packing density than in the other
direction, said apparatus comprising a sending unit for
transmitting voltage pulses as said digital data via the
signalling wire-pair (signalling line voltage variation),
a reactive circuit arranged to deform rectangular vol~age
pulses produced for transmission as said digital data and
two substantially corresponding receiving circuits one of
which is connected to the sending unit and arranged at the
transmitting.end to provide a synchronising signal as a
result of detection of each transmitted pulse, the other
receiving circuit being for use at the receiving end for
pulse detection and synchronisation so that timing signals
at transmitting and receiving ends can be made mutually
synchronous .
Due to deformation of the pulses forming the binary digits,
the deformation consisting essentially in rounding of the
pulse cornersr the noise voltagP occorring in the speech
wires as a result of signalling is reduced in comparison
with the use of rectang-llar pulses. Consequently, the
per~issible n~ise voltage nee~ not be exceede~ even with
;
~s~
very short subscriber lines. For most applications with
two-way alternate sign~lling, a low transmission frequencyr
for example 250 Hz, is used. With such a frequency, the
harmonics are in a range which is preferable for the ear.
Due to the deformation introduced and the associated
attenuation of the harmonics, the transmission level can
be raised without at the same time increasing the speech
line noise voltaye. The result is a reduction of
susceptibility to noise.
Since the pulses registered by the receiving circuits are
used to provide synchronization pulses, there is no longer
a phase shift between the drive signal for the sending
unit and the pulse actually transmitted, which is caused
during deformation by means of the reactive circuit.
Since synchronization takes place relative to a predetermined
direction with each pulse to be assigned to-this direction
within a data block, the required clock pulse can be derived
from an oscillator which, in the simplest of designs, need
not have a high degree of frequency stability.
The control of display elements at the telephone sets
requires transmission with a greater packing density than
in the opposite direction since active data is only ever
transmitted from the telephone set when a key is actuated.
At other times, for example, 0-bits corresponding to the
quiescent value of the line are sent. Preferably said
deformation is carriad out for those pulses which are
generated by the amplitude variation of the signalling line
voltage and serve to transmit information from the central
device to the telephone sets.
An embodiment of this inven~ion will now be described, by
way of example, with -eference .o the accompanying drawings
in which:-
~ ~5029L
~ 5Figl is a block circuit di~gr~m o~ PBX ~stem e~bodyin~
this invention,~nd
Fig2 is a set of graphs illustrating the operation of the
system shown in Figl.
The block circuit diagram of FIG 1 shows the basic structure
of a small private branch exchange in which two-way alternate
signalling takes place between central device and individual
telephone sets~ Only the details necessary for understanding
the invention are shown.
In the small private branch exchange shown in FIG 1, the
individual telephone sets TSl to TSx are connected in radial
configuration (star-arrangement) to a central device ZSt via
a four-wire subscriber line Ltg. Each subscriber line
consists of a speech wire pair a, b and a signalling wire
pair c, d. The wires a, b are used for transmitting the
speech data and ringing current. The wires c, d are used
for two-way alternate signalling between the individual
telephone sets and the central device, while at the same ti~e
supplying the necessary supply voltage to those units of each
telephone set which are associated with the signalling and
which are shown combined for telephone set TSl in a digital
transmission unit BS. This supply voltage is fed by current
transformer supplied by the mains voltage, which in the same
way can also deliver the supply voltage to be connected to
the speech wires ~or supplying the microphone. This is
indicated by the circuit symbol signifying a multiple
correction at one output of the unit SE.
The speech circuit SS is connected to the speech wires a, b
leading ~o each telephone set; the speech circuit SS, apart
from processing the speech data in connection with the
~502~
- 6 ~
handset H~, c~ntains a terminating cixcult. Internal
connections can be estahlished between the individual
telephone sets and the possibility exists of connection to
an external line EL, which can be a line to the public
telephone network or to another private branch exchange.
For example, for a total of 16 telephone sets which
together with the central device form the small private
branch exchange, four external lines can be provided. The
type of connection desired can, for example, be signalled
by the actuation of the pushbutton keys provided for the
purpose on each telephone set. For telephone set TSl the
possibility of initiating the transmission of certain data
by operating keys is indicated by the key pad TB. The key
Tw is intended to symbolize the keys of a conventional number
selection keyboard, while the key Te represents a series of
keys by means of which specific additional operating proce-
dures can be initiated. If, for example, depression of a
key intended ror the purpose - which may be one of the keys
Te - gives a signal of the intention to set up an external
connection, then subsequent operation of the number selection
keys results in output of the assigned selection data. In
systems with normal dial pulse generation by loop
interruptibn, this, controJ,led by the control unit Pz in the
central device, can be effected by the unit IW. ~he unit
MF provides the possibility of delivering the necessary
selection data in a known manner in a multiple-frequency
co~e. The control unit Pz also supplies the connectinq
commands for the switching matrix network KN, by means of
which the connection can be switched through by appropriate
actuation of its crosspoints. If an internal connection to
another telephone set of the private branch exchange is
desiredr the control unit issues appropriate connecting
commands for the switching matrix network.
~502~
-- 7 ~
sefore being processed, the data transmitted from a telephone
set can be b~lffer-stored by the control unit Pz in a me~ry
which is not specifically illustrated. Transmission of this
data can take place by means of binary digital signals,
whose binary values are characterized by a predefined
quiescent current value or by a drop in the signalling ~ine
current. This current drop can be brought about in every
telephone set by the sending device ~, which in turn is
controlled by a control unit Pt which first receives and
processes the stimuli coming from the keys. This control
unit is also used to register the lifting of the handset
and to pass information to this effect to the central device.
The sending device S can consist essentially of an electronic
switch by means of whIch a first o~ two current regulator
units, namely current regulator unit IRl wh~ch serves to
supply the control unit Pt, is switched to an inactive state.
During this time, the powex supply to this control unit is
maintained by means of a storage element contained in the
regulator unit.
In the unit BS of each telephone set there is a second
regulator circult IR2 through which visual display elements,
referenced by the display element L, are supplied with power.
These display elements can be light emitting diodes by means
o~ which, for example, the engaged condition of the
respecti~e Pxternal lines or the engaged condition of the
internal lines is indicated. These L~D's can be connected
in series in order to reduce their current consumption.
Those LED's which are to be switched off can each be
e~fectively bypassed by means of an electronic switch.
These swit~hes are con~rolled by the control unit Pt, which
for its part receives appropxiate connection information
fxol;l tne central device via receiving circuit Et.
A constant current is consumed by the regulator circuit
IR2. The current flowing via the c, d wires of the
signalling line therefore has a value which essentially
corresponds to the sum of the current drawn by both
regulator uni~s. Addea to this current value is the current
consumed by the sending device S and by the receiving
circuit Et, which in comparison is negligibly low. If, for
example, the regulator unit IRl is switched off by the
sending device, then the current flowing in the signalling
wire-pair is reduced by the value of the current otherwise
consumed by this regulator. Consequently, a modulation
of the current flowing on the signalling line takes place.
The binary values of the digital data to be transmitted are
therefore formed by the current drop, which can correspond
to a l-bit, and, for example, by the quiescent current value
of the signalling line, which then corresponds to the O-bit.
This data coming from the te]ephone set can be gathered
separately in a data block, data blocks then being
transmitted alternately to the central device and from the
central device to the telephone set. The data to be
transmitted from a telephone set can also be interleaved
with the data transmitted from the central ~evice bit-by-bit
within a data block which is to be formed.
Tne digital data transmitted from the telephone sets is
registered by a receiving circuit El to Ex coupled to one
wire of each signalling wire-pair and passed to the control
unit Pz in the ceniral device. There is therefore a
separate receivlng circuit for each teleph~ne. ~his receiving
S~2'~
~ g
circuit can consist essentially of a resistor inserted in
the d-wire and a transistor or operational amplifier to
evaluate the voltage drop across this resistor. On the
basis of the data received, the control Ullit intiates the
functions which are to follow.
~he data transmitted from the central device ZSt to the
individual telephone sets, mainly involving connection
information for visual display elements, are transmitted as
binary digital signals via the relevant signalling wire-pair.
The binary values of the entire information for a telephone
set which is to be gathered in a data block are formed by
a quiescent voltage value and by a voltage drop.
In many cases the same data has to be transmitted to the
individual telephone sets. This can be achieved by means
of a common sending circuit Sz control~ed by the control
unit Pz, whereby the sending circuit is multiple-connected
at the output end with one wire o~ each of the signalling
wire-pairs. In the practical example the sending circuit
is coupled to each c-wire. The sendin~ circuit Sz
conta~ns a voltage regulator SR, via which a predetermined
7 DC voltage supplied by the unit SE is applied to the
signalling wire-p~ir. This serves as the supply voltage
for the units coupled to this line in the respective
telephone sets.
The active binary value corresponding for example to a l-bit
is intended, as already mentioned, to be formed by a
determinate drop in the voltage on the signalling wire-pair-
The bit pattern corresponding to the information to be
transmitted in a data block is transmitted by a corresponding
.
~5~
~- 10 --
trans~itter bit timer to the sending circuit Sz by the
control unit P~. The transmitter bit timing is applied via
the base resistor R5 to the base of the transistor T2
providecl as a level converter. At each active transmitter
bit timing pulse for a bit pattern shown, for example, in
graph a of FIG2, the transistor T2 is driven. ~s a result,
driving potential is applied to the base of the transistor
Tl via the load resistor R4 and the resistors R2 and R3. ~y
turning this transistor on, the Zener diode ZDl is bypassed
Consequently, the potential at the so-called earth connection
of the integrated regulator circuit SR is reduced by the
value of the Zener voltage. The output voltage of the
regulator circuit follows this drop. At each transmitter
bit timing pulse which is applied within the framework Of
the bit pattern predefined by the control unit, the voltage
across the signalling wire-pair drops by a determinate value,
for example 3 V. Since individual telephone sets can be
coupled directly or via only very short subscriber lines to
the central device, it h~s to be ensured that no inadmissibly
high noise voltage occurs on the speech line when the
pulses are transmitted. Since a low transmission frequency
of for example 250 Hz can be used for signalling, the
harmonics in the case of rectangular pulse generation lie
in a range in which the ear is very sensitive. In the
sending circuit there are RC logic elements by means of
which a deformation of the transmitted pulses is achieved.
This consists essentially in a rounding of the corners of
the pulses to be transmitted. This is achieved by means
of time circuits formed from the resistor Rl and capacitor
Cl and from the resistors R2, R3 and capacitor C2. The
capacitor C3 connectea across the voltage regulator serves
to suppress the oscillation tendency of the regulator. If
~ ~s~
the individual telephone sets are intended to xeceive
different data, this can be effected by means of a simple
selection circuit Sl tc Sx, provided accordingly for each
telephone set. By means of this additional sending circuit
it is them possible to insert an additional data bit in a
position, predetermined for each respective telephone set,
in the data block transmitted by the common sending circuit
Sz. This can be effected by means of--a voltage drop on the
d-wire which leads to the ~elephone set which is to xeceive
the information. ~s a result, the entire relevant
information in the data block in question is ~valuated only
by the receiving circuit contained in this assigned telephone
set, while being ignored by the control units of the other
,elephone sets which evaluate pulses received. The selection
circuit which in each case is coupled to a d-wire can, as
shown for the circuit Sl, be in the form of a ~ener diode
ZD2 inserted in this ~re; this Zener diode can be bypassed
by means of transistor T3 which i5 driven by the control
unit via resistor R6. If this transistor is turned on by a
drive pulse supplied by the control unit, then the voltage
across the signalling wire-pair is reduced by the value of
the Zener voltage. In this way~ therefore, it is possible
to insert a l-bit in the data block for a specific
telephone set, while for all other telephone se~s there is
a O-bit in this position. These latter telephone sets are
so arran~ed that this received data block is not evaluated~
If it is intended for all the telephone sets to evaluate
information, then the common sending circuit Sz in the
central de~ice can transmit the appropriate selection bit
in the daia block at the same time. Selection of a
specific telephone set by inserting a selection bit
defined by a voltage drop is possible, since in the telephone
5~
- 12 ~
set no reference potential is fixed ~or the wires of the
signalling wire pair,
In the telephone set, the receiving circuit Et inserted in
the signalling line evaluates the voltage variations
occurring on the wires of the relevant signalling wire-pair-
In this way it is possible to determine the data bits defined
by voltage drops. The receiver of the incoming data bits is
very simply in the form of the transistor T6, whose emitter
is connected to the c-wire while its base is connected via
the base resistor Rl~ to the plus side of the capacitor C5
connected to the d-wire. ~lth t~ansistor T7 turned on, this
capacitor C5 is charged via Zener diodes ZD3 to a voltage
which is lower than the voltage on the c-wire by the value
of the Zener voltage. If the sending circuit in the central
device causes a voltage drop, then the voltage drops for
example by 3 V, while the voltage across the capacitor CS
keeps the base potential of the transistor T6 roughly
constant. If the Zener voltage of the Zener diode ZD3 is
chosen such that together with the base emitter voltage of
the transistor T6 it is less than or at most equal to the
voltage drop which ha~ taken place, then the transistor is
blocked. At the end of each voltage drop, the transistor
rr6 becomes conductive again. Each voltage drop is therefore
transmitted to the control unit P_ as an appropriate
control pulse. Eor this purpose, the voltage drop occurring
at collector resistor R15 is used. The collector circuit
also contains a delay circuit consisting of th~ resistor R16
and the capacitor C6. This delay circuit serves the purpose
of stopping momentary drops in the signal voltage due to
noise from belng evaluated.
As already explained, there is a deformation of the active
~ ~S~)~4
signals, for~ed b~ volta~e drops, which are transmitted by
the sending unit Sz of the central device ZS_. This is
ef~ected by means of xeactive components and serves,
especially with regard to short subscriber lines, to reduce
the noise voltage on the speech lines. In the practical
example, the corners of each pulse transmitted are rounded
b~ RC networ~s formed from the resistor Rl and capacitor
Cl, and from the resistors R2 and R3 and capacitor C2. ~s
a result of these RC networks, a phase shift occurs between
the transmitter bit timing pulse which is supplied by the
control unit Pz and the signal ~ransmitted due to this
timing pulse. Two-way alternate signalling, in which the
data is either sent one block from the central device to
the relevant telephone set and then one block in the
opposite direction, or in which the data bits to be assigned
alternately to the two directions are interleaved bit-by-bit
within a da~a block, requires accurate synchxonization of
the t~mes of transmission in the circuits described. The
clock pulse used as a basis for transmitting the data bits
in the telephone ~et must agree with the clock pulse which
aetermines the transmission of the data bits in the central
device. For this purpose, mutual synchronization of those
units from which the clock pulse is derived is necessary.
In the practical example, the clock rates required for the
control unit Pz of the central device 7st are derived from
the oscillator arrangement Oszl. For the control unit Pt
of the telephone set TSl, the necessary clock rates are
derived ~rom the oscillator arrangement Osz2 with divider
circuits, which are not illustrated further, interposed.
The same applies to the other telephone sets TS.
The endeavour is to construct the individual units of
small private branch exchanges in a cost-effective manner.
~850~
~ 14 -
Consequentl~ simple oscillator circuits are used for which
thexe`are no exacting demands with regard to their frequency
stability. However, in order to adhere to the required
clock accuracy~ it is necessary for them also to be
synchronized within each data block transmitted, these
synchronization pulses being derived from the pulses trans-
mitted in a given direction. The transmitter bit timing and
the pulse received in the individual telephone sets as a
result of this transmitter bit timing cannot be used for this
synchronization, because the phase shift mentioned above
occurs on account of the RC networks provided for the purpose
of deforming the send pulses. In order ~o eliminate this
phase shift, the same simple receiving circuit as used in
the individual telephone sets is coupled to the send line of
the central device. In this way it is possible to effect
the necessary timing in the central device for the arrival
of signal pulses which are transmitted from a telephone set
~ue to the two identical receiving circuits,a predetermined
edge o~ the transmitted pulse, lor exam~le the descending
pulse edge, is recognized simultaneously by them.
Consequently, synchronization between the oscillator
arrangement in the central device and the oscillator
arrangements in the individual telephone sets can be re-
established by recognition of this pulse edge, which then in
each case results in a synchronization pulse. It is there~ore
possible to repeat synchronization frequently within each
data block, so that cheap oscillator circuits with a greater
tolerance can be employed.
The receiving circuit Ez additionally connected to the send
line in the central device/ in the same way as the receiving
circuit Et which is provided in each telephone set, contains
a Zener diode, namely the Zener diode ZD4, for voltage
~85~2~
- 15 -
evaluation. A certain Zener current is maintained for this
Zener diode ~ means of a high-value resistor R7. Transistor
T4 coupled to the Zener diode on its emitter side and via
base resistor R8 is controlled by the voltage drop of the
Zener diode. This transistor T4 corresponds in terms of its
function to the transistor T6 in the receiving circuit Et.
A collector resistor of the transistor T4 is designated R9.
The voltage drop occurring across this resistor, which in
each case de~ines a transmitted pulse, controls the
transistor T5 via resistor R10. The resistor R10 in
conjunc~ion with capacitor C~, forms a delav circuit which serves
the purpose of preventing pulse spikes from ta~ing place.
The transistor T5 serves solely as a level converter. The
voltage drop occurring at its collector resistor ~11 is
supplied as a synchronizing signal to the control unit Pz
via the resistor R12. This control unit can then reco~nize
for example each descending edge of a transmitted pulse.
The same edge is recogni7ed with negligible delay times
simultaneously by the control unit Pt on account of the
received pulse transmitted to it in the same way. Each
control unit then derives from ~his a synchronization pulse
~or the oscillator circuits Oszl and Osz2 respectively.
The receiving unit Ez, unlike the receiving unit E , does
not contain those components provided in unit Et for
specific matching to the line conditions. In the case of the
receiving circuit Ez, which is arranged at the beginning of
the line relative to the sending direction, the influence
o~ the transmission line can be aisregarded.
The pulse aiagram in graph a of FIG 2 shows an example of a
bit pattern for a data block to be transmitted from the
central device. Each bit pattern shown corresponds to the
transmitter bit timing applied by the control unit Pz to the
50~
. 16
com~on sending circuit ~z. The data block, which is trans~
mitted continually and not only when there are changes of
information, can begin with a synchronization bit S~ which
can have twice the n~rmal bit duration in order to distinguish
it from the other data bits. The following data bit Se
drawn in a broken line can be the selection bit which has
been described and which is inserted into the data block
by one of the selection circuits Sl to Sx in order to select
a specific telephone set. In the example, the bit sequence
"O,lrO,l" follows on. The position of these bits in the
data block determines the selection address for individual
visual display elements. This bit sequence determines the
state of ill~nination of the display elements. The
transmission of a l-bit, for example, will indicate that the
assigned display element is to be switched to the active
state. If the element is an LED, then it is thereby put in
to its illuminated state. Thi-s is ensured by appropriate
drive signals from the contro~ unit of the telephone set
concerned.
,,
An acknowledgment bit Q and a parity bit P can ~ollow. In
the pæactical example illustrated in graph a F~G 2, an
interval of the same length in each case is inserted betwee~
the data bits. During this interval, a data bit sent from
the telephone can be inserted as a O-bit or a l-bit during
bit-by-bit ~nterleaving.
Graph b in FIG 2 shows the potential profile on the signalling
wire-pair res~-lting from the transmitter bit timing
predeter~ined by the control unit as per graph _. As already
explained, each clock pulse leads to a drop in the signal
voltage. This drop from the signal voltage quiescent value
Ur to the value Us thereby defines each l-bit. A phase
:
:`
~1 ~&5(~4
shift t_ exists between each transmitter bit timing pulse
and the pulse actually transmitted. This phase shift is
produced by the RC networks which are connected into the
sending circuit in order to deform the transmitted pulses
before t~ansmission from the sending end. Consequently, the
necessary reduction of noise voltage is also achieved in
those cases where a telephone set is connected to the central
device via a very shori subcriber's line. The transmitted
pulse is received during its pulse time tz both in the
receiving circuit at the sending end Ez and in the receiving
circuit Et and fed to the relevant control unit Pz or Pt.
Each descending edge can be used for synchronization of the
existing oscillator circuits. This is indicated in each case
by the reference Sl in graph b. During the time Ts, a data
bit can be sent from the relevant telephone set to the
central device.
