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Патент USA US3030455

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Apnl 17, 1962
E. G. HITCH ET AL
3,030,445
COMMUNICATION SYSTEMS
Filed June 23, 1959
2 Sheets-Sheet 1
1.1aw
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‘
I
INVENTORS
ERIC GIBBON HITCH
BRIAN ALBERT PELLS
CLIFFORD HENRY JAMES FLEE'I‘WOOD
nited States area-t0
1C6
Patented Apr. 17, 1962
2
1
3,030,445
COMMUNICATION SYSTEMS
3,030,445
W
_means according to the function required of it at any
time, it may also be desirable for the switching to be
under control of the master equipment so that, for in
Eric Gibbon Hitch, Middlesex, and Brian Albert Polls
and Clifford Henry James Fleetwood, London, Eng
land, assignors to Siemens Edison Swan Limited, Lon
don, England, a British company .
Filed June 23, 1959, Ser. No. 822,279
Claims priority, application Great Britain July 11, 1958
6 Claims. (Cl. 179-1)
stance, the extension equipment can only be set for trans
mission when permitted to do so by the master equip
ment and is at other times set for reception: the master
equipment would then be able to transmit to the exten
sion at any time.
An object of the invention is to avoid the requirement
10 for a separate energy supply at the or each extension
equipment, while at the same time permitting the exten~
This invention relates to communication systems per
sion equipment to be under control of the master equip
mitting the transmission between spaced locations of in- 1
ment, all without using more line connections than are
telligence in the form of electrical signals, and more par
required for the transmission of intelligence between
ticularly to such a system of the kind in which a so-called
“master” equipment is connected to at least one “exten~ 15 them.
With this object in view it is now proposed in accord
sion” equipment for communication therewith and where
ance with the present invention that the amplifying means
in the transmission of intelligence between the master
in the extension equipment, having two, alternative,
equipment and an extension equipment can be effected
modes of connection for respectively amplifying incom
in only one direction at any time. The connection be
tween master and extension equipments may be made 20 ing and outgoing A.C. intelligence signals, shall be ener
gisable by D.C. energy fed to it from the master equip
over a single line wire and completed by way of an earth
ment over the same line connection as is used for the
return circuit. With this mode of connection, however,
transmission of the intelligencesignals, and that the ex
excessive cross-talk may be received at‘ the equipments
,tension equipment shall also include a switching device
due to the induction into the single wire of unwanted
signals. For this reason it may usually, though not nec 25 which is operable in response to one polarity of said
D.C. energy, but not in response to the opposite polarity,
essarily always, be preferred to provide between the equip
to change the connection of the amplifying means from
ments a two-wire connection which is balanced about
one of said modes to the other. In this way the master
earth, since unwanted signals would then tend to be
equipment can control the connection of the extension
induced equally into the two line wires, being of the
same polarity with respect to'earth, and would thus tend 30 equipment amplifying means for ampli?cation of incom
ing or outgoing signals, that is, for transmission or recep
to cancel each other in the balanced two-wire connection
tion, simply by determining the polarity of the D.C. en
to give an appreciable decrease in the resultant cross-talk
,ergy which it feeds to the extension equipment. As will
present therein.
.
,be appreciated, appropriate means such for instance as
It is not uncommon in a communication system of the
above kind for the connection between master and ex 35 a recti?er bridge, would have to be included in the ex
tension equipment in order to ensure that the D.C. en
tension equipments to be made over “land-lines,” that
ergy as applied to the amplifying means will be of the
is line wires leased from a national or other telephone
correct polarity for its operation, irrespective of the
authority. Where this is the case, however, it may be
polarity of the energy as actually received from the mas
necessary when designing the system to take into con
ter equipment.
sideration certain requirements governing the use of
land-lines, which requirements, inter alia, may specify
the maximum electrical energy that land-lines are per
mitted to carry.
Consequently, if the system is to provide speech com
In carrying out the invention, it is contemplated that
the amplifying means in the extension equipment would
advantageously be a transistor ampli?er so that the D.C.
energy required to be supplied from the master equip
munication on a broadcast'basis in both directions, that 45 ment can be kept to a minimum. Preferably, such am
pli?er would include at least two transistors connected
is, audio reception is to be effected at both master and
in push-pull and giving Class A ampli?cation: this would
extension by means of loudspeakers, the energy level
reduce the modulation of the D.C. energy and so sim
required by intelligence signals, in order to actuate a
plify feed-back problems.
loudspeaker to produce the desired audio output there
Suitably the switching device in the extension equipment
from, may be higher than the maximum energy level 50
allowed to be transmitted over the land-lines. In such
a circumstance, therefore, “loudspeaker-to-loudspeaker”
working may be provided for by including at both the
master equipment and the extension equipment respec—
tive amplifying means which serve to vamplify outgoing 55
intelligence signals to an energy level commensurate with
that permitted for transmission and also to amplify in
coming signals to a yet higher energy level suitable for
may be a relay operable by the D.C. energy and con
trolling change-over contacts effective to provide appro
priate switching of input and output circuits of the am
plifying means according as the extension is to be set
for transmission or reception, such relay being polarised,
as by means of a half-wave recti?er connected in series
‘with an energizing winding thereof, so that it operates
only when‘the D.C. energy is of requisite polarity. With
this arrangement, a single electroacoustical transducer
also provide compensation for attenuation of the trans 60 may be employed which is normally connected to an out
put circuit of the amplifying means to function as a loud
mitted signals by the land-line, such compensation being
speaker, but which by operation of the relay change
required particularly if the master and extension equip
actuating the loudspeakers. The amplifying means can
ments are a considerable distance v[apart and a relatively
over contacts referred to can be‘connected instead to an
long land-line is needed to interconnect them. Asre
input circuit of the amplifying means to function as a
tends to be costly, especially where a considerable num
transistors, might ‘be used.
Instead of using a polarised relay as the
gards such amplifying means in the ‘extension equipment, 65 microphone.
switching device, it is possible that a switching arrange
the provision of a suitable energy supply for it at the
ment of an electronic nature, for instance one employing
location of the extension equipment is inconvenient and
In order to prevent the master equipment from over
ber of extensions are involved. With the same amplify
hearing
the extension without the sanction of the latter,
70
ing means used for both transmission and reception in
as could happen if the extension equipment could be set
the extension equipment, some form of switching being
for‘ transmission under control of the master equipment
provided for establishing connection to the amplifying
3
3,030,445
alone, it may be desirable to providethe extension equip
4
ment: the make contacts Id and k2 are both open when
ment with some means which require to be operated by
a person at the extension before amplifying means there
the control switch is in a normal position and closed when
it is in an operated position. Connected across input
at can be set for transmission. To this end the exten
sion equipment may include a control switch which, un
circuit comprising the energising winding r1 of. a change
less it is operated, prevents the switching device from
responding irrespective of the polarity of the received
D.C. energy.
terminals rtl and 112 of the recti?er bridge RF is a series
over relay RL, which constitutes the ‘switchingdevice con?
trolled from the master equipment, a recti?er Rf5, and
the contacts k2. ‘ The recti?er Rf5 permits current ?ow
As regards the master equipment, the reversal of the
through the winding 1'1 and consequential operation of
polarity of the D.C. energy may be effected therein by 10 the relay RL, only when the line wire Lla is positive and
means of polarity reversing contacts controlled by a
the line wire Llb negative, that is, when the master equip
switching relay or other changeover means which itself
ment is ‘set for reception.‘ It is evident, however, that
is controlled by contacts of a receive/transmit switch.
such current will now only when the contacts k2 are
Amplifying means, preferably a transistor ampli?er, and
a dual=pt1rpose transducer, similar to those provided for 15 closed.
Output terminals H3 and H4 of the recti?er bridge RF
‘the extension equipment may be provided for the master
are connected to leads LN and LP which serve as negative
equipment also, with further contacts of the receive/
and positive supply leads over which the D.C. energy
transmit switch serving to provide appropriate switching
received over theline wires'Lla and L1]; is extended to
of the-input and output circuits of this amplifying means.
a transistor ampli?er ‘TRA. 'The ampli?er TRA is nor
In addition to amplifying signals both incoming to and 20 mally connected for reception, but can be connected for
‘outgoing from the master equipment, the master equip
transmission by vmeans of change-over contacts RL1,
ment amplifying means may also be arranged ‘to be
operable as an oscillator generating a tone frequency
signal by connecting its input and output circuits to
.gether.
RL2 and ‘RL3 which are operated on operation of relay
RL. The ampli?er TRA and the manner in which it can
be switched either for reception or transmission will ‘be
This tone frequency signal may serve as a 25 dealt with presently.
calling signal either in respect of the master equipment
‘calling an extension equipment, or in respect of an ex~
tension equipment calling the master equipment, the in
The bridge recti?ersRfl-Rf4 are so poled that when
the D.C. energy fed over the line wires Lla and Llb makes
them respectively negative and positive (that is, when the
‘terconnection of the input and output circuits of the
master equipment is set for transmission) current can ?ow
amplifying means being e?ected in the former instance 30 from the positive line wire Llb into the positive supply
by contacts controlled by a ringing switch included in
lead ‘LP over the path: line wire Llb, primary winding p2,
the master equipment and in the latter instance by con
.and recti?er RB, and from the negative supply lead LN
tacts controlled by a calling relay which can be energised
to the negative line wire Lla over the path: line wire Llm,
in the master equipment by the establishment of a line
primary winding p1 and recti?er Rf4. The ampli?er TRA‘
loop at the extension equipment.
35 will therefore be energised whenever the master equip
In order that the invention may be more fully under
ment is set for transmission. When, however, the master
stood a particular embodiment thereof will now be de
equipment is set for reception, the line wire Lla being
scribed with reference to the accompanying drawings in
then positive and the line wire Llb negative, the current
which:
path between the negative line wire Llb and the supply
FIG. 1 illustrates in conventional .manner the circuit
lead LN is now ‘by way of the primary winding p2 and
'of an extension equipment; and
the recti?er Rf2, while the current path between the
FIG. 2 likewise illustrates the circuit of a master
positive line wire Lla ‘and the supply lead LP is by way
equipment suitable for controlling a number of exten
of the primary winding p1, the'recti?er Rfl and the con
sion equipments constituted as illustrated in FIG. 1.
tacts k1. Therefore at such times as the master equip
Referring to the drawings, the extension equipment
ment is set vfor reception, the contacts k1 must be closed
of FIG. 1 would in use be connected to a master equip
before the ampli?er TRA can be energised, that is the
ment over a'pair of line wires Lia and Llb which are
used both for the transmission of AC. intelligence sig
nals between the equipments and for the supply of D.C.
control switch requires to be in its operated position.
Consequently it is not possible for the master equipment
to listen-in to the extension equipment unless the control
energy to the extension equipment from the master 50 switch in the latter has been operated, thus giving protec
equipment. These lines are also used for the transmis~
tion against unsanctioned listening-in to the extension.
sion of calling signals as will be described later. The
Furthermore, since the relay RL can be operated only in
D.C. energy is controlled by the master equipment to
response to D.C. energy received over the line wires Lla
be of one polarity when that equipment is set for trans
and Llb with polarity corresponding to the master equip
mission .and of the opposite polarity when it is set for 55 ment’ being set for reception, the result is obtained that,
reception. It will be assumed that the controlled polarities
even though the control switch may be operated, the
of the D.C. energy are such that the line wires Lla and
ampli?er TRA is connected for transmission from the
Llb are respectively negative and positive when the mas
extension equipment only if the master equipment is set
ter equipment is set for transmission, and are respectively
for reception. The ampli?er TRA thus remains connected
positive and negative when it is set for reception.‘ Other 60 for reception at such times as the master equipment is set
extension equipments such as shown in FIG. 1 may be
for transmission, the master equipment therefore con
likewise connected to the master equipment over respec-v
tive pairs of line wires (Lla, Llb, L2a, L2b . . . Lna,
trolling the direction of transmission.
Considering now the transistor ampli?er TRA and its
Lnb, FIG. 2) for. operation therewith on a similar basis.
manner of operation. When the extension equipment is
Considering the extension equipment circuit shown in 65 set for reception, circulation of AC. intelligence signals
FIG. 1, line wires Lla and Llb are connected to the
primary side of an input transformer T1, which has two
transmitted by the master equipment, or of a tone fre
primary winding p1 and p2 serially connected between
quency signaltransmitted as a calling signal, takes place
in a primary receiving circuit which follows the, path: line
the line wires Lla and Llb with an intervening recti?er
wire Lla, primary winding p1, recti?er Rf4, supply lead
bridge network RF of conventional form consisting of 70 'LN, a capacitor C1, supply lead LP, recti?er Rf3, primary
winding p2, to line wire Llb. The recti?e'rs Rf3 and RM
four recti?ers Rfl-Rf4. There is included in one arm
o?er lowimpedance to the received signals, being at this
of the recti?er bridge RF, in series with the recti?er Rfl,
time biased, into conduction by the transmitting polarity
make contacts k1 which, together with make contacts k2
gangedthereto, are .controlled .bya control switchtnot
of the received D.C., energy, while the capacitor C1 con
otherwise represented) included in the extension equip 75 nected across the supply leads‘LP aiid‘L‘N'has a' large
6
‘ capacitance in order also to provide a low impedance path
for the received A.C. signals while at the same time act
as a ?lter which removes these A.C. signals and also any
spurious switching pulses which may be received, from
the DC. energy as applied to the ampli?er TRA over the
supply leads LP and LN. From the primary windings p1
and p2 of the input transformer T1, the received A.C.
signals are induced into serially connected secondary
winding s1 and s2 thereof, whence they are applied to the
base of an input transistor TR1 by way of an attenuation
winding s4. ‘From there, they are applied through the
now operated contacts RL1 and the resistance pad RP to
the windings s1 and s2 of the input transformer T1,
whence by induction into the windings p1 and p2 of this
transformer, they are transmitted over the line wires
Lla and Llb to the master equipment. As will be ap
preciated, when the‘ extension equipment is thus set for
transmission the windings s1 and s2 of transformer T1
serve as primary windings and the windings p1 and p2
serve as secondary windings.
Turning now to FIG. 2, the circuit of the master equip
ment shown therein includes a transistor ampli?er TRB
which is similar to the ampli?er TRA just described and
like it can be switched to amplify either received signals
R95 and a ?xed resistor Rs6, the relay contacts RLZ
(unoperated), and a capacitor C2. The base of the tran 15 or signals to be transmitted. In this instance, however,
the switching of the ampli?er TRB from a receiving func
sistor TR1 is connected at the junction of two resistors
tion to a transmitting function is effected by means of
Rs7 and R98 connected in series across the supply leads
switch contacts swl, sw2 and sw3 controlled by a receive/
Lp and LN, the base thus being biased to the potential of
‘transmit switch (not otherwise represented), in conjunc—
this junction, while the emitter of the transistor TR1 is
connected to the positive supply lead LP through a resis 20 tion with change-over relay contacts RLAl controlled by
a switching relay RLA and corresponding to the relay
tor Rs9 across which is shunted a capacitor C3 offering
contacts RL2 in FIG. 1. The contacts sw2 and sw3 cor
a low impedance return path to the received signals.
respond in function to the relay contacts RL1 and RL3
The collector of transistor TR1 is connected to the
in FIG. 1. The relay RLA also has two polarity revers
supply lead LN through the primary winding p3 of a
transformer T2 by which the transistor TR1 is trans 25 ing contacts RLA2 and RLA3 which according to their
matching pad RP (comprising series resistors RS2 and R93
and shunt resistor Rs4), the relay contacts RL1 (unop
erated), a potentiometer comprising a variable resistor
former-coupled to ‘a push-pull output stage comprising
condition (unoperated or operated) determine the polar
two further transistors TRZ and TR3. The transformer
TZ-has a centre-tapped secondary winding s3 connected
wires Lla Llb, L2a L2b . . . Lna, Lnb, connecting the
ity of the DC. energy as applied to the pairs of line
master equipment to respective extension equipments.
at its ends to the bases of transistors TRZ and TR3 respec
tively, and at its centre-tap to the junction of two resistors 30 The line wire pairs Lla L1b, L2a L2b . . . Lna Lnb,
are connected through padding resistors Rp, which serve
. R910 and Rs11 connected in series across the supply leads
to match each pair of line wires to, say, 300 ohms, to
LP and LN: the potential at this latter junction is applied
through‘ the appertaining half of the secondary winding s3
the moving contacts of respective pairs of change-over
contacts Kla Klb, K2a K212 . . . Kna Knb, which pairs
as a biasing potential for the bases of the two transistors
. TR2 and TR3. The emitters of the transistors TRZ and 35 are controlled by respective extension keys, push-buttons
or other such switches, not otherwise represented. The
' TR3 are connected together and through a resistor R912
master equipment also includes a cancellation switch hav
to the positive supply lead LP, while the collectors of
ing break contacts Kxl and change-over contacts KxZ,
these two transistors are connected to opposite ends of a
centre-tapped primary winding p4 of an output trans
and a ringing switch having make contacts krl and krZ,
_ former T3. The centre-tap of primary winding p4 is con 40 and break contacts kr3. Preferably, the extension switch
nected to the negativesupply lead LN, thereby affording
es and the cancellation switch are mechanically coupled
connection of this supply lead to the collectors of the
(in some known manner not shown), such that operation
' transistors TRZ and TR3 through the respective halves of
of an extension switch causes release of the cancellation
the winding p4. The output transformer T3 also has a
switch, while operation of this latter switch causes release
tapped secondary winding s4 which is connected at one 45 of any operated extension switch. Thus although in FIG.
end, a, to the positive supply lead LP, at the other end, b,
2 the cancellation switch contacts Kxl and Kx2 are
to the ‘front contact of the relay contacts RL1, and at its
shown as being unoperated (as are the extension switch
tapping, c, through the relay contacts RL3 (unoperated)
contacts), they will be assumed normally to be operated
to an input terminal ta of an electro-acoustical transducer
and to assume their unoperated position on operation of
TX. A second input terminal tb of the transducer TX is
one of the extension switches. The ringing switch oper
connected directly to the positive supply lead LP.
ates its contacts independently of the cancellation switch
With the extension equipment set for reception (as
and the extension switches.
shown), the received signals as applied to the base of
The master equipment is supplied with DC. energy
the input transistor TR1 are ampli?ed by this transistor
over positive and negative supply leads Pm and Nm
and passed to the push-pull transistors TRZ and TR3 for 55 which may for instance be connected to receive as the
further ampli?cation, after which they are extended to
DC. energy the recti?ed output from a suitable A.C.
' the transducer TX, effectively connected across the por
tion ac of the secondary winding s4 of the transformer T3.
source.
Secondary positive and negative supply leads
Pa and Ne are respectively connected to the supply leads
When the extension equipment is set for transmission,
‘ Pm and Nm by way of respective choke coils CI-Ip and
namely when the relay RL is operated, the contacts RL1 60 CHn, these leads Pe and Ne serving to extend the DC.
connect the matching pad RP across the ends a and b of
the secondary winding s4 of the output transformer T3,
contacts RL2 render the potentiometer comprising resis
tors RsS and Rs6 ineffectual, and contacts RL3 disconnect
energy, through the polarity reversing relay contacts
RLA2 and RLA3, to further leads La and Lb of which the
lead La is connected in common to the back contact of
each of the contacts Kla, KZa . . . Kna, while the lead
the transducer TX from the secondary winding s4 and 65 Lb is connected in common to the back contact of each
connect it instead through the now operated contacts RL2
of the contacts Klb, K2b . . . Knb.
to the capacitor C2 at the base of the input transistor
Examination of the circuit will show that when the
TR1. In order to prevent switching noises from reach
relay contacts RLA2 and RLA3 are unoperated, the lead
ing the transducer TX, the contacts RL3‘ should change
Fe is connected to the lead La through the contacts RLA2
' over before the contacts RL1 and RL2 do so. Thus in 70 and the lead Ne is connected to the lead Lb through the
— this condition of the extension equipment, A.C. intelli
contacts RLA3, whereas when these contacts are operated
gence signals produced by the transducer TX are applied
(relay RLA energised) they connect the lead Pe to the
RLZ
(by-passing
the
po
. through the operated contacts
lead Lb and the lead Ne to the lead La. In consequence,
tentiometer RsS, Rs6) to the ampli?er TRA, and after
when a pair of extension key contacts (say Kla and Klb)
'- ampli?cation appear in the output transformer secondary 75
3,030,445
7
8
are operated the line wires Lla and L1b become respec
tively positive and negative if the relay RLA is unoperated
and respectively negative and positive if this relay is
position of the receive/ transmit switch causes operation of
relay RLA so that the polarity of the D.C. energy as ap
operated. The leads La and Lb also serve to carry A.C.
extension equipment for reception. In the receiving posi
tion of the receive/transmit switch, the relay RLA is un
plied to the line wires Lla and Llb is such as to set the
intelligence between the ampli?er TRB and the extension
operated and the polarity of the line wires is such as to
permit the extension equipment to be set for transmission
key contacts Kla Klb, Kza K2b . . . Kna Knb, and the
function of the choke coils CHp and CHn is to prevent
by operation of the control switch contacts k1, k2 (FIG.
signals from reaching, the supply leads Pm and Nm.
1) thereat. It is evident that instead of using the ringing
Each of these choke coils has connected across it a non
linear impedance element Rx, such as that known‘ by the 10 switch to call the attention of the extension, this may be
done simply by operating the receive/ transmit switch and
Registered Trade Mark “Metrosil,” which offers a low irri
making some distinctive sound, for example speaking or
pedance to induce voltages built up across the appertain
whistling, which will be picked up by the transducer TX’
1ng choke-coil when the polarity of the D.C. energy as
applied to the leads La and Lb is reversed by the relay
contacts RLA2 and RLA3.
Capacitors Ca and Cb are 15
provided in order to prevent the D.C. energy from reach
lng the input transformer, T1’, of the ampli?er TRB.
The master equipment also includes: a main indica
and transmitted to the extension.
When it is required to call the master equipment from
the extension equipment of FIG. 1, the control switch in
the latter is operated and its make contacts k1 loop the
line wires Llpa and Llb at the extension equipment. In the
master equipment a connection exists, on the one hand, be
tion lamp LX which is permanently connected across the
supply leads Pm and Nm in series with a limiting resistor 20 tween the line wire Llb and the positive supply lead Pm by
way of switch contacts Klb (unoperated) and the lead Ld,
Rr, thereby to provide a visual indication when these
and on the other hand, between the line wire Lla and the
supply leads are carrying the D.C. energy; indication
negative supply lead Nm by way of switch contacts
Kla (unoperated), the indication lamp L1, the cancella
lamps L1, L2 . . . Ln which pertain respectively to the
several extension equipments connected to the master
equipment and which give a visual indication whenever
tion switch contacts KxZ (normally operated) and the
energising winding rlb of the relay RLB. These two
the appertaining extension is calling; and a calling relay
connections are therefore joined by the loop at the ex
RLB which controls four change-over contacts RLB1
tionsion equipment so ‘as to complete a circuit for the
RLB4 and is operated when an extension equipment calls
D.C. energy, thereby causing operation of the relay RLB
the master equipment but only if the master equipment
30 and the lighting of the lamp L1. The lamp L1 gives a
is not then in communication with another extension.
visual indication that the extension is calling, ‘and in addi
The operation of the master equipment is as follows:
tion the relay contacts RLB1 interconnect the leads Lo
to connect a particular extension to the master equipment,
and Ld, thereby completing the energising circuit for the
which for the sake of description willybe assumed to be
ampli?er TRB in the same manner as do the cancellation
the extension connected to the line
Lia and Llb,
the extension switch contacts Kla and Kib are operated. 35 switch contacts Kxl, when operated, while the relay con
This results in the cancellation switch contacts, Kxl and
tacts RLB2 interconnect the input and output circuits
KxZ, assumed to be normally operated, being restored to
their unoperated position shown, in which the make con
erating oscillator as already described: in this instance the
of the ampli?er TRB to cause it to function as a tone gen
resistor R0 and the capacitor C0 are by-passed, so that
the generated tone is different in fraequency from that
tacts Kxl interconnect two leads Lc and Ld and there
by connect the negative supply lead Nm to the ampli?er
TRB to energise the latter, the positive lead Pm being
permanently connected to this ampli?er. The change-over
contacts KxZ are ineffective at this time. Following this,
generated when the master equipment is calling. The tone
signal generated by the ampli?er TRB ‘is fed to the trans
ducer TX’ by way of the switch contacts sw3 (unop
erated), the ringing switch contacts kr3' (unoperated),
operation of the ringing switch causes the make contacts
kr'l, on closure, to Complete an energising circuit for the
energising winding rla of the relay RLA between the
positive and negative supply leads Pm and Nm. Con
tacts RLA2 and RLA3 are thereby operated and result in
D.C. energy being applied to the line wires Lla and
Llb with polarity corresponding to the master equipment
being set for transmission, that is, the line wire Llc be
comes negative and the line wire Lib positive. Also, the
ringing switch make contacts kr2, on closure, inter
connect the input and output circuits of the ampiitier TRB
and a limiting resistor Rt, to produce an audible calling
signal in addition to the visual signal produced by the
relevant lamp (Ll). Normally the resistor Rt is shunted
by the relay contacts RLB3, but in the condition just
described,these contacts are opened so that the level of
the tone signal applied to the transducer TX’ is reduced.
It may be desirable to have some means remote from
the master equipment of indicating when an extension
equipment is calling. To this end a remote indicator
such as a lamp or a buzzer (not shown) may be connected
through a feedback circuit consisting of a resistance R0 55 to terminals tt which are energised by relay contacts
RLB4, thereby to operate this indicator, when an exten
and a capacitor C0, thereby causing this ampli?er to func
sion calling condition obtains. In response to an indica
tion as an oscillator transmitting a tone frequency calling
‘ tion that an extension is calling, the relevant extension
signal to the extension equipment: the values of the resistor
switch is operated, causing restoration of the cancella
R0 and the capacitor C0 serve to determine the fre
quency of oscillation. During the time that the tone sig 60 tion switch and thus release of the relay RLB. The master
equipment can then be controlled by the receive/ transmit
nal is being transmitted the ringing switch break contacts
switch as before. The cancellation switch contacts Kx2
kr3 disconnect the transducer TX’ from the output of
connect a resistance Rv to the supply lead Pm in place
the ampli?er TRB. Restoration of the ringing switch after
of the relay winding rlb so that another extension equip
a burst of tone signal has been transmitted resets the
master equipment for reception and in releasing the relay 65 ment looping the previously traced connection to call
the master equipment while the latter is in use will light
RLA changes-over the contacts RLAZ and RLA3= to're
the relevant indicating lamp, but will not affect the am
verse the polarity of the. D.C. energy as applied to the
pli?er TRB.
line wires Lla and Llb, that is, the polarity is now such
What we claim is:
1. In a communication system including a master equip
ment, at least one extension equipment, and a line con
' nection interconnecting the master equipment with each
as to permit transmission from the extension. The master
equipment can then listen for an acknowledgement from 70
the extension. The receive/transmit switch (sw1/2/3)
is thereafter used to control the direction of transmission:
the contacts swz and sw3t determine the connection of
1. the’ ampli?er TRB for transmission or reception as ‘the
extension equipment and serving to convey both A.C.
intelligence signals between the master equipment and the
case may be, and the contacts .swl, in the transmitting 75 extension equipment and also D.C. energy from the master
3,030,445
equipment to the extension equipment, each extension
equipment comprising amplifying means energisable by
said D.C. energy and having two modes of operation, the
10
wherein the master equipment amplifying means has input
and output circuits and also has means for establishing
between said output and input circuits a feedback inter
one for amplifying incoming intelligence signals and the
other for amplifying outgoing intelligence signals, the
combination in each extension equipment of polarity sen
sitive means determining the mode of operation of said
amplifying means according to the polarity of said D.C.
energy, said polarity sensitive means permitting operation
connection converting said amplifying means into an os
intelligence signals between the master equipment and the
pli?er which, like the transistor ampli?er included in each
ter equipment to the extension equipment, each extension
nection, the one for amplifying incoming A.C. intelligence
signals and the other for amplifying outgoing intelligence
signals, together with a receive/transmit switch operable
cillator for generating a tone frequency calling signal,
said feedback interconnection establishing means includ—
ing a ringing switch having contacts operable to establish
said feedback interconnection in respect of the master
equipment calling an extension equipment, and a calling
in the outgoing mode in response only to a particular 10 relay having contacts operable to establish said feed
back interconnection in respect of an extension equipment
polarity of said D.C. energy, and a control switch which,
calling the master equipment each extension equipment
unless it is operated, prevents said polarity sensitive means
having means for looping the line connection and said
from producing said outgoing mode of operation even if
calling relay at the master equipment being connected
said energy has said particular polarity.
2. In a communication system including a master equip 15 for operation in response to such looping of the line con
nection.
ment, at least one extension equipment, and a line con
5. A communication system as claimed in claim 2,
nection interconnecting the master equipment with each
wherein the master equipment includes a transistor am
extension equipment and serving to convey both A.C.
extension equipment and also D.C. energy from the mas 20 extension equipment, has two alternative modes of con
equipment comprising a transistor ampli?er having input
and output circuits, and said ampli?er being energisable
to select a particular mode of connection for said master
by said D.C. energy and having two modes of connection,
the one for amplifying incoming intelligence signals and 25 equipment ampli?er, and polarity-reversing contacts con
trolled by said receive/ transmit switch and connected to
the other for amplifying outgoing intelligence signals,
apply said D.C. energy to said line connection with the
the combination in each extension equipment of a polar
polarity appropriate to connection of the extension equip
ised relay which is operable by said D.C. energy in re
ment ampli?er in the alternate mode to that selected for
sponse only to a particular polarity thereof, change-over
contacts controlled by said relay and eifective on opera 30 the master equipment ampli?er.
6. A communication system as claimed in claim 2,
tion thereof to change the connection of said ampli?er
wherein the master equipment transistor ampli?er has in
from its said one to its said other mode by appropriate
put and output circuits and also has means for establishing
switching of its input and output circuits, and a control
between said output and input circuits a feedback inter
switch connected in series with said relay and effective,
unless operated, to prevent operation of said relay in re 35 connection converting said ampli?er means into an os~
cillator for generating a tone frequency calling signal, said
sponse to said particular polarity of the D.C. energy.
feedback interconnection establishing means including a
3. A communication system as claimed in claim 1,
ringing switch having contacts operable to establish said
wherein the master equipment includes amplifying means
feedback interconnection in respect of the master equip
which, like the amplifying means included in each exten
sion equipment, has two alternative modes of operation, 40 ment calling an extension equipment, and a calling relay
having contacts operable to establish said feedback inter
the one for amplifying incoming A.C. intelligence sig
connection in respect of an extension equipment calling
nals and the other for amplifying outgoing intelligence
the master equipment, each extension equipment having
signals, together with a receive/transmit switch operable
means for looping the line connection and said calling re—
to select a particular mode of operation for said master
equipment amplifying means, and. polarity-reversing con 45 lay at the master equipment being connected for operation
tacts controlled by said receive/transmit switch and con
nected to apply said D.C. energy to said line connection
with the polarity appropriate to operation of the extension
equipment amplifying means in the alternate mode to
60
that selected at the master equipment.
4. A communication system as claimed in claim '3,
in response to such looping of the line connection.
References Cited in the ?le of this patent
UNITED STATES PATENTS
2,802,055
Wildhagen ..,___,....,....__._..,__ Aug. 6, 19517_
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