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Патент USA US2407150

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Sept. 3,>ii946.‘
Filed Sept. 18, 1943
8 Sheets-‘Sheet 1
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Patented Sept. 3, 1946
‘ Charles Gillings, Charles Edmund Beale, and
Teseo Bruno Dante Terroni, Liverpool, England,
assignors, by mesne assignments, to Automatic
Electric Laboratories, Inc., a corporation of
, Delaware
Application September 18, 1943, Serial No. 502,884
-> _ ‘In Great Britain October 15, 1942
v7 claims. ' (o1. 179_27>
The present invention relates to telephone or
sation could take place in both directions of
like signaling systems and'is more particularly
concerned with the setting and control of auto
Although the use of, astopper valve provides
matic telephone switching apparatus over long
a good degree of protection against ‘disturbing
trunk routes involving mixed systemswf Voice vCA currents generated at the sending end interfering
frequency and direct current dialling and super
with the vV. F. receivers, its presence in the line
vision. The long trunk routes envisaged may
include thermionic repeaters and echo sup
pressors and the invention is therefore’ particu
larly applicable to the‘method of V. F. signaling
described in our‘application Ser. No. 489,136, ?led
'May 31, 1943, now Patent No. 2,376,352, dated
May 22, 1945.‘
is nevertheless 'undesirable‘in certainv circum
stanceaparticularly-in the case of non-metered
calls.‘ Generally‘ speaking {this type of call is
routed through to an.operator's position and the
terminating apparatus issuch that it does not
send back an. answering signal so that ‘the oper
According to this method of V. F. signaling, the
ator may talk to the calling party and give him
i the information required without metering the
numerical direct current impulses received at the
‘call against him. When, however, such a call
sending end of the trunk are‘converted into voice
frequency coded signals which are transmitted
over the trunk and re-converted at the terminal
end into' numerical direct current impulses.
is originated over] a V. F. trunk including a
stopper-valve, itis necessary to arrange the ter
minating apparatus ‘to send back an answer sig
jnalin-order to bring about the removal of the
Furthermore, each voice frequency coded signal 20 stopper valve‘ to enable conversation to take place.
is preceded by a pre?x signal which is the same
> ‘As will be appreciated this is particularly un
for all codes and serves the purpose‘of testing
desirable in the case of automatic calls because
the incoming equipment and also switching any
a the answer signal brings about automatic meter
echo suppressor in the route into the correct
ing‘ of the call. against the calling party and it
direction of ‘transmission preparatory to the
is therefore necessary in such circumstances for
the operator tomake out a credit slip for such
calls in order ‘thatrthey shall be deducted from
sending of the coded signal which is character
istic of the D. C‘. train of impulses to be trans
.Nthe calling subscriber’s account. From an oper
‘This method of signaling gives a good degree of
ating point of view such arrangements are unde
speech immunity in that by reason of their 30 sirable and it is one of the advantages of the
length and combination the signals used rarely
present invention that they have been avoided.
if ever occur in speech, In fact the degree of
4 immunity against false operation is so gOOd with
this method of signaling that the use of so-called
stopper valves which have hitherto been gener
ally used in V. F. systems to prevent the exten-'
sion of disturbing signals forwardly over the
trunk are no longer considered necessary and
In order to give a good-‘degree of speech irn
munity for supervisory‘signals which are trans
mitted over the trunk after the connection has
‘been set up, it is arranged according to the in
ventionthat‘the apparatus measures‘ such sig
nals as regards their frequency, their v‘duration
and, their spacing. , The sequence is such that it.
have not been provided. Previously such stopper
is unlikely to be reproduced? fortuitously. and
valves were included in the apparatus at the
sending end of the trunk line and served to pre
vent disturbing currents which might be gener
there is therefore little likelihood of false opera
ated by the calling party from passing out to
the trunk line and so interfering with the oper
tion from’disturbingsignals.
A furth'er advantageous feature of the inven
tion which is particularly desirable in connection
with‘ tandem working is‘ that upon the ‘receipt
ation of the valve receivers connected‘ to both’ 45 at one exchange of ‘the common pre?x signal
which precedes the code signal a blocking device
ends of» the trunk. Since the stopper valve‘ is a
‘ is connected in circuit with‘the trunk line lead
one-way device,- it offers no obstruction to signals
.coming. backwardly over the line to the calling
ing to'a following ‘exchange, said blocking device
end and accordingly, therefore,‘ any toxic signals
by introducing considerable attenuation into the
received during the setting up of the connection
speaking circuit preventing the code signal from
were received quite freely.
When the connec
tion was completely set up and the called partly
answered, an answer signal was sent backwardly
over the trunk‘ to bring about the removal of the
,_ stopper .valve‘at the sending end so that conver
actuating?apparatus at said .following exchange.
The invention will be better understood from
M the following description of one method of carry
ing it into effect, reference being bad to the ac
- ‘ companying drawings comprising Figs. 1-9.
m. s. pulse of X frequency which serves to ter
these drawings, Figs. 1-4 show the circuit ar
rangements of a sleeve-controlled outgoing relay
set, Figs. 5 and 6 show the circuit arrangements
of an incoming relay set, Figs. '7 and 8 form a
trunking diagram of various ‘combinations of
minate the sending of the repeated Y pulses from
the incoming relay set. Conversation now takes
place and when the called party clears, repeated
‘Y pulses similar to those forming the answer sig
trunk routes to Which the invention maybe use
nal are transmitted from the incoming relay set
back to the outgoing relay set where they bring
fully applied while Fig. 9 shows an alternative
about the lighting of the operator’s supervisory
circuit arrangement including certain modi?ca
lamp. When the operator clears in response to
tions which are necessary if the incoming relay
10 the clear signal, the outgoing relay set sends for
set is required to respond to 'busy ?ash signals.
ward to ‘the incoming relay set a clear signal
Before the detailed circuit .operations are de
comprising a ‘two-seconds X pulse followed by
scribed, a description will ‘be given of .the ‘gen
a 300 m. s. Y pulse which causes the incoming
eral method of working with reference to the
‘relay ‘set to clear down and release the train of
trunking diagram, Figs. '7 and -8. 'In this dia
automatic switches at exchange C.
gram exchanges A-H are shown interlinked by
In case the operator has to clear in face of
V. F. and D. C. trunks. Like ‘pieces .of apparatus
ringing, busy or NU tone, the clear signal under
in the various exchanges have been given like
these circumstances is lengthened to a six-sec
designations, for example, MB is manual board,
onds X 'pulse followed by a 300 m. 5. Y pulse.
O/G is outgoing relay set, 1/0 is incoming relay
set, VF is voice frequency receiver, A—A BS is 20 The object of the long X pulse is to ensure that
the clear signal will get through ‘in face of NU
auto-to-auto relay set, while VF. 'TRK and
tone which is ‘interrupted at the source for one
D. C. TRK are voice frequency and direct cur
second after the tone has :been transmitted for
rent inter-exchange ‘trunks respectively.
'?ve seconds.
Assuming that the operator at exchange B
'The proposed voice frequency digit :code is as
plugs into the ‘jack ‘at the manual board MB lead
ing to 'a free outgoing relay set ‘O/G, her super
visory 'lampwill ?icker and the 'outgoing‘relay set
will send a 100 milli-seconds (m. s.) “X voice fre
quency‘pulse forward over the trunk to the in
coming relay set 1/0 at exchange C to‘bring about
the seizure ‘of an ‘associated ?rst selector.
operator now operates the dialling key whereupon
the supervisory lamp 'is extinguished and when
she dials the required 'subscriber’s number, the
impulses are received and. stored on uniselectors
in the ‘outgoing ‘relay ~set. Each train of impulses
received at the ‘outgoing ‘relay set is re-trans
mitted over the trunk in coded form vmade up
of a combination ‘of four voice‘frequenc'ies, here
inafter referred ‘to as ‘W, X, Y and Z. Each coded
digit transmitted is preceded by a preparatory
signal comprising all ‘four frequencies, W, X, Y
and Z which are effective "in the incoming ‘relay
The ‘proposed voice frequency supervisory sig
nal code is as follows:
seizing pulse _______________________ _, 100 m. s. X.
Coded prepare pulse _________ __
Codcd~dzg1t pulse ___________________ __~
. s."\VXYZ.
. s. W to Z.
Subscriberanswer Signal- ---------- -- {£8 E: ghee (repeated).
Acknowledgment signaL. __________ __
set 170 to prepare the voice frequency receiver ‘F Called subscribcrclearsignal.__g____
thereat for operation. ‘The prepare pulse also
Operator clear signal_______________
takes control of ‘any echo suppressors in the route
to ‘turn ‘them .‘into the correct direction for trans
440 m s. X.
ghee (repeatcd)
Operator clear on‘no reply _________ __ {gbsoe‘gngsyx'
mitting. Conveniently the 'W','X_, Y, Z‘frequencies
used are of 900, 750, 600 and 500 cycles ‘respec
As will‘be seen from the trunking arrangements
.of exchange C, the outgoing relay set may be
The coded digit comprising not more than three
of the frequencies W to Z is decoded at the incom
ing relay set and converted into a marking on
the bank of a uniselector which counts the send
ing out of ‘a corresponding train of loop impulses
to set the automatic switches in exchange C.
.After an interval which is measured in the out
going relay set and is su?icient to allow for the
setting up and hunting time of the automatic
switch in exchange C, the second prepare and
taken into ‘use from any one of three sources,
namely, manual board, automatic local subscriber,
or incoming relay set from a distant exchange.
In the case of seizure from an automatic local
subscriber, the operations are substantially sim
ilar to those already described for seizure from
the manual board, except that resistance battery
is connected to the sleeve of the manual board
jack to busy the outgoing relay set against seiz
ure from this source.
code signals are transmitted and are dealt with
In caset'he outgoing relay set in exchange C'is
taken into ‘use via an incoming relay set in the
same exchange, a battery signal over both lines is
‘When all digits have been dealt with and the
switching equipment at exchange C has been set
to connect with the required subscriber, the call
‘extended backwardly ‘from the outgoing relay set
ing operator will receive ringing tone, busy tone,
to the incoming relay set to cause the incoming
or iNU tone ‘to denote the state of the called line.
When the called subscriber answers, repeated
relay ‘set to switch the lines straight through and
cut out ,of circuit the decoding and retransmitting
apparatus. At the same time in the outgoing re
Y pulses of ‘.140 m. s. duration spaced 36.0 m. s.
apart are sent ‘back from vthe incoming relay set
in exchange C to ‘bring about the extinguishing
of thesupervisory lamp inthe operator’s cord cir
cuit at exchange B. \‘Vhen this is done, an ac
C lay set the lines are extended by Way of a repeat
ing coil bridge to the incoming .relay set in ex
change D and the storing and code sending ap
paratus in the outgoing relay set of exchange C is
7 cut out‘of circuit, Code signaling therefore takes
knowledgment signal is transmitted back from
the outgoing relay set comprising a single Q40 I5 place directly from the outgoing relay set of ex
change'B to the incoming relay set of exchange‘ D.
In case the call from exchange C is‘to be routed ‘
T84.‘v Relay CO atcontacts e03 disconnects re
lay SR and also at contacts col and 002, Fig‘. 2
over a D. C. ‘trunk to exchange E, access will be
removes the line termination resistance TER and
had from the incoming selector to theline side
of the auto-to-auto relay set A-—-A RS, thus cut
connects the trunk line through to the VF sig
naling circuit via the repeating coil RPCI. In
ting this relay set out of circuit.‘ rl‘his arrange
connection with the terminating resistance TER,
ment is desirable to avoid two repetitions of the
it should be mentioned that for single-way trunk
impulses in the same exchange. The incoming
‘working the jack points Ill and H are strapped
relay set extends earth forwardly to hold the
together as shown. For bothway trunk working,
automatic switches in exchange C.
10 however, this strapping is removed and the jack
If now the call to exchange E is extended over a
VF trunk to exchange G,‘ signals passed between
the outgoing relay set in exchange E and the
incoming relay set in exchange C bring about
points Ill and ii are cross-connected to the ter
mination in the incoming relay set. Relay FXY
in operating, at contacts fang/ll brings up relay CS
and at contacts fats/2 causes the switch TS to‘self
the setting up of straight through connections 15 drive over its bank TS3 to the third position.
and the disabling of the coding and impulsing ap
During the period that the switch is self-driving
paratus. Signaling therefore takes place directly
over the two strapped contacts, X frequency is
from the outgoing relay set in exchange B to the
applied to transformer VXF via contacts fc2, fxyl
incoming relayv set in exchange G so that the
and cal and terminating resistance VTR and
conditions become similar to the VF to VF case 20 then extends via contacts csl and 082 and con
tacts col and 002 to the trunk line to form a seiz
In case the connection from exchange E is ex
ing signal to the incoming relay set at the dis
tended over a D. C. trunk to exchange F and then
tant exchange. With switch TS in position 3, re
over a VF trunk to exchange H, a different set of
lay FXY releases to terminate the X pulse after
conditions‘arises because an auto-to-auto relay 25 a period of the order of 100 m. s.‘ and at contacts
set at exchange E is now interposed in the trunk
h2g3 operates relay FC which looks and at con
already described.
route. It is therefore now impossible to signal by
means of the simplex battery connection between
the outgoing relay set of exchange F and the in
coming relay set of exchange 0 to cause these .
sets to switch over to straight-through connec
tacts I03 causes the switch TS to step to position
4~from which it drives to position 6 over bank T85
and interrupted earth connected to conductor 13.
Relay FXY, in releasing, at contacts fan/4 releases
‘relay CS.
tions. On the other hand, unless means are pro
vided to prevent it, the VF signals from the out
going relay set of exchange B can ?nd a circuit
When the operator throws the dialling key, bat
tery is connected to both lines and relay RR,
Fig. 1, is operated over its upper winding, while
'by way of the condenser bridge in the auto-to 35 at the same time a high resistance isintroduced
auto relay set of exchange E to the incoming relay
into the sleeve circuit to, extinguish‘the calling
set of exchange H and this would bring about the
supervisory lamp. ‘ Relay RR at contacts rrl
generation of two trains of impulses for each train
operates relay A which in turn'o-perates relay B,
of impulses transmitted from the outgoing relay
Fig. I3,1;0 prepare certain’ impulsing circuits and
set in exchange B. To prevent this the incoming
operate relay BR, Fig. 4. Relay BR at contacts
relay sets are provided with blocking devices in
brl drops relay RR and at contacts brl and M2
the line circuit which function almost instanta
connects relay A to the tip and ring lines extend
neously upon reception of the VF prepare signal
ing ‘to the operator’s dialling circuit and also at
preceding each code and thus prevent the code
contacts 1213 disconnects the ?icker earth circuit
signals reaching‘ the incoming'relay set at a dis- & and connects up relay KR into the sleeve circuit;
tant exchange such as exchange H. Thus the
‘sequence of impulsing insofar as setting up of
automatic switches in exchange H is concerned is
as follows. VF signaling between exchanges B
and C; conversion at the incoming relay set of '
exchange C to D. C. impulses which extend by way
of exchange E and the auto-to-auto relay set
‘ this relay however will not operate owing to the
high resistance already introduced therein. When
the dial is‘turned off-normal, balanced battery
on the lines is replaced by a loop through the dial
impulse springs and relay A continues to hold.
When the dial is released, relay A responds to the
impulses which it repeats at contacts al, Fig. 3,
threat to outgoing relay set of exchange F. Re
conversion takes place at exchange F to VF sig
via relay C to the driving magnet DSAM of the
?rst digit storing switch DSA so that wipers of
nals which extend to exchange H and are there in U! this switch are stepped to a ‘corresponding posi
changed back again to D. C. impulses to set up the
tion. Relays B and C hold operated during the
impulsing and the latter at contacts 02, Fig. 4,
operates relay OS to prepare the VF signaling
volved in setting up a connection over the equip
circuit at contacts csl and 082, Fig‘. 2. “Relay C
ment shown, when the operator plugs into jack 60 also at contacts cl, Fig. '3, energises the driving
JCK, Fig. 1, if the cord circuit speaking key is
magnet DDM of the digit distributor switch DD,
‘ automatic switch train in exchange H.
Referring now to the detailed operations in
thrown relay M is operated over the sleeve circuit
and in turn operates the relief relay MM, Fig. 4.
Relay MM at contacts mm I, Fig. 1, connects flick
but as this switch is of the reverse drive type
its wipers are not moved at this time.
At the end of the ?rst series of impulses, relay
er earth to the upper low resistance winding of
A remains energised and relay C releases after its
relay M to ?ash the lamp in the operator's circuit
to provide an indication that she is connected to
a dialling trunk, and at contacts mm2, Fig. 4, con
nects earth to the P conductor incoming from
selector levels to guard the relay set against seiz
ure over this path and also operates ‘relay SR.
Relay SR- at contacts srl causes the switch TS to
self-drive from its home position to position l
where relay CO is operated over the oil-normal
‘ slow period and at contacts 0! completes a circuit
‘bank TSI and relay FXY is operated over bank. .
over bank DSAI, Fig. 3, for operating relays CNA
and CNR in series. If the digit dialled is ‘5 or less,
relay CNA operates over‘ its left-hand winding and
as a result a short interdigital pause is provided
‘between the retransmitted digits as will appear
subsequently. Relay CNR at contacts cnr2 ad~
vances the: sending control switch SC from its
home position to position I where relay ST is
operated over its two windings in series. Relay
-ST thereupon at :contacts stl short-circuits‘its
left-hand high resistance winding to self-‘drive
the sending control switch SC‘ to position 4, at
contacts 'st2, Fig. 4, maintains relay CS and at
contacts Sl§3——8i3 causes a prepare pulse of
WXYZ frequency to be connected via the trans
former VXF to the trunk line to the distant ex
change. When the sending control switch SC
reaches contact 4, relay ST releases after its
relay EP would operate and lock to earth over
‘bank and wiper DSAI. Accordingly, when the
switch DSA reaches its home position, thus meas
uring 01f part of the interdigital pause, relay EP
will hold during the slow period due to its short
circuited upper winding and at contacts 6102 will
cause the switch DSA to make another revolution.
vDuring this revolution relay EP releases but relay
Z holds to bring 'about a substantial increase in
slow period thereby disconnecting the WXYZ pre .10 the interdigitalpause. 'For example, if the digitYO
had been dialled, the .interdigital pause would be
pare pulse and at contacts stl operates relay S
made up‘ as follows: fourteen steps of the switch
DSA plusan additional revolution making 39 steps
all gives a period of approximately 780 ms.
‘To-this must be added sixteen steps of the switch
SC say 320 ms. plus a further 260 ms. represent
ing the transmission time of the prepare and
code pulses of the second digit giving a total in
terdigital pause of approximately 1,360 ms. The
nectsthe code pulse to the line, ‘this pulse being
dependent for its composition on the setting of .20 object of vthis extra pause period is to ensure that
the distant selector has sufficient time to perform
the switch DSA as fully described in the previ
its level-selecting function plus its trunk-hunting
ously mentioned speci?cation so that it is thus
function before the next decoded digit is trans
characteristic of the digit dialled. Relay S also
mitted to the succeeding switch.
at contact 81 short-circuits its high resistance
left-hand winding to self-drive» the switch SC‘ to
When all stored digits have been transmitted
contact :9 where relay Z operates over its high re
in code, relay CNR ?nally releases and at contacts
cnrZ opens the kick-on circuit for the sending
sistance lower winding and at contacts 24 dis
connects the code pulse from the trunk line. The
control switch SC, while at contacts curl, Fig. 4,
duration of ‘the code pulse is measured by the
it brings up relay BS, Fig. 2, which has no func
tion‘at this stage.
time taken for switch SC to self-drive from con
Although four digit storing switchesDSA-DSD
tact 4 to contact 9 which is approximately 100
ms. Relay Z holds relays CNA and CNR at
have been provided, it is possible that in prac
tice a lesser number may be found satisfactory
contacts 2] and at contacts 22 causes the switch
depending upon the speed at which storing and
DSA to self-idrive to its home position where relay
retransmitting in ‘code can be accomplished and
Zreleases and at contacts 22 causes the switch SC
'over its'two windings in series with magnet SCM.
The duration of the prepare pulse is thus meas
.ured by the time takenfor the switch SC to step
from contact i to contact '4 plus the slow release
period of relay ST which ,gives a total ?gure of
approximately 160 ms. Relay S at contacts s5
.holds relay CS and at contacts s3, Fig. 2, con
also to drive to its home‘ position, while at con
tacts 23 relay-CS is ‘released.
If in the meantime the operator has dialled the
second digit on to switch DSB, (notshown) re
lay CNR will be maintained operated in series
with a relay CNB, .not shown, and accordingly
when the switch SC comes into its home position
it will again self-drive to contact -I where relay
ST will re-operate to-send out the prepare pulse
of WXYZ frequency followed by the code pulse
which .is picked up from the setting of the Wipers
upon the number of digits to be dialled. As
shown the digit storing switches DSA—DSD are
taken into use in cyclic repetition, that is tosa-y,
when the fourth digit has been stored on the
switch DSD, the switch DSA which will by this
time be ‘normal is again taken into use to store
the ?fth digit and so the storage cycle repeats
itself. With the IONA-CND digit storing circuit
shown in Fig. v3, however, it is possible that if
the ?fth digit dialled is received before all the
?rst four digits have been sent out in coded V. F.
form, the DSA switch, on which ‘the ?fth digit
of the switch DSB which is assumed to besimilar
to the switch DSA already described.
is received, will take precedence and so will up
It should be explained here that if, as :has been
‘set the cyclic function. This trouble may be
assumed, the digit dialled was 5, the interdigital 50 simply overcome by providing, in the vCNR relay
pause provided between the two code digits trans
individual battery feeding vcircuits to the three
mitted to line will be measured :by 20 self-driving
relays CN-A-—CNC, break contacts of relay CND
steps of the switch DSA, say 400 m.s., plus 16
which are paralleled by make contacts of each
self-driving steps of switch SC‘, say 320
relay concerned. As regards relay CND this will
a total vof ‘720 ms. approximately. During this . remain directly connected to relay CNR as shown
period ?ve machine-generated pulses are delivered
in Fig. 3. With these arrangements the opera
at the distant exchange to the incoming selector
tion in turn of the relays CNA—CNC‘ is not in
after which hunting must take place to select an
terfered with and on operating they will lock in
idle trunk in the level selected. While the selector
dependently of relay CND, but as soon as relay
is hunting, the incoming decoding relays are re
CND is operated in response to the fourth digit,
leased to receive the second prepare and code sig
none of the preceding relays can be operated in
nals which will require a time period of 160 plus
response to a ?fth and subsequent digits. This
100 ms. giving a total of 260 ms. which may be
condition obtains until the fourth digit has been
added to the 720 m.s. already mentioned to give
sent out in coded V. F. form after which the bat
a total of approximately 980 ms. for the complete ' tery feeding circuits of ‘the three relays CNA
interdigital pause during which the incoming se
CNC are recompleted. If by this time the ?fth
lector must be set to the required level and per
digit has been dialled, the 'DSA switch will have
form its hunting operation.
been already set accordingly and relay CNA will
nowr re-operate to provide for the sending out of
If, however, the ?rst digit dialled had been 6
to 0, a longer interdigital pause would be required , this digit. Similar remarks would apply to the
vsixth and seventhdi‘gits, while the ‘eighth digit
to enable the incoming selector to perform its se
would ‘function-on relay CND in the same manner
quence of operations and this is derived as fol
lows. ‘Relay CNA under these conditions would
as for the fourth vdigit. By this means it is con
:be operated over its right-hand ‘winding in series
templated that up to ‘ten digits could be safely
with relay CNR so that when relay S operated,
handled by the four digit switches DSA—DSD.
2,407.’, 150.
When the operator restores her dialling key at
ing of relay RR from the high resistance wind-‘t
the end- of dialling, the current in the'sleeve cir
ing of relay M so as to dim the supervisorylamp
in the operator’s circuit. If relay RR has been
held operated up to this time on its lower winding,
cuit is increased and the battery applied to the
tip and ring conductors is removed shortly after
wards. Relay KR, Fig. l, thereupon operates and 5 it new releases.
locks up at contacts lcrl while at contact lcrZ it
; Conversation now proceeds, and when the
releases relay A. Relays B and BR in turn re
called party clears, the incoming relay set at the
store after their slow periods and the former at
‘ distant exchange transmits Y pulses of the
contacts b3 advances thedigit distributors-witch
‘ same typegas for thecalled party answer signal.
DD to its‘ mid positionpreparatory to the oper 10 At the outgoing end relays M, MM, KR, CO, FC,
ation of relay AA at a later period. Relay BR
BS, SA and SB are held operated and the switch
at contacts br3 connects the earthed low resist
TS is in position 6. Accordingly the ?rst 140 m. s.
ance lower winding of relay RR into the ‘sleeve
pulse of Y frequency releases relay BS and oper
circuit and though relay RR receives insu?icient
ates relays MS and MT as already described.
current‘to, operate the ‘supervisory lamp again 15 During the following 360 m. s. break period relay
glows under this condition. Relays M, MM, KR,
BS is ‘ operated and releases relay MS but not relay
CO,_.FC and BS remain operated and the line
MT so that when the next Y pulse is received
condition‘ is now such that the operator may
and relay BS again releases, during the slow re
listen to tones and if necessary speak without the
lease period of relay MT‘ relay SY operates over
necessity for a called‘party answer signalg but 20 contacts $172 and at contacts syl completes a
no supervision is given.
locking circuit for itself-for the remainder of
If Lthe operator restores her cord circuit speak~
the Y pulse. Relay SY also at contact sy3 oper
ing key the sleeve circuit current is increased.
ates relay SZ and at contacts syZ releases relay
SA which at contacts sa3, Fig. 1, disconnects bat
relay at contacts rr2, Fig. 1, maintains a suit 25, tery fromthe‘ring of the jack for backward su
able V. F, termination‘ on the trunk line, in place
pervision, and at contacts saB restores earth via
of the operator's telephoneset. ,
the low resistance winding of relay RR to the jack
When the called; party answers, the incoming
sleeve circuit to vre-light the operator’s calling
relay set at the distant exchange transmits 140
supervisory lamp. g
m. s. of Y frequency‘ over the trunk followed by
As long as the Y pulses persist, relay SY re
a, space period of 360 m. s.‘ and this signal is re
mains held because its release time is 400-450
to‘ a value sufficient to operate relay RR and this
peated until acknowledged by the transmission
m. s., i. e. longer than the 360 m. s. breakperiod
of an X signal from the outgoing relay set. Upon
the reception of the Y frequency, relay Y oper
ates in the YF receiver VFR shown schematically
and consequently if the called subscriber should
again remove his receiver, the Y pulses will cease
and after its slow period relay SY releases and
disconnects relay SZ. During ‘the slow release
period of relay SZ relay SA re-operates and locks
in series with relay SB to restore the speaking
as a dotted rectangle in Fig. 2 and opens the cir
cuit to relay BS which releases after a slow period
of 80-110 m. s. To release relay BS therefore the
Y frequency must be applied for a sufficient pe
riod and. whenvrelay BS has released, relays MS
and MT are operated in- turn and hold for the
remainder of'the Y pulse. When the Y pulse is
?nished, relay BS re-operatesand at contacts
bsl opens, the circuit for relay MS which has a
When the operator clears by removing the plug
from ‘the .jack in response to the clear signal, re-g
lays M and M in the sleeve circuit release and
the latter releases relay KB. The release of relay
MM at contacts mm2, Fig. 4, steps the switch TS
from position 6 to position 1 by way of bank TS3,
whereupon relay BR operates in series with relay
SR over bank TS4. Relay SR on operating at
contacts. sr3 re-operates ‘relay CS and these re
release time‘of 300-400 m. s. Relay MT has a
release time of 200-300 m. s. ' If the break period
is of the correct duration, namely 360 In. s.,then
thisperiod plus'the 80-110 m. s. release time of
relay BS, which releases in response to he next
lays together bring about the application of an
Y pulse, will be. ,su?‘ic-ient to cause relay MS to 50 X frequency pulse forwardly over the trunk‘by
release but not relay MT. _ Hence on the recep
way of contacts sr2, csl and 082. Since relay SB
tion of the next Y pulse the "subscriber answer”
is operated, the switch TS continues to self-drive
relay SA‘will be operated ,over contacts sy2,‘ sal,
from position ‘I to position I5 via bank TS3 and
sb2, ms], mtl, syl, Ybsl‘, ac5, yl and cm! to earth.
contacts sb4 and then steps by means of inter
At the end of a, the Y pulse, or when relay MT re 55 ruptedearth on lead 13 from position I 5 to po
leases, relay SB which has been short-circuited
sition [9 via bank TS5. Relay FXY is then up
, hitherto operates in series with relay SA and locks.
Relay SB operates relay AA, Fig. 3, in the self
driving circuit of the‘switch DD and relay AA
at contacts aa2, Fig. 4, reoperates relay CS and 60
at contacts aal, Fig. 2, applies an X pulse ac
knowledgment signal to the outgoing trunk.
The duration of this acknowledgment signal is
measured by twelve steps of the switch, DD plus
the release time of the relay AA which releases
when the DD switch reaches the home position
and which inv turn releases relay CS. When the
Xpulse is received at the distant incoming relay
set, the transmission of the Y‘ pulses ceases and
relay BS remains operated.
'1 It should be mentioned that relay SA at con
erated over bank 'I'S?, contact l9, and serves to
disconnect the X frequency at contacts .‘fxyl,
Fig. 2, after a period of application of two sec
onds and at the'same contacts to connect up Y
frequency to the trunk. When the wiper leaves
position ill of bank TSII, relays SR and BR are
caused- to release after their slow periods and on
the release of the latter, earth is extended over
bank‘ TS3 to step the switch to position 20. Relay
‘FXY then releases after its slow period and thus
terminates the Y signal after a period of applica
tion of 300 m. s. and also releases relay CS.
Switch TS then steps from position 20 to position
70 23. via bank TS3 and contactsiacl, and then slow
~»steps from earth on lead l3 over contacts 23
and ‘24 via bank TS5 to reach its home position
tacts sa3, Fig. 1, connects battery via resistances
YP and YQ to the ring of the jack for through
supervision and alsoat- contacts sa?, Fig. 1, re
moves the short-circuit by way of the lower wind 75
where relays CO, FC and SB release. During the
forward; application of Y frequency relay BS will
have released and at the end of‘the signal relays
cuit. When the called party- answer signal is re
SY and SZ release in, turn. Relay QQ disconnects
ceived‘ in the form’ of repeated 140 m. s. Y? pulses
as’ already described; relays‘ Ms and MT check‘
the length and spacing of the pulses- and if- thesev
moved from the P conductor leading to the se
lector levels when the TS switch reaches its in are’ correct allow~ relays SA and SB to operate.
Relay SA at’ contacts saA and‘ sa5, Fig. 1, re
home position the relay set is free for further use.
verses the A relay connections towards‘ the callConsideration must now be given to the con
ing partyfor; supervision, while relay SB operates
dition when the operator clears before receiving
relay AA to cause an X pulse acknowledgment
the answering signal or in face of any tone (ring
ing,_ busy or NU) or on a non-metered call. Any 15) signal to be transmitted forwardly to the distant
exchange. This’ signal lasts for- a period meas~
of these conditions are characterised by- the non
uredby the homing time of the switch DD-via the
operated condition of relay SB so that when the
low resistance relay
plus the slow release pe
operator withdraws the plug and releases relays
the busy condition from the sleeve of the jack
at contacts 005, Fig. 1, and; as‘ earth is also re
riod of relay AA which is disconnected when,
M and MM, the switch TS is caused to slow step
from position ‘I to position‘ l9 byway of inter ., switch DD arrives at its home position. The re-r
ception of the X" pulse acknowledgment signal‘ at
rupted earth supplied over lead [3. Accordingly,
the distant exchange causes the backward Y"
therefore, the X pulse which precedes the Y pulse
answer signal to- be disconnected and‘ conversa
is lengthened from 2 to 6‘ seconds, after which
tion may now takeplace.
it is followed by the normal 300 m. s. Y pulse.
In the. case of a call via selector levels from
The object of the long X pulse under these con- 13
an incoming VF relay set, earth is connected to
ditiOns is to ensure that the clear signal will be
both the negative andpositive lines and serves to
received. at the distant incoming end when, for
operate relay AG; Fig. 1, while the normal for
example, NU tone is being transmitted back
ward5 earth on the P conductor‘ operates relaywardly. The NU tone‘ is transmitted for ?ve sec
onds and vdisconnected for‘ one second and this :7 . SR.
arrangement ensures that the clear signal which
is being extended forwardly shall be able to
break in‘and register at the incomingrelay set.
When the outgoing relay set is taken into use
over the negative, positive‘ and P trunks extend- _..
Relay'AQ at contacts ac2 andv ac3'connects
the negative and- positive lines from the selector
level through to the outgoing-trunk line- while
relay SRsteps switch- TS from the home position
to contact I where relays CO, CS’ and‘ FXY are
operated-a Relay~CO operates-BS, drops SRand'
disconnects the trunk‘ line termination» TER.
ing from selector levels, the call may have been
originated- either byv a subscriber on» the same
Relay FXY extends a 100 m. s. X‘seizing pulse for-..
exchange or via an incoming V. F; relay set from
a distant exchange, say 13). In the ?rst case the
subscribers dialled impulses are converted by
the switches DSA—-DSD into VF prepare and
ward overthe trunk to the distant‘ exchange andv
steps, the switch» Ts to» contact 3'. In this posi->
tion relay- FXY isv disconnected and when it releases after-its slow period the X‘ seizing pulse‘
is terminated. Sincerelay AC is- operated‘, switch
TS self-drives-~ to position 20 via bank‘ TSZ‘ and
code pulses; while in the» second case, where
through working is required, the outgoing relay
remains; there until the end of the call‘ so that as
set‘ merely transmits the X seizing signal-"to the
idstant' incoming VF-relayset afterwhich further
prepare and‘ code pulses are. transmitted directly
through the outgoing relay set to the distant in
it leaves position llv while relayeFXY is still'oper
ated, relay F6 is not operated. The incoming-v
V‘F’prepa-re and code pulses are thus extended‘
directly throughthe outgoing; relay set to the dis
tant exchange; Relay CQ busies the- jack by
into?use from a- subscriber in the same exchange, 45 connecting battery to» the sleeve circuit, while
switch TSatpbankTSd holdsearth'on the P wire’
earth extended forward over- the. private con
coining VF= relay set without modi?cation.
' In the case when the outgoing relay-set is taken
ductor? operates relay-‘SR, Fig. 4, which causes
the switchTS to- self-driverover bank TS3 to the
to‘ busy the outgoing; relayvset against seizure
oventh'e selector- levels. When~the connection is:
setup and the- called’ party-answers, repeatedv Y
Relay C0v operates relay BS; and drops 50 pulses‘ are extended‘ from'the' distant exchange
directly through’ the- outgoing-relay set and- are;
relay SR, while relayFXYextends a 100 m. s. X
?rstiposition where'it-operatesrelays 00', CS and
acknowledgedby-a 440m; s. X‘ acknowledgment
signal extendingfrom-the callingend of the con
nection directly) through the outgoing relay set.
relayFXY is- disconnected and after» its- slow: pe 55 It-will be- noticed‘that the‘ Y_ relay_- of the-VF re
ceiver' VFR» is- operated; from the Y pulse but as
riod-releases to- terminate the X pulse. Relay
relay1AC is operated at- this-time‘ the- response of
FCQis then operate-d and-locks up and‘ serves'to
step TSto- positionA; from which it slow steps ' relays'MS, MT. SA and SB is prevented» by con
tacts1ac5, Fig. 2. When the calling end of the
from interrupted‘ earth on conductor; l3; to posi
tion?g. Relay FC also brings about-the opera 60 connectioniscleared; earth isremoved from the
negative andipositive- lines thus releasing relay
tion of relay A, Fig. 1, over the calling subscrib
AC which allows/switch TS to self-drive overcon-l
erls loop-and relay A in turn brings up relaysB
seizing-pulse forwardly overhthe; trunk to the
distant incoming relay set. Relay FXY also
causes switch ‘TS to self-drive-to-position 3 where
and-BRto prepare the circuitforthe reception
tacts 20-23. via- bank‘ 'rs3- after which it a slow
steps ~ to its home . position.
of thedialledrimpulses. From this point thecall
proceeds as for an operator call already- described; 65 If» thevoperator inadvertently withdraws the
plug. while the called-1 party is stilli'connected and
that is-to say, relay-A respondsvto-thedialled-im
possibly is speaking at rthistime, switch’ TS will;
pulses‘ which are stored-via the digit distributor
run» home- from~ position 6 > to extend the clear‘
switch-DD on the coding, switches DSA--DSD.v
signal of two seconds'X followed‘ by 300 In, s;,Y'
Subsequently under’ the control ofthe sending
'sw-itchSC the coded digits-aretra-nsmitted to- line 70 and release the equipment in the outgoing, relayv
in. theform of prepare and» code pulses- and are
received on decoding apparatus at the incoming
relay ‘set in the distant exchange. RelaygCO un
der. these, conditions busies the manual board jack
by. connecting. resistance battery’ to» the sleeve-cirJ
set.- If_ however the called-party is,_ still talk
ing at this time; any echo suppressor in the con
nection will be held; against theclearing; signal
which may_=the_refore fail to get throughto the
distant‘ end ofltherconnectionz
It‘ is therefore
necessary'when the calledfparty ‘?nally clears to
arrange for‘the-switch‘fI‘S to perform another
revolution in order that theblearingsignal may
be sent‘forward to the distant end of the con
nection. Accordingly, thereforefwhen the called
party replaces his receiver, Y pulses are trans
mitted backwardly over the connection, these
a duration of approximately 160 m. s.‘ and is
followed immediatelyby the code pulse which
comprises frequencies W, X, Y and Z in suit
able combinations having a duration of'lOO m. s.
In the example in question the digit 5 comprising
the ‘frequencies W and Z is assumed to have been
transmitted so that relays IX and IY are re
leased and in turn drop their relief relays XR and
YR. The short-circuit is thus removed from
lowed by 360 m. s. space period. The ?rst Y
pulse operates relays MS and MT and during 10 relay CB which immediately operates from the
earthed‘ contacts M12 in series with relay CA,
the ?rst 360 m. s. break period relay MS releases .
an‘d‘at contacts c174 and 0191 looks relays WR
so that at the commencement of the next 140
and ZR corresponding to the code which has been
m. s. Y‘pulse relay SA is operated and at con
transmitted. Relays WR to ZR have a pyramid
tacts 3012, Fig. 4, causes the switch TS to step
from its home position to contact I. Relay, SB 15 chain of contacts (not shown)v connected to the
bank SS2 of the sender switch SS, and since the
is operated and locked when the Y pulse is?n
code ‘relays WR and "ZR are operated at this
ished and from then on the switch TS makes
a complete revolution during which the clearing
time, earth is connected to the sixth contact of
the'bank SS2 ‘in order to terminate the sending
signal comprising a two-second X pulse fol
lowed by a‘ 300 m. s. Y pulse is transmitted to the 20 after ?ve impulses have been transmitted to
the incoming selector as will appear subsequent:
trunk line to release the distant equipment.‘ It
1y.’ Relay CB also at contacts c225 releases relay
should be mentioned that during the revolution
GK and operates relays PC and P, the latter
of the switch TS the‘ X seizing pulse is again
looking over its own contacts. Relay PC at
transmitted while the switch is passing over the
early contacts of the bank, but under these cir 25 contacts pcI operates relay CC which at con
tacts ccI prepares a circuit for relay IG which
cumstances this signal produces no switching
operates. when the constantly driven impulse
function at the distant exchange.
springs 66%M next open. Relay IG> at contacts
Referring now to the circuit changes produced
z'g2 drops relay K and at contacts igI extends
at the incoming relay set, Figs. 5 and 6, when the
comprising’ as usual 140 m. s. of Y frequency fol- .
100 m. s. X seizing‘ pulse is received over the 30 the impulse springs 66%M to the driving magnet
SSM of the sender switch. Relay IG also at con
trunk line from the outgoing relay set, relay IX,
not ‘shown but assumed to be located in the valve
receiver IVliRyoperates and in turn brings up
the relief relay XR. Relay XR at contacts m3
tacts ig2 transfers the forward, holding loop ex
tending to the selector to the impulse springs
33%M which are closed at this time, since they
operates relay K which locks up‘at contacts kl, 35 are out of phase with the magnet impulse springs
56%M'which‘ have just opened to bring about
at contacts k2 operates relay PY,‘ Fig. 6, and‘ at
contacts k4 extends earth forwardly over the P
the operation of relay IG. Accordingly, the send
, ing sWitch‘SS now steps its wipers until they
encounter the marking‘ earth connected to the
switch. At the end of the/X seizing pulse, relay
IX in the valve receiver releases and in turn 40 sixth contact of bank SS2 over contacts of the
coding relays WR and ZR, while at the same time
drops relay XR which at contacts :rr4 operates '
conductor to the incoming selector to busy this
the impulse springs 33 %M transmit ?ve im
pulses to the incoming selector as described in
the previously mentioned speci?cation. Relay
ing selector,‘ at contactsnnl operates relay KK
over the negative line in series with one wind 45 SZ operates when wiper SS2 comes into its sixth
position and locks over contacts seI to the hom
ing ‘of the A relay in the incoming selector and
relay, NN. Relay NN at contacts nn? further
earths the P conductor extending to the incom
at'conta‘cts nn3 operates relay GX, Fig. 5. Relay
GX at contacts 9034, Fig. 6, connects battery by
ing. bank SSI.
Relay SZ also at contacts s22
short-circuits the 33%M springs to terminate the
impulses to the selector and at contacts 823
way of a high resistance YF to the centre point
of the repeating coil which ?nds a circuit to 50 releases. relays CA and CB which have been
held in series. Relay CB at contacts cbt drops
earth at the centre point of the transformer
relay‘IG which at con-tacts ig4 causes the sending
which connects the valve receiver IVFR ‘to line.
switch SS to.,-home, while the holding circuit for
A small current is thus causedto ?ow by way of
the selector is again transferred to relay KK
various relay contacts connecting with the line
circuit which are thus “wetted” to prevent co 55 which‘ re-operates. Relay CB also re-operates
relay GX and releases relays WR and ZR as Well
herer trouble. No further operations take place
as relay PC after its slow period which is fol
at this stage and the incoming selector is now
lowed by the release of relay CC also after its
ready to receive the train of impulses.
slow period. Relays K, PY, NN, KK, GX and P
" When the outgoing relay set transmits the 160
m. saprepare pulse of WXYZ frequency as al 60 remain held.
After a pause long enough to permit the in
ready described, relays IW, IX, IY and IZ oper
coming selector to hunt over the selected level,
ate in the V. F. receiver and in turn bring up
which pause is determined in the outgoing relay
their relief relays WR, XR, YR and ZR. Over
set as already described, the next prepare and.
the series connected contacts :01'4, yrd, 213 and
wr3 relay CA is now operated, whereupon the 65 code pulse is transmitted and the operation is
botliway line termination comprising resistances
YB, YC, YJ, YD and YE and condenser QA
which normally imposes a loss of the order of 3
db, is converted into a combined line termi
nationand in?nite attenuation loss pad ‘which 70
prevents the passage of signals so that the line ‘
is blocked at this point. The purpose of this
arrangement will be described later in connec
tion withtandein calls. '
repeated until all the impulse trains have been
transmitted. It will be appreciated that the pre
pare pulse of WXYZ frequency ensures that any
echo suppressors in the connection are switched
into the correct direction and provides a test that
all the responding relays in the incoming V. F.
receiver are ‘operating satisfactorily. If this is
not so‘ it will be impossible for a complete con
nection to be set up and there is no danger of
-' ‘The prepare pulse, as already described, has 75 connection being made to the wrong party.
The connection is thus fully set up and the
called party is rung. When the called party
Wardly to the automatic switches. When the
Y pulse is ?nished, relay NN releases and drops
answers, battery is reversed over the forward
trunk so that relay KK releases and relay EE op
erates. Relay EE at contacts eeZ brings up relay
IR which at contacts irZ-ir4 alters the connec
tion to the loss-pad to render it ine?ective and
at contacts irl drops relay K. Relay K opens the
circuit of relay PY and during the slow release
period of the latter, relay MA is operated and
locks up over contacts mal and gx'l. Relay MA
at contacts WW3 and mad, Fig. 5., connects up
the secondary winding of the transformer VSF
to the backward trunk and at contacts ma2 con
nects relay FY, Fig. 6, to conductor 14 which ex
tends to interrupted earth having a periodicity
relays P and K which open the holding. circuit
for relay CR, The forward holding loop is opened
and the earth on the P conductor is removed
on the release of relays N and CR thus initiat
ing the release of the operated automatic
switches, and the incoming relay set is now free
for further use.
In regard to tones which may be encountered
during the setting up of the call such as ringing,
busy and NH, these are passed back along the
trunk line in the normal manner except that the
line termination which is in circuit under these
conditions imposes a 3 db loss but this does not
seriously reduce the tone level.
The NU tone
of 140 m. s. on and 360 m. s. off. Relay FY there
is broken at‘ the source for one second in every
fore pulses to the interrupted earth and extends
six‘ to allow the release signal to obtain control
backward over the trunk repeated 1140 m. s. Y
of any echo suppressors which may be in circuit
pulses with 360 m. s. spacing.
20 and thus get through to its destination. As ex
When this signal has been accepted at the out
going relay set, the latter returns a 440 m. s. X
plained above if the release signal is sent, before
the called party has answered, it comprises 6
pulse acknowledgment signal which operates re
seconds of X frequency followed by 300 m. s. of
Y frequency. This long signal insures a certain
break-in during the I second silent period of the
NU tone.
Considering now the operation of the incoming
tacts grl drops relay MA which connects the line
relay set under these conditions, since the called
through for conversation and disconnects the re
party has not answered, relay K will still be oper
peated Y signal. Relay GY in operating cuts the 30 ated and hence the X signal operates relay PC
earth pulse connection to relay FY. Since the
which brings up relay CC and from this point
repeated Y signal is measured at the outgoing
the release procedure is identical with that pre
relay set both for length and spacing, it will be
viously described for the normal operator clear.
appreciated that this gives the desired degree of
In case the incoming selector switches through
irmnunity against false Y signals which may be
to a short-circuited trunk, arrangements have to
generated by speech or line noise. Similarly, the
be made to prevent this short-circuit masking the
X acknowledgment signal is measured against
VF receiver and so preventing the incoming relay
the release time of relay GX (300-400 m. s.) to
set from accepting the release signal of 6 sec
lay IX in the valve receiver and in turn the relief
relay XR. Relay XR at contacts m3 opens the
circuit of relay GX and operates relay GY. Ree
lay GX' releases after its slow period and at con
provide the required degree of immunity against
onds X followed by 300 m. s. Y. In the face of a
spurious X frequencies on the line. At the con-7
short-circuit from the selector, neither of relays
clusion of the acknowledgment signal, relay XR
releases, relay G-X re-operates and relay GY re
KK and EE is operated so that on the release of
When the called party clears, battery is again
reversed over the forward line circuit so that :
relay KK re-operates and relay EE releases. Re
lay EE at contacts eeZ drops relay IR‘ to re-intro
duce the line termination and also operates relay
MA. Relay MA connects interrupted earth to
relay FY which pulses to send back to the out 50
going relay set the clear signal comprising 140
m. s. Y pulses with 360 m. s. spacing. As already
relays CB, PC and CC‘ in turn relay SC operates
over contacts nus, 1001, 003, kkl, eei and p4 and
locks up at contacts .901. At contacts s02 and s03,
Fig. 6, relay SC disconnects the forward trunk
leading to the short-circuit and under these con
ditions the line termination is maintained since
relay IR is not operated and the clearing signals
can be satisfactorily received from the outgoing
relay set.
In the case of tandem calls over two VF trunks
in series, for instance from exchange B to ex.
explained, in the outgoing relay set this signal
change D, Figs. 7' and 8, the following operations
results in the release of relay SA which discon~
take place. The digits dialled cause the selector
nects battery from the ring of the outgoing jack 55 to seize an outgoing relay set to another VF
and lights the calling supervisory lamp at the
channel so that battery is returned from the AC
relay in this outgoing relay set over the negative
operator’s position. Relays NN, KK, GX, P and
and positive lines to operate both relays EE and
MA remain held,~while relay FY is slowly pulsing;
KK. Relay EE operates relay IR as already de
When the operatorv clears, the outgoing relay
set sends a clear signal comprising a two-seconds
scribed in connection with. the called subscriber
X pulse followed by a 300 m. s. Y pulse; The X
answer condition, and relay IR releases relay K.
pulse operates relays 1X, XR and GY in the in
As relay K is also operated at this time, how
coming relay set and the latter opens the circuit
ever, relay PY is held operated and no circuit is
for relay F'Y to terminate the transmission of the
completed to operate relay MA and initiate the
called party clear signal. Relay XR also? opens (i5 sending of the repeated Y answer signal back
the circuit of relay GX suf?ciently long for it to
wardly over the trunk. With relay IR operated
release whereupon‘ it drops relay MA and oper
the loss pad and line termination are removed
ates relay CC over contacts‘ 9333. When the X
at contacts ir2-ir4 while any circuit for relay
pulse is ?nished, relays IX, XR and GY release
CB is disconnected at contacts‘ 2'1‘5' so that the fol
and relay CC commences» to release slowly. The 70 lowing VF' signals pass directly through the in
300 m. s. Y pulse now follows and serves to op
coming relay set without any‘ conversion or repe
erate relays IY, YR and CR Relay CR locks to
tition. Under these conditions before they in-'
the earthed contacts 172' and transfers the hold—
coming relay set can be releasedr it must receive
ing circuit for relay NN to the Y pulse and also
a long X pulse to drop relay GX followed at the
connects earth to the P conductor extending for 75 proper interval by a short Y pulse to operate
relay CR. ‘The X portions of the subsequent pre
pare and‘ codeipulses‘ although‘ picked up by the
necessity for the provision of the in?nite attenua
tion loss pad in the incoming relay set.
VF receiver in the'incoming relay set are not long
enough to bring about the-release of relay GX. .
Considering now the circuit operations, it will
be‘ remembered that when‘ the VF prepare im
. pulse is received at the incoming relay set at the
Furthermore, the ‘440mm; X pulse acknowledg
ment signal which is sent from the originating
end in response to the? called party answer‘ signal
will bring‘ about the release of relay‘ GX,‘ but as
this is not followed by ‘a Y signal the‘temporary
release of relay GXI brings‘ about no material‘ 10
change in the condition of the incoming relay. set.
When the release signal comprising alongvX pulse
followed by a short Y. pulse is received, the in~
coming relay set releases as already ‘described.
second exchange (C), relay CA operates almost
immediately and provides an in?nite attenuation
loss pad which in effect-short-circuits the trunk
across a resistance bridge to prevent the passage
of V. F. signals beyond the incoming relay set.
However, during the operate time of relay CA
at the incoming relay set in the second exchange
(C), relay CA in the incoming relay set‘ at the
fifth exchange‘ (H) may operate but will at once
In the case of tandem calls over VF and D. C; 15 release when the CA relay at the ?rst incoming
relay set has operated and inserted the loss pad
in the trunk. It will be recalled that relays CA
and CB lock in series during the reception of the
code pulse and while the loop impulses are being
trunks in series, for instancefrom exchange B
to exchange E, when the‘D. C. outlet is'seized in
the second exchange (C), access i's'had to the out
going side'of the normal a‘uto-to-auto ‘repeater,
A—A RS, Fig. 7, which is thus cutout of circuit
in order to avoid two impulse repetitions in the
same exchange. The incoming relay set there
transmitted so that neither the prepare nor the
code VF signals are transmitted to the incoming
relay set in the ?fth exchange.
It will be appreciated that the desired block-.
fore’ extends earth forwardly ‘over the P con
ing effected could also be obtained by breaking
ductor to hold‘the selector train in the second
exchange‘ (C) and also accepts code signals and‘
the connection between the incoming and out
going sides of the incoming relay‘set. The ar
transmits forward loop ‘l impulses to set up’ the
rangement according to the invention whereby
automatic switch train at the third exchange
considerable attenuation is introduced‘ is prefer
(E). When the called party connected‘ to the
able however particularly when combined with
thirdexchange answers, ‘battery is reversed over
the line to‘operate relay BE in the incoming relay 30 the line termination as it involves fewer relay
contacts and also it has the advantage that
set at the second exchange (C) which thereupon
sends repeated Y pulses back to‘ the ?rst exchange
trouble due to dirty contacts would not put the
(B) ‘as already described. The- further opera
circuit out of action but at the worst would in
tion and release of the incoming relay set are as
troduce a-loss of the order of 3 ‘db; ~ ‘ l
already described.
i In the case of non-metered calls which are usu~ ‘
ally of the enquiry type‘such as service, intercep
tion, enquiry or speaking to B operators at man
ual exchanges, it will be appreciated that with‘
In the case'of tandem calls over vVF, D. C. and
VF trunks in‘series for instance from exchange
B to exchange G, when connection is made with
the outgoing relay set in the third exchange (E),
battery over both lines is extended backwardly
over the trunk to operate relays'KK and EE in
the incoming relay set at the second exchange
the circuit arrangements as described conversa
tion can take place immediately without‘ the ex
tension ‘of an answer signal and therefore the‘
automatic metering of non-metered calls
avoided. It will be seen that a 3 db loss pad (re
(C) which thereupon cuts‘ out its repeating ‘ap
paratus and provides a straight-through pair of
trunks as already described. Accordingly, VF
signals from the outgoing relay set‘in the ?rst
exchange (B) now pass directly without repeti
tion to the incoming relay set at the fourth ex
mains in the connection-under these conditions
owing to the absence of ‘any'battery reversal
which would operate relays‘ EE and IR but this
does not unduly interfere with speech and fur
thermore if more than one incoming‘ relay set is
’ change (G) which converts them to D. C. im
included in the connection the loss pad is re
pulses‘ for setting up the train of switches at 50 moved in all incoming relay sets except the one in
this exchange to establish connection with the
the terminating exchange where no battery is
wanted subscriber. ‘ The connection therefore be‘
returned over the negative and positive lines to
comes in effect VF to VF since there is no ‘auto
effect the removal of the pad, and consequently
to-auto repeater in circuit at the second ex
the overall extra loss never exceeds 3 db on this
change (C) and the switches here are‘ held for 55 type of call.
wardly from the incoming relay set‘ at this ex
Referring now to Fig. 9, this shows the modi?
cations necessary to the incoming relay set to‘
In the case of tandem calls over VF, D. 0., D. C. ‘
adapt it to function correctly if busy flash‘ sig
and VF trunks in series, for instance from ex
nals in addition to the normal busy tone are en
change B to exchange H, a different set of condi
tions arises in that an auto-to-auto repeater‘will ‘
60 countered.
be taken into use at the third exchange (E) which
prevents battery on, both lines from the‘ outgoing
relay set in the fourth exchange (F) ?nding a
circuit back to theincoming relay set in the sec
ond exchange (C), to cause this to provide a
straight through pair‘ of lines as was the. case in
the VF—D. C.-—VF connection.‘ In the absence
of any means‘to prevent it therefore, there is a
danger'that the V.'F.’code signals from‘ the origi
nating exchange may be received both at the in
coming relay set in the second exchange (0) and
the incoming relay set in the ?fth exchange (H)
and this would result in ‘two trains of impulses
being delivered for each digit. This explains the 75
For this purpose an additional relay
TH is provided and the connections to the line
termination and loss pad are suitably modi?ed.
On through calls when the incoming relay set
is required to provide a straight through con
nection and disable its repeating apparatus, re
lays K and EE operate as before from battery
on both lines through relay AC in the outgoing
relay set, and as this battery is now made positive
instead of negative, relay‘ TH is operated via the ,
metal recti?er MBA and locks up.‘ Relay IR; is
also operated from relay EE and with all four
relays operated the line termination and loss pad
is disabled and a straight through connection is
In other circumstances where the incoming ‘
relay set is the terminating one in the connec
tion, relay KK will be operated while the call is
being set up. If a busy line is encountered, busy
erate automatic switches to extend a connection,
means in eachyequipment operated only in case
tone is returned as explained and if this is ac
companied by busy ?ash batteryon the positive
line relay EE is also operated. Since however,
relay TH is not operated under these conditions
since the battery in question is a negative bat
tery, relay IR in operating does not disable the
line termination and loss pad, so that balanced
conditions are maintained to the line ampli?er
which is prevented from “singing” during the
What we claim as new and desire to secure by
Letters Patent is:
_ 1,. In a telephone system wherein a connec
tion is established to, a called station by auto
matic switches controlled over a voice frequency
signaling trunk including means responsive to a
disconnect at the calling end to. transmit a for
ward signal to release said switches, a source of
impulses comprising alternating current of- de
termined voice frequency interrupted at prede
termined intervals accessible to the called end of
said trunk, means in said trunk responsive to a 25
disconnect at the called station to connect said
source of impulses to the trunk as a clearing sig
nal whereby the calling station disconnects and
the forward signal is transmitted, and means re
sponsive to said forward signal to disconnect the
clearing signals from the trunk before the said
switch release is initiated.
2; In a telephone system wherein a connection
is extended over a trunk in which the functional
operations including superyisory and, releasing
fQI'LSaliQ 0031180310124. are cQlllil‘olled by sisnalsfrom
a second trunk is seized having such equipment
therein in a tandem connection for preventing
the translation of received ‘voice frequency sig
nals in the ?rst equipment and causing the trans
mission of the received signals to the second
seized trunk without translation, said meansre
sponsive only to a flow of current over the two
speaking conductors in parallel.
5. In a telephone system, a trunk line, means
for transmitting over said line a plurality of
coded signals each'comprising impulses of a plu
rality of different voice frequency sources and a
varying number of said frequencies in accord
ance with received signals and other control sig
nals of varying number of impulses of determined
frequency duration and spacing in accordance
with other received signals, a plurality of receiv
- ers connected successively to said line and re
sponsive to a received signal for translating it to
a signal corresponding to said received. signal, a
blocking meanscontrolled from each of said re
ceivers to prevent the further passage of said
received signal current over said line to the suc
cessively connected receivers, means in each re
ceiver effective on certain of said coded signals
for translating said signal and operating said
blocking device to prevent said other receivers
from effectively completing said particular code
translation and means in each receiver responsive
to other received signals to complete the corre
sponding translation without operating said
blocking means.
6. In a telephone system, a trunk line includ
ing voice frequency responsive equipment, means
a plurality of sources of alternating currents of
for extending av connection thereto and for trans
different frequencies, means in said connection
mitting coded digit signals of voice frequency
to transmit; impulses of said, currents of deter
currents thereto to extend a. connection there
mined: frequency duration and spacing over Said 40 over and for transmitting other coded voice fre
trunk in accordance with said functional opera
quency signals thereto, a blocking device in said
tions'controlled thereover, a signal comprising
trunk, each digit signal including a portion ef
impulses one of said frequencies repeated at a
fective to include the blocking device in the trunk
determined rate to control the supervisory func
to block further progress of voice frequencies
tional operation, a second signal comprising an
over the connection, and each other signal pass
other frequency of determined duration, means
ing over the connection without blocking.
in said trunk to transmit said second signal to
7. In a telephone system, a voice frequency
stop said supervisory signal and means in said
signaling trunk terminated on a responsive re
connection to transmit a subsequent pulse of
peater including means for transmitting and re
said; ?rst signal within a determined spacing, after 50 ceiving over said trunk a signal to seize said re
said stoppage signal is effective to control the
peater and another signal to release said repeat
functional operation of releasing said connection.
er, a. switch operating line controlled from said
3., In a telephone, system, a connection includ
repeater, means responsive to the seizure of the
ing- a trunk line over which voice frequency sig
repeater to connect said line to said trunk, said
nals are transmitted to establish a connection
repeater responsive to other voice frequency Sig~
and in which a clearing signal of voice frequency
nals received over the trunk to control‘ a connec
is normally sent tothe calling end when thecalled
tion, and to repeat said release signal thereover,
station hangs up and a release signal of Voice
holding equipment in said repeater controlled over
frequency is then transmitted from the calling
said connected line to prevent a false release of
end to release the connection, the provision of
said trunk before said line is disconnected, means
means responsive to a release signal transmitted
effective in the event of a short circuit on said
from the calling end before a. clearing signal. is
line whereby said holding equipment and said
received for causing the transmission of the clear
other signals are disabled, means in- said repeater
ing signal to act as a release signal to release the
effective only when said equipment is disabled
‘i. In a telephone system, a plurality of voice
frequency responsive equipments each associated
with a trunk line and each normally responsive
to received voice frequency coded signals to trans
late the same into direct current impulses to op
to disconnectsaid line from said trunk to permit
said release signal to be effective to release‘ said
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