Note: Descriptions are shown in the official language in which they were submitted.
:~36732
This is a division of copending Canadian Patent
application Serial No. 238,198 which was filed 23 October 1975.
Background of the Invention
This invention relates to time division telephone
switching systems and more particularly to the implementation
of key telephone service in time division private branch exchanges.
Time division PBXs conventionally employ solid state
crosspoint~ which are operated for a fraction of a second
called a time slot and which provides fox the transmission
of voice samples without, however, the ability to provide a
d.c. or metallic conduction path between the subscriber's station
port circuit and the trunk port circuit. The sleeve lead
conventionally found in space division switching systems
is also not normally present. This normally causes no undue
problems when the switching network is simply handling ordinary
telephone sets. However, when a key telephone set having
a hold button and one or more illuminated line pick-up keys
is served a problem arises. Briefly, when a call involving
such a set is placed on hold by the key set user a holding
bridge is inserted across the tip and ring conductors. The
2û holding bridge is physically located in a key telephone unit
that is interposed between the telephone set and the line
circuit appearance in the switching network.
The operating of the hold button at the key telephone
set causes an A relay in the 40û-series equipment to release
and thereby insert the winding of a loop-monitoring L-relay
in series with the holding bridge. When the station user
removes the hold condition by operating the pick-up key for
the line, the A relay is reoperated, the L-relay is;released,
and the holding bridge is removed. Other contacts o~ ~he A
relay control other relays that steer the key button lamp
--1--
~36 ~'3Z
illumination potential between a source of steady current
when the line is picked up to a source of wink lamp illumina-
tion current when the line is placed on hold.
When such a holding bridge arrangement is employed
in a conventional step by step or crossbar PBX and the call
is abandoned by the party at the remote end of the trunk,
the on-hook state of the trunk is ref~ected by a change
in the d~c. state of the line circuit and the holding bridge
is removed and the lamp illumination state changed to reflect
the true state of affairs. When such a holding bridge
arrangement is employed with a time division PBX however the
holding bridge is not released and the lamp illumination
continues at the wink rate giving the station user the erroneous
impression that the remote party is still being held.
Heretofore the problem has either been ignored or
has re~uired the use of the rather complicated special scan
rate distinguishing circuitry. The line circuit was required
to be provided with equipment to distinguish between the
scanning rates present for held lines and the interruption
of scanning by the processor when it determined that the call
had been abandoned. It would be
1~36~732
advantageous to provide a somewhat less complex arrangement
at the line circuit for dealing with the changes occurring
on the trunk side of the PBX network and for reflecting
those changes at the station side.
It would also be advantageous to provide a means
for releasing the hold bridge at a line circuit when the
call is abandoned at the remote end of the trunk, or by the
other party in an intercom call.
Summary of the Invention
The foregoing and other objects of my invention
are achieved in one illustrative embodiment in which the
subscriber's line circuit is equipped with a ringing control
relay that is normally operated by the central processor to
apply ringing to the tip and ring leads of the subscriber's
station. The ringing relay is advantageously provided with
a back contact that normally isolates the ringing current
from the line circuit transformer and thus from the time
division hybrid.
In accordance with my invention, when the central
processor detects that the call has been abandoned by the
party at the remote end of the trunk circuit, either on the
abandonment of a held condition or otherwise (since the
difference cannot be distinguished), it reoperates the
ringing control relay in the line circuit. The ringing
control relay, however, is operated at a time when the
ringing generator would normally not be connected to the
line circuit, i.e., in the so-called silent interval of
ringing. The back contact of the ringing relay, when
operated under the control of the processor during the
silent interval of ringing, interrupts the d.c. talking
current to the subscriber's loop. The interruption in this
-- 3 --
1~36~32
current to the subscriber's loop causes the holding bridge
to be released and lamp illumination to be switched from
winking to steady just as if the holding bridge arrangement
had been used with a line circuit appearing in a step by
step or crossbar PBX. Moreover the break in the d.c.
current occasioned by the reoperation of the ringing control
relay and the release of the holding bridge by the key
equipment circuit permits the line relay in the line circuit
to report the line state as idle to the line circuit
scanpoint where it is sensed by the scanner just as if it
~ere an ordinary on-hook line served by the time division
PBX.
The silent interval of ringing may be utilized in
either of two fashions according to whether the ringing
phase is assigned to a line by the processor from a
register storing an indication of currently available idle
ringing phases or whether ringing phase is predetermined by
association of the line circuits in a group with a particular
ringing phase bus. In either case ringing is frequently
supplied to a group of line circuits through a common
ring-trip and disconnect circuit. This invention also
applies when a single common ringing voltage bus serves
the entire system.
In the preferred embodiment ringing is assigned on
the basis of currently available ringing phases, up to four
lines in the group served by the common ring-trip and
disconnect circuit may be rung, each of the,lines receiving
its active one second of ringing at a time., If less than
four lines in the group are assigned to active ringing,
there are idle ringing phases available. During any of
these idle ringing phases, a line requiring call abandon
-- 4 --
~36732
correction may have its line circuit ringing relay operated
just as i it were to receive active interval ringing but
at the same time a disconnect relay is operated in the common
ring-trip and disconnect circuit so that no ringing current
is actually applied. The disconnect relay may be operated
at this time because it corresponds to a portion of the
silent interval for any of the other stations in the group
because they are assigned other ringing phases. It is thus
an aspect of this embodiment that another of the line
circuits in the same~group may be receiving normal ringing
phase signals during the active phase of ringing while a
particular line circuit in the group may have its ringing
relay and the disconnect relay in the group ring-trip and
disconnect circuit operated during the silent interval of
that ringing phase to accomplish hold-abandon correction.
In accordance with one aspect of the present
invention there is provided a line circuit including in
combination, a line relay, a pair of tip and ring conductors
for connecting said line relay to a telephone set, a ringing
control relay havir.g a set of transfer contacts including a
make contact for normally applying ringing current to one of
said pair of tip and ring conductors and a back contact for
disconnecting said ringing current from said line relay, and
means for operating said line relay during a predetermined
interval of time when ringing current is not applied to said
make contact.
DESCRIPTION OF THE DRAWINGS
The present invention taken in conjunction with the
invention described in copending Canadian patent application
serial no. 238,198 which was filed on 23 October 1975 will be
described in detail hereinbelow with the aid of the accompanying
--5--
1~3673Z
drawings, in which:
FIG. 1 is a block diagram o~ the groups of key
telephone line circuits and the processor of a time division
PBX arranged in accordance with my invention;
FIG. 2 shows the arrangement of a conventional key
telephone set and prior art 400-type key telephone unit in
association with the contacts of the line circuit ringing
relay and other details of the line circuit appearing in an
illustrative time division switching private branch
exchange;
FIG. 3 shows the details of a group ring-trip and
disconnect circuit;
FIG. 4 shows the line port shift register ringing
relay control flip-flop and interconnections to the data
address buses of the time division switching system; and
FIG. 5 shows how FIGS. 2 through 4 are to be
arranged.
General Description
.
Referring now to FIG. 1 there are shown the
relevant portions of a time division private branch exchange
arranged in accordance with one illustrative embodiment of
my invention. The telephone sets of the PBX are divided into
several sroups of which only telephone 561 of the first
group and one telephone 564 of the last group are shown
explicitly. Fully to illustrate the advantage of my invention
the telephone sets are shown as being key telephone sets although
some may also be ordinary, non-key sets. While keysets are
usually equipped with a plurality of pick-up keys PUK-, for
simplicity only a hold key H and one pick-up key PUKl and PUK2,
respectively, has been shown at each telephone set. Associated
with the pick-up key (or button) shown at each telephone set
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1~3~732
is a respective button illuminating lamp Ll, L2.
The line corresponding to the respective pick-up
key at each of telephone sets 561, 564 is accorded access
to the time division communications bus via a respective
line circuit 101-1, 101-n. Each line circuit contains a
hybrid and a filter circuit, not shown in detail, a coupling
transformer T-, a time division switch ~N- and digital control
therefor operated by the central processor 900. The time
division switch SW-, when operated, accords the line circuit
access to the time division communications bus to other
line circuits or to one of a plurality of trunk circuits such as
trunk circuit 700.
Associated with the battery feed circuit at the
primary winding of each line circuit's coupling transformer
Tl, T4 is a respective line relay LCl, LCn, whose respective
contact, LCl-l, LCn-l, places a signal condition on the line
scan point bus via line circuit gate SS to inform the processor
900 as to the switchhook state of the line. Likewise, each
trunk circuit 700 provides a status input to processor 900
via trunk scan bus as well as having an appearance in the time
division communications bus.
The line circuits are assigned in groups so that
a common ring-trip and disconnect circuit 300-A, 300-J may
serve several line circuits. In one illustràtive embodiment,
a group may contain up to 32 line circuits. Details of the
common ring-trip and disconnect circuit are shown in FIG. 3.
A common ringing generator RG may be employed to
apply continuous ringing to all of the common ring-trip and
disconnect circuits 300-A, 300-J in the illustrative PBX.
In each of these circuits there is provided, in accordance
~36 ~r~z
with the principles of my invention, in series with the
ringing supply lead RSG-l, RSG-J a respective back contact
RD-l RD-J whose function will hereinafter be described. The
ringing generator lead from circuit 300-A multiples to all
of the other 31 station line circuits in the same group as
line circuit 101-1.
Any line circuit in a group served by a particular
group ring-trip and disconnect circuit such as circuit 300-1
may be selected to have ringing applied to its associated
telephone set by the processor ~00 addressing the S input
of its respective ringing flip-flop, such as flip-flop 102-1
in line circuit 101-1. When the ringing flip-flop is set,
the line circuit ringing relay, such as relay RG-l, is
operated and at the make contact of its transfer contacts
extends the continuity of the ringing generator lead RSG-l
to the ring lead R of the telephone set. When the line
circuit ringing relay is operated the back contact of its
transfer contacts prevents ringing current from flowing
through coupling transformer Tl.
Ringing current will normally be applied to the
telephone set such as set 561 so long as flip-flop 102-1
remains set thereby maintaining ringing relay RNGl operated.
Since the normal ringing code is one second on followed by
a silent interval of three seconds, four different stations
in the group served by ring-trip and disconnect circuit 300-A
can effectively be rung in a four-second interval. Similarly,
four different telephone sets in the group served by ring-
trip and disconnect circuit 300-A may effectively be rung
in any four-second interval. For example, if flip-flop 102-1
is operated during the first second of a four-second interval
to apply ringing to telephone set 561 corresponding flip-flops
-- 8 --
1~36~73z
~: of three other line circuits in the same group served by
circuit 300-A may be operated one during each of the next
ensuing three seconds of the four second interval.
During the time that flip-flop 102-1 is set to
maintain the line circuit ringing relay RG-l operated, the
d.c. state of the loop path to telephone set 561 is monitored
by circuitry of circuit 300-A represented by the dotted path
(shown detailed in FIG. 3) between contact RD-l and group
ringing bus RSG-l. If, during the active interval of
ringing for line circuit 101-1, the station user places
telephone set 561 in the off-hook state, the switchhook
contacts of set 561 will, in the conventional manner,
complete a d.c. path to the ring conductor R. The presence
of the d.c. path is detected by the circuitry represented by
the dotted portion of the ring-trip and disconnect circuit
and that circuit clears the ringing flip-flop by applying
a clear signal to its input C. Details of the aforementioned
ring-trip circuit are disclosed in FIG. 3.
Let it be assumed that one of the stations, not
explicitly shown, which is served by one of the line
circuits in the same group as line circuit 101-1 is to
receive ringing during the first, one-second-long ringing
phase. Ringing current is applied from ringing generator RG
to circuit 300-A and therethrough over back contact RD-l to
ringing bus RSG-l which makes ringing current available to
the 32 line circuits of the first group. Processor 900 as
will hereinafter be more fully explained addresses the
ringing control flip-flop in that line circuit and transmits
a set command signal to the "S" terminal thereof.
The ringing control flip-flop when so set operates
_ g _
~ ~36r~3z
and maintains operated the line circuit ringing relay. The
make contacts of the transfer contacts of this relay apply
the ringing current from bus RSG-l to the ring lead of the
telephone set causing the bell in the set audibly ~o ring.
~t the end o~ the one-second interval, processor 900
readdresses the ringing control flip-1Op in the l~ne
c~rcuit, this time transmitting a clear command to the "C"
terminal thereof~ The flip-flop is reset causing the
ringing relay to release and stop the audible ringing at
the telephone set. The ringing control flip-flop will not
again be addressed by processor 900 until the onset of the
next four-second interval at which time an active one-
second of ringing will again commence for this line circuit.
Let it be supposed that telephone set 561 is not
a called telephone, as has just been described for another
of the telephone sets in the group of line circuits served
by circuit 300-A but is a telephone that is involved in a
communications connection with trunk circuit 700 and a
distant party served by the central office at the remote
end of trunk 700. Let it be further assumed that the
station user at set 561 has placed the call on hold by
operating the hold key H.
The hold condition in a time division switching
system may be administered by the processor without the
necessity of maintaining a physical hold bridge across the
tip and ring conductors and if the key lamp illumination
control circuitry is an integral part of the line circuit
no problems arise in controlling lamp illumination under
the abandoned call conditiOn.
--10-- ;
~3~32
However, the inclusion in the line circuit of
digital logic circuitry capable of administering the hold
condition and other key telephone service such as music on
hold and common audible control is a "deluxe" type of
telephone service whose availability to customers depends on
their willingness to pay the higher tariffs charged
therefor. For customers desiring to use the older, pre-
existing types of key telephone system terhnology, the
well-known 400-series key ~elephone unit equipment manufactured
by the Western Electric Company must still be provided. This
type of relay control circuitry senses the continuity of the
"A" lead on the key telephonè set and provides a hold bridge
across the tip and ring conductors when the A relay is released
and also steers the appropriate lamp illumination currents
to the pick-up key lamps of the telephone set.
Heretofore, the use of the 400-type key equipment
in time division switching systems has resulted in a
somewhat undesirable aspect of circuit operation in that
when the party at the remote end of trunk 700 abandoned the
call after the release of the A relay in the 400-type
equipment inserted a holding bridge between conductors T and
R, there was no easy way of changing the lamp illumination
signal delivered through thè~400-series equipment since
there is no provision made in the 400-series equipment for
use with common control equipment and specifically no way to
connect the 400-series equipment to the processor which as
described above has become aware of the abandonment of the
call by i~s scanning of the trunks. One approach to the
solution of this problem employs a circuit for sensing
--11--
~3673Z
differences in scanning rates.
In accordance with my invention, however, when the
processor 900 ascertains via scanner 901 that trunk 700 has
abandoned the call, it addresses the ringing control flip-
flop in the line circuit involved on the call during a time
when the common ring-trip and disconnect circuit serving the
line circuit is not involved in delivering ringing current
to any other line circuit.
At the same time, processor 900 addresses the
ring-trip and disconnect circuit 300-A serving the line
circuit and transmits a set command to the S terminal of its
ringing disconnect flip-flop 303-1. The setting of flip-
flop 303-1 operates ringing disconnect relay RD-l. The
operation of ringing disconnect relay RD-l at its back
contacts prevents ringing current from being applied to any
of the line circuits served by circuit 300-A.
Advantageously, the addressing of the ringing
flip-flop 102 in the line circuit and of the ringing
disconnect flip-flop 303-1 in the ring-trip and disconnect
circuit 300-A may be performed by a processor 900 under
stored program control and in some applications this may be
the preferred embodiment. To simplify the discussion,
however, processor 900 is shown as containing hard-wired
logic circuitry for accomplishing the addressing of the line
circuit ringing control flip-flop during the silent interval
of ringing.
Processor 900 contains a counter whose outputs ~1
to ~4 are successively activated at the onset of each one-
second interval to define intervals of active ringing.
Normally the outputs of counter 901 control (multiple)
output gates 902-1 through 902-4, in sequence, to gate the
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~Q3~732
addresses of the four lines to be rung from registers 904-1
through 904-4 to the address leads of bus SYB~SA. At the
same time an inpulse is delivered to the data lead (not
explicitly shown) of bus SYBUSA which is connected to the
S input of the ringing control flip-flops 102-1 th~ough
102-n of the line circuits 101-1 through 101-n. The
address and data leads of bus SYBUSA are not individually
shown in the drawing nor are the decoder circuits associated
with line~circuits 101-1 through 101-n inasmuch as the use
of address and data leads are in a bus to line circuits and
the use of decoders.
When ringing is to be assigned to a called line,
the number of the called line is applied by the processor
to the line group decoder 901 via the lower input of OR
gate 901A. The manner in which a processor obtains the
number of a line required to be rung is, of course, well
known in the art and need not be here described in detail.
Accordingly, no circuitry is shown as being explicitly
connected to the lower input of gate 901A. Line group
decoder 901 decodes one or more of the digits of the line
number address delivered to it through gate 9QlA to select a
group of four ringing control registers serving the group of
lines that includes the line requiring ringing. At the same
time decoder 901 enters the number of that line into an idle
one of line address registers 904-1. Line group decoder 901
decodes one or more of the digits of the line number address
delivered to it through gate 901A to select a group of
four-ringing control registers serving the group of lines
that includes the line requiring ringing. At the same time
1~3~3Z
decoder 901 enters the number o~ th~t line into an idle one
of line address registers 904-1 through 904-4 via the upper
input of a respective one of idle access gates 903-1 through
903-4 whose lower input is enabled by idle access control
circuitry Cnot shown). Idle access control circuitry,
sometimes referred to as lockout circuitx~y, being well-known
need not be described herein.
Four line~address registers are shown in each group
so as to provide one register for~each ringing phase. The
contents of the line address registers are read out to the
address bus when the respective ringing phase occurs. Thus,
the contents of line address register 904-1 is read out to
the address leads of bus SYBUSA during ringing phase ~1 and
the contents of line address register 904-4 is read out
during ringing phase ~4.
In the prior art, when the trunk scanner detected
the on-hook condition of the trunk serving the remote party
on a held call it signaled the processor (therein called a
"controller") to discontinue sampling of the held line and a
"missing sample" detector, which was provided as a part of
the line circuit responded to the absence of sampling pulses
to change lamp illumination from wink to steady. In my
embodiment, however, when the processor 900 is informed by
the trunk scanner 901 that the trunk 700 serving the held
call has gone on-hook it does not issue a command to alter
the scanning of the held lines line circuit. Instead, the
address of the line involved on the call with the trunk is
obtained from line address register 907 and transferred to
register 908 which is provided in accordance with my
invention to hold the address of the line requiring hold
-14-
lQ36~3Z
abandoned correction. The contents of register 908 is then
entered through the upper input of OR gate 901A to line
group decoder 901 instead of through the lower input of
gate 901A as was the case for a line requiring ringing.
However, when an address is entered through the upper input
of gate 901A, lead 909 is activated by register 908 responsive
to its being loaded with the line address requiring hold
abandoned correction. Decoder 901 selects the one of
group A through group J register circuits that serves the
particular line. The selection of one of the register
circuits, such as the group A circuits enables the input
AND gates 903-1 through 903-4 of the selected register
group to receive the line address in the same manner as it
did when operating to furnish the group A through group J
register circuits with the address of a line requiring
ringing. The one of input AND gates 903-1 through 903-4
which is enabled at its lower input by the idle access
circuitry (not explicitly shown) causes the contents of
-register 908 to be entered into an idle one of line address
registers 904-1 through 904-4.
The enabled one of gates 903-1 through 903-4
enables a corresponding one of flip-flop input AND gates
910-1 through 910-4 which allows the associated one of
flip-flops 905-1 through 905-4 to be set with an indication
that its associated one of line address registers 904-1
through 904-4 contains not the address of a line requiring
ringing but the address of a line to be processed for hold
abandoned correction. Thus flip-flops 905-1 through 905-4
are only set for hold abandoned processing. The set one of
flip-flops 905-1 through 905-4 permits the associated one of
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1~3ti~32
output AND gates 912-1 through 912-4 to be fully enabled on
its respective ringing phase. When that ringing phase
occurs, OR gate 914 passes an enabling signal to the upper
input of address control gate 915. The lower input of
address control gate 915 is enabled by the same output of
line group decoder 901 which selected the appropriate one of
group A through group J register circu~ts. When output
gate 915 is thus enabled, it delivers a setting signal to
the appropriate lead of SYBUSB that is associated with the
set input S of the one of ring-trip and disconnect circuits
300-A through 300-J serving the line group containing the
line having the abandoned hold condition. For example, if
AND gate 915 in the group A register circuit is enabled
corresponding to the entry of a line address requiring hold
abandoned correction into any one of registers 904-1 and
904-5, gate 915 will deliver a set impulse during the
corresponding one of ringing phases ~1 through ~4 to the
set input S of flip-flop 303-1 in ring-trip and disconnect
circuit 300-A. The setting of ringing disconnect flip-flop
303-1 causes relay RD-l to be operated. The operation of
relay RD-l at its operated back contact disconnects
superimposed ringing from bus RSG-l serving the line group.
The operation of ringing disconnect relay RD-l in circuit
300-A does not remove active interval ringing from any of
the group A line circuits since the address of the line
requiring hold abandoned processing was entered into one
of registers 904-1 through 904-4 not assigned an active
ringing phase. The entry of that line number in the idle
one of registers 904-1 through 904-4 causes the line
30 address to be transmitted to SYBUSA through the associated
one of output AND gates 902-1 through 902-4 during the
- 16 -
1~36732
~`
corresponding one of ringing phases ~1 through ~4. In themanner previously described for the case when the address was
that of a line requiring ringing, the application of the
line address code to SYBUSA is decoded by the board and
centerboard decoders (not shown) associated with line cir-
cuits 101-1 through 101-n to deliver a setting signal to
the S input of the addressed one of the ringing control
flip-flops 102-1 through 102-n in the addressed one of the
line circuits. The setting of that ringing control flip-
10 flop such as flip-flop 102-1 of line circuit 101-1 causes
its associated one of ringing control relay RG-l to be
operated. The operation of ringing control relay RG-l at
the make contact of its transfer contacts RG-l connects
the ringing bus RSG-l to the ring lead of the line circuit
but since the source of superimposed ringing has been dis-
connected from ringing bus RSG-l by operation of relay RD-l
in circuit 300A, ringing is not applied to the line. At the
operated back contact of its transfer contacts ringing
control relay RG-l interrupts the continuity of the ring
20 lead R of the line circuit toward telephone set 561. Opening
the ring lead opens the holding path for the L relay in the
400-D circuit allowing that relay to release. The release
of the L relay in the 400-D circuit removes the holding
bridge resistor Rl in the manner described in U.S. Patent
3,436,488 which issued on April 1, 1969 to Robert E.
Barbato et al. Release of the L relay also causes the
release of the C relay (winding not shown) whose transfer
contacts C disconnect the source of (wink) interrupted
lamp illumination from the key telephone set lamp Ll
30 associated with pick-up key PUKl.
-- 17 --
1~36732
Accordingly, I have shown a means for re-employing
the ringing control circuitry in the line circuit during an
unassigned ringing phase for changing the continuity of the
line circuit when the party at the remote end of the trunk
has abandoned the call. It will be noted that this
arrangement uses much of the same circuitry as would be
re~uired to control a line for ringing and thus provides
a superior alternative to prior art attempts to correct
the hold abandoned condition.
Further details of the analog and digital portions
of the line circuit are shown in FIGS. 2 and 4 and the
complete ring-trip and disconnect circuit serving a group
of 32 line circuits is shown in FIG. 3. FIGS. 2, 3, and 4
are to be arranged as shown in FIG. 5.
In FIG. 2 the telephone set 561 and 400-D key
telephone unit have been repeated from FIG. 1. The line
circuit of FIG. 2 contains a pair of hybrid amplifiers Al
and A2 which operate in conjunction with the time division
switches 201D and 201S to extend two-way time sampling
communications between the tip and ring TR, the line circuit
loop and on time distribution buses SUM, DIST of the time
division switching network. The description of the hybrid
and the control of the time division switches 201D, 201S,
being described elsewhere and not being a part of this
invention, is omitted herefrom. In FIG. 2 the path for
ringing is seen to extend from ringing supply bus RSG-l over
the make contact of the transfer contacts of ringing control
relay RG-l to lead R and the 400D circuit. Ringing current
is continued over make contact A2 of the 400D circuit in the
well known manner to the ringer of the telephone set 561.
- 18 -
~36~32
When a hold condition is applied by the stationuser at telephone set 561, the operation of the hold key
opens the continuity for the A relay in the 400-D key
telephone circuit. The release of the A relay at its
released ~ake contact A2 inserts the winding of relay L
in series with the`ring lead as described in the above
identified U.S. Patent 3,436,488. The xelease of relay A
through mesne circuitry, not shown, also inserts the holding
bridge resistor Rl between the tip and ring conductors. So
long as the holding bridge remains in the circuit, line
relay LCl in circuit 200 is maintained operated over a
path which may be traced from ground at its lower winding,
battery feed inductor BF the tip lead T to the 400-D circuit
and the holding bridge circuitry therein indicated by the
dotted path to resistor Rl, the winding of relay L in the
400-D key telephone unit to the ring lead R, the released
back contact of the ringing control relay transfer contacts
RG-l through the upper half of the battery feed inductor BF
and the upper winding of line relay LCl to battery.
As previously described in connection with FIG. 1,
when the processor determines that the party at the remote
end of the trunk circuit has abandoned the call, it applies
a set signal to the S terminal of ringing control flip-flop
RGFF, FIG. 4, which flip-flop when set operates ringing
control relay RG-l. When relay RG-l, FIG. 4 is operated
by the ringing control flip-flop RGFF, the back contact of
its transfer contacts RG-l opens the holding path for line
relay LCl and also for relay L in the 400-D telephone circuit.
The release of line relay LCl at its released make contact
LCl-l in the lower portion of circuit 200, FIG. 2, removes
~ - 1 9
)3673~
ground from lead LCG. Removal of ground from lead LCG
enables the scan point bus input gate SS to deliver an on-
hook signal to scan bus SS thereby informing the processor
that the line is on-hook. As previously described, the
release of relay L in the 400-D key telephone circuit allows
the interrupted (wink) lamp illumination ~o be removed from
- l9a -
iQ;~6~'3Z
the line lamp assoclated with circuit 200 on key telephone
set 561.
The remaining circuitry in FIG. 4 including time
slot shift register 401 and the busy/idle gate BI and the
selected busy/idle gate SBI previously known.
Referring now to FIG. 3, there is shown a first
and a last of the ring-trip and disconnec~ circuits 300-A,
300-J, respectively, each ~or serving up to 32 line
circuits. Each circuit, such as circuit 300-A, includes a
current sensing resitor Rl in series between the source of
10 20-cycle superimposed ringing and the associated ringing
supply bus RSG-l that serves the group of line circuits.
Capacitors Cl, C2, and C3 have a low a.c. impedance at the
20 Hz ringing frequency as compared to resistors R2, R4, R5,
and R7 and this network forms a filter at 20 Hz to render
transistors Ql, Q2, and Q3 insensitive to the a.c. component
of ringing. Resi;stors R6 and R7 form a voltage divider to
ground with respect to the negative 48-volt battery of the
superimposed ringing source that maintains their junction
point less negative with respect to ground than resistors R2
and R3 connected to the emitter of transistor Ql.
20 Accordingly, transistor Ql is normally on. Resistors R9,
R10, and Rll form a voltage divider such that when
transistor Ql is on, transistor Q2 will be turned on.
Transistor Q2 in the on condition maintains transistor Q3 on
and, in this condition, inverter RTC* maintains a low signal
on lead RT-l. This low signal has no effect on the ringing
control flip-flop RG of circuit 400, FIG. 4.
When the subscriber goes off-hook during the active
--20--
interval of ringing, the d.c. current through current
sensing resistor R1 back-biases transistor Ql turning it off
which in turn turns off transistors Q2 and Q3. Inverter
RTC* at this time applies a high signal to lead RT-l. The
high signal on lead RT-l is applied through inverter RT* in
circuit 400, FIG. 4, which applies a low signal to the C
input of the ringing control flip-flop RGFF. The low signal
resets flip-flop RG causing relay RG-l to be released and,
at the released make contacts of its transfer contacts RG-l
in FIG. 2, to disconnect the telephone set from bus RSG-l.
It should be noted that circuit 300-A detects
answer only during the active interval of ringing since only
during that interval is it connected to the ring lead R of
the ringing telephone set. This causes no inconvenience
however inasmuch as the ringing is immediately interrupted.
As previously described in connection with FIG. 1,
circuit 300-A includes a ringing disconnect relay RD-l
having break contacts RD-l in circuit with the ringing
supply bus RSG-l. Relay RD-l is operative by the flip-flop
including cross-connected NAND gates RDFS and RDFC. Relay
RD-l is operated by the application of a low signal to lead
RDS-l* by the processor and is cleared by the application
of the low signal to the lead RDC-l*.
While I have thus shown an arrangement for dealing
with the hold abandoned condition in a private branch
exchange having a time division switching network controlled
by a processor 900 having wired logic registers and gates
arranged as shown in FIG. 1, it is to be understood that the
connection of the line address registers to the gates may
also be made under the direction of stored program control
without departing from the spirit and scope of my invention.
- 21 -
In particular, my invention can be used in othertypes of.systems, not making use of a centralized ring-trip
detecting circuit, by causing the common ringing current
generator to have a short interval of zero voltage, during
which the ring relay can be activated on all circuits
requiring hold abandon correction, thereby causing hold
bridges in those lines to be removed. Further and other
variations will become apparent to tho~e of skill in the
art.
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