Note: Descriptions are shown in the official language in which they were submitted.
'7 ~1~
This invention relates to apparatus for interfacing
a subscriber station t~ a two-wire telephone network, and more
particularly to such apparatus that provides individual line
service features on a multi-party s~rvice line.
The use of multi-party service lines (MPS) is
common in rural applications and in other areas where user
population density is low. In order to provide telephone
services at a reasonable cos-t to subscribers, a predetermined
amount of telephone traff.ic must subsist. ~n MPS telephone
line provides a reasonable sol~tion to the problem by ensuring
a favorable ratio of subscribers per system.
In the case of a rural telephone application, a
two-wire telephone network provides a subscriber loop through
which all subscriber stations are connected to a telephone
central office. A major function of the telephone central
office is to provide a source of power for the network in
the form of a common talking battery and also to provide
sw.itching arrangements by which any subscriber station may
contact another station on the same network or any other
station on other networks with which the central office can
communicate.
A telephone network that is common to all sub-
scriber stations is economical to establish and maintain
since only two wire conductors are involved. There are,
howeverl related problems which have been accepted in -the
past in exchange for the principal feature of economy.
~ significant limitation of pr.ior art MPS telephone
systems is that privacy is an unavailable feature. A-t any
given time, one or all of the subscriber stations may be
connected in parallel across the telephone network
L ~
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which provides an opportunity for eavesdropping. Since it
is frequently the case that communica-tions be-tween stations
are of a private nature, :it is apparent -that the ability
to eavesdrop without ready detection comprises a serious
system limitation.
The fact that all MPS line subscribers are connected
in parallel across the network creates a problem when a
revertive call is to be placed, i.e., when a call is placed
from a calling station to a called station on the same side
of a two-wire subscriber loop. A revertive call when placed
in accordance with prior art teachings normally requires
the calling station to go on hook after dialing the called
station in order to free the line for a ringing signal sent
out by the cen-tral office to the called station. A problem
frequently encountered is failure of the call to ring through
which occurs when some other subscriber s-tation goes off hook
just as the calling station goes into its on hook mode.
In some MPS switching systems of the prior art,
supervisory signal paths are used together with separate
communication paths, the signal paths being used to establish
and maintain connections, perform busy tests without
interfering with the communication path and for performing
other necessary signal functions. The addition o more
conductors, however, complicated the system and tended to
defeat the economic reasons for establishing the MPS system.
Other MPS systems, notably of -the type used in
rural areas, use coded rin~ing siynals that are sent out on
the line and simultaneously energize all ringers of -the
subscriber stations in a predetermined portion of the network.
So as not to energize all ringers of the subscriber stations
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simultaneously with each ringing signal, and to reduce -the
ringing current load, ringers are often distribu-ted between
either the tip and ring conduc-tors of the line, or between
either the tip and ring and ground. In this way, -the simul-
taneous ringing of subscriber stations is res-tricted to those
stations having their ringers connected in parallel. This
feature therefore reduces the -total number of rings received
by any given sub~criber station over a given interval.
Nevertheless, all of -the s-tations havin~3 their ringers
connected in parallel will riny in unison each time any
rin~g signal is applied to that part of the network.
Another Limitation of some known MPS systems may
be seen in the interference that is injected in a communication
path when a calling station inadvertently dials a number
while the network is in use.
An object of the present invention, therefore, is
to provide apparatus to be used in conjunction with a
subscriber station on a two-wire telephone network in order
to provide party-line privacy.
The invention also provides apparatus that permits
fully selective ringing and ringer isolation from the network.
A further provision of the invention is apparatus
th
-that interfaces individual ones of subscriber stations
the telephone network and provides the party-line privacy
with a break-in capability.
A further provision of the invention is apparatus
that may be remotely connected and disconnected from a central
office of the telephone network in response to a supervisory
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~L2~2 7i~
signa] sent out on the n~-~work.
Another provision of the invention is to provide
apparatus that permits revertive calls, e.g., calls made on
a multi-party two-wire rural line from a calling station to
a called station on the same side of the line.
Still another provision of the invention is appar-
a-tus that locally yenerates s~eci~l su~ervisory an~ test tones
to be applied selectively to at leas-t one of the subscriber
station-and telephone network.
Another provision of the invention is apparatus
that i5 self-contained, having a rechargeable battery that
is charged from the telephone line, and which requires no
external power source for i-ts operation.
A further provision of the inven-tion is apparatus
providing limited access to the -telephone line and wherein
only the called party is allowed to have direct access thereto.
The invention also provides apparatus of ~lectronic
modular construction that is small and compact, and which
interfaces any type of standard subscriber station equipment
~ ~he two-wire telephone network, including rotaryl Touch-Tone
(Trade Mark) or decorator telephone sets.
Another provision of the invention is electronic
apparatus that may be readily fabricated by means of printed
wiring and integrated circuits arranged on plug-in cards
and the like.
Still another provision of the invention is
apparatus that permits revertive calls without requiring the
calling station to go on hook after dialing the number.
Yet another provision of theinvention is apparatus
that provides an audlble alarm for subscriber stations that
2 7~ L~ ~
are not permitted direct access to the network duriny use
thereof andwhich return to an on hook cond:ition without breaking
in on the line, the alarm denoting when -the network i5
available.
~he problems and disadvantayes associated with
the prior art may be substantially overcome and the objectives
and provisions of the present invention may be achieved by
recourse to an apparatus for interfacing a subscriber station
~1 ~I't~
~ t~ a two-wire telephone network. The apparatus comprises
means for detec-ting an on hook and oEf hook mode of the station,
means for determining the availability oE the network to
establish a bi-directional communication path with the station,
conditioning means responsive jointly to the mode of the
station and the availability of the network ~Eor selectively
conditioning the apparatus to an active or non-active state
and means responsive to the status oE the apparatus for
connecting the station across the network in the active state
and for disconnecting the station thereirom in the non-active
s-tate.
The invention will now be more particularly
described with reference to an embodiment thereoE shown, by
way of example, in the accompanying drawings in which:
Fig. 1 is a block diagram of a two-wire -telephone
network including a plurality of subscriber stations and
~Jl~h
apparatus for interfacing each s-tation t-e the network in
accordance with the invention;
Fig. 2 is a block diagram oE the interfacing
apparatus appearing in Fig. l;
Fig. 3 is a more detailed block diagram oE the
-- S
apparatus of Fig. 2; and
Figs. 4-7 are schematic di2gr~msindicating with
greater particularity -the circuits comprising each block of
the diagram shown in Fig~ 3.
Prior to entering into a detailed description of
the circuit configuration of the apparatus of the present
invention, an initial description will be clevoted to the
general structure and function of such apparatus in order
to provide a more comprehensive understanding o its utility
and operating featuresO
The apparatus of the invention of which an
exemplary embodiment is to be later described, is referred
to herein as a rural interface device (RIU) 100 which is an
electronic device that has been developed to meet the require-
ments of multi-party telephone networks. According to Fig. l,
each RID 100 is serially connected with a corresponding
~ J J'th
subscriber station 101 to interface the station t~ a two-wire
telephone network which originates at a central office 103.
It will be observed that the P~ID lO0 may be connected to a
multi-party service (MPS) line with a maximum of four parties
per line that engage in shared service. The RID lO0, however,
i5 not restricted to a maximum of four parties and greater
nur~ers may be used in view of the modest current require-
ments of each RID unit.
Selective coding in the RID 100 of the present
invention is easily set by way of strapping, later to be
described in greater detail, which limi-ts the selective rincJing
decodiny capability of the system to eiyh-tcodes. This
limitation need only apply, however, in the event that only
tip and ring conductors of -the network are used. If requlred,
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the system capability may be lncreased using the same eiyht
codes by employiny a ground re-turn ancl connectiny the
subscriber sta-tions between ground and a tip conductor 104
on the one hand, and ground and a ring conductor 105 on the
other hand. Fig. l shows this feature where a fifth station
is connected between the conductor 105 and ground. Even if
only the tip ancl ring conductors are used, the decodincJ
capacity may be increased by increasing the number o straps
and related circuits to accommodate extra ringing codes.
In accordance with the objectives of the invention,
the RID 100 has been provided as a compact package that is
self-contained and which requires no external power source.
As later described in more precise detail, the RID lO0
includes a bridging input for connection across a two-wire
telephone line 102 and also includes an output for serial
connection with a standard telephone, shown as a subscriber
station lOl in Fig. l.
The circuit structure of the RID lO0 is solid--
state and includes a plurality of integrated circuits (I.C.'s)
-that are mounted on plug-in cards to facilitate maintenance
when required. As a result of these features, bo-th installation
time, and maintenance, if it is needed, are rninimal.
A block diagram of the RID lO0 in Fig. ~ shows
input connections to the tip and ring conductors 104 and 105,
respectively, of the line 102 that origina-tes at the central
office 103. It will be noted that the input connections
bridge the line 102 and lead to a voltage sensor 201 that is
connected between the tip and ring conductors. Line termin-
ating circuitry is indicated generally at 202 and will be
further described in greater de-tail in a following description
-- 7 --
of the exemplary e~odiment oE the RID 100.
It has been disclosed that the RID 100 has an
output that is serially connected with the station 101. In
Fig. 2, it will be observed that a sing:Le pole double khrow
(SPDT) relay switch 203, inserted serially with a tip con-
ductor 104' of the station cable, provides that the station
101 may be connected directly across the line 102 or serially
wi-th the RID 100 in response to the operating condition of
an activate relay 204. As may be seen in Fig. 2, the contacts
of the switch 203 are in an ACTIVE position. This means
that the station 101 is serially connected with i-ts RID 100
and is consequently disconnected from the ~ip conductor 104
of the line 102. The condition of the RID 100 is such that
it places the station 101 in an idle, ON HOOK mode in which
`~ the switch 203 disconnects a station ringer (not shown) and
an automatic number identifica-tion (ANI) circuit 220 from
the line 102~
Terminology herein referring to an active circuit
st~te is denoted by capital letter~. ~or example when the
line 102 is busy its condition is indicated as BUSY.
Conversely, when the line is inactive and therefore not busy
its condition is indicated by a bar, i.e., BUSY.
Reference will also be made to various outpu-ts in
the circuits herein to be described as being high under
certain conditions and alternately low under other predeter-
mined conditions. It is to be understood that a low condition
is defined by a signal voltage having an amplitude ~ 0 and
that a high condition occurs when the signal volta~Je ~0.
Described as a self-con-talned unit, the RID 100
does not require connec-tion to a separate source to derive
7'~
its operating power. The power requirements for the P~ID 100
are obtained fxom the talking ba-ttery (not shown~ which is
located at the central office 103. Thus, the RID 100 obtains
its source oE operating voltage from the DC voltage appearing
across the line 102. An appropriate reduc-tion in voltage is
provided by way of the voltage sensor 201 o~ Fig. 2. The
line voltage is also used as a charyiny source ~or a
stand-by battery 216 which provides the RID 100 with an
emergency source of power in the event of temporaxy loss of
talking bat-tery potential.
Various operational fea-tures of the RID 100
will next be discussed in terms of the block diagram of Fig. 2
in order to provide a better understanding of the working
characteristics imparted to the station 101. These character-
~1''~
istics permit interfacing the station ~3 the telephone network
in a way that provides individual line service Eeatures on a
MPS line that ser~es a plurality of stations 101. The ex~mple
in Fig. 1 shows five such stations.
Referring again to Fig. 2, it will be observed
that means are provided for determining -the ava:ilability of
the line 102 to establish a bi-directional commun:icLItion
path with the station 101. In this regard, the voltage
sensor 201 is connected across the tip and ring conductors,
104 and 105 respectively, and produces an output signal that
is applied to a status detector 205 which interprets the
signal. In order to decide the condi-tion of the RID 100,
i.e., whether it should be ACTIVE or ACTIVE, a current
sensor 206 provides means for detecting an ON HOOK and
OFF HOOK condition of the station 101 and produces an output
signal that i9 coupled as a second input to the s-tatus
_ g _
7'~0
detector 205. Accordingly, when the station 101 is OFF HOOK
and the condit.ion of the line 102 is B~'SY, then arl output
from the status detector 205 i5 applied to an input of control
log.ic 207 circuitry which energizes the relay 204 and places
the switch 203 in the ACTIV~ mode. As a result, the tip
and ring conductors, 104' and 105' respectively, oE the
station 101 are direct].y connected to corresponding conductors
of the line 102 and a call may be placed at this time.
In the even~ that some station 101 of the network
has gone OFF HOOK so that the line 102 is BUSY, the relay 204
is not energized by the control logic 207 and as a result the
RID 100 remains in the ACTIVE mode. Thus, there is no
connection for the station 101 of Fig. 2 to the line 102.
Concurrently, an output from the control logic 207 is applied
to an input o a local tone generator 208.
It will also be seen in Fig. 2 that a second output
of the control logic 207 is applied to a timing circuit 209
which is also coupled to a clock 210 to receive timing pulses.
A second input to the local tone generator 208 is obtained
from the timing circuit 209 which enables the generator 208
~o
to output a BUSY signal of ~ Hz at 60 ipm that is coupled
through the switch 203 and the current sensor 206 to the
station 101.
A power conservation feature of the RID 100
provides that local loop current only flows from a connect
switch 211 and -therefrom through the switch 203 ! the stat-
ion 101~ and the sensor 206 when the s-tation goes OE'F HOOK.
The OFF HOOK condition is determined by the sensor 206 which
produces a corresponding output that is applied to an input
of the detector 205. The detector 205 in turn actlva-tes the
1.0 -
control logic 207 which then produces a first input to a
command circuit 212, a second input thereto being obtained
from the clock 210. The circuit 212 is -there~y conditioned
to enable the switch 211 through which the loop current flows.
The connect switch 211 is similarly actuated when
the station 101 is in the ON HOOK mode and a ringing
condition is sensed on the line 102 by a ring sensor orde-tector
221. Assuming that the ringing code is that of the station
101, the detector 205 responds to the first detected ring
output from the detector 22I by actua-ting the control logic
207 and a ring analyzer 213. Upon receipt of a valid ring
code, the analyzer 213 in turn enables a code comparator 214
-that produces an output which is coupled to the control logic
207. The relay 204 is actuated,i placing the switch 203 into
the ACTIVE mode and connecting the called station 101 across
the line 10~ to apply ringing voltage to the station 101.
When the station 101 responds by going OFF HOOK, a communi-
cation path is established with ~he calling station.
In the event tha-t the line 102 is BUSY, and -~he
station 101 goes OFF HOOK, a BUSY tone is generated by the
local tone generator 208 as described in the foregoing remarks.
The RID 100 will remain ACTIVE and the station 101 wil]. not
be given access to the line.l02. Should, however, the sta-tion
wish to break in on the line, as would be the case in -the
event of an emergency, a hook flash is generated which is
recognized by the RID 100, causing it to go ACTIVE. For
purposes of this desc.ription and a more detailed descrip-tion
~o follow, the hook flash is caused hy going ON HOOK for a
period of from 600 ~o 1600 milliseconds (ms). By flashing
the switch hook, the relay 204 is latched. The BUSY tone is
7 ;~ ~
concurrently cancelled and the station 101 breaks the existing
privacy of the line. Surrep-titious eavesdropping is pre-
vented, however, since the loss of privacy is announced by
an intrusion tone produced by a C.O. tone generator 215 of
the RID 100 and applied across the line 102. The tone
generator 215 is deactivated and the tone subsequen-tly re-
moved from the line 102 when the station 101 that invoked
the break-in feature goes back to -the ON HOOX condition.
A significant feature of the RID 100 is its
facility for interfacing all of the stations 101 with the
line 102 in a manner that fully provides individual line
service features for each station even though they are con-
nected across a MPS line. This is a revertive call feature
in which a call may be placed between two parties located
on the same side or across the same par-ty line.
To place a revertive call, a station 101 goes
OE'F HOOK and dials the calling number. The call is placed
in the usual way through the central office 103 and, since
the calling party is OFF HOOK, a BUSY tone is placed on the
line 102 from the central office. On hearing the BUSY tone,
the party at the calling station creates the hook flash
which is sensed by the detector 205 and enables -the RID 100,
causing it to actuate the relay 204 and to set the switch 203
to the ACTIVE stateO The line 102 is thus Ereed and the
central office 103 rings the called party. At this time the
calling station 101 hears a ring-back -tone tha-t is in s-tep
with the ringing voltage, the ring-back tone being generated
by the generator 208 and fed back to the station 101 via the
switch 203. When the called station answers, the condition
is sensed by the voltage sensor 201 which produces an output
- 12 -
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to enable the RID 100 to operate -the relay 204 and to
reconnect the calling s-tation 101 across the line 102 through
the switch 203.
Should the calling station go ON HOOK before the
revertive call has been completed, this condition is detected
by the status detector 205. After typically 2 seconds,-the
status detector 205 will enable the control logic 207 and
sub~equently the command circui-t 212 to actuate a termina-tion
relay 218 causing a switch 404' to place a balanced termina-tion
219 across the line for typically 2 seconds. At -the central
office 103, this termina-tion will simulate an Mæs subscriber
being connected to the line and ringing voltage will cease.
Should the called station not answer its ring and should the
calling station go ON HOOK, this condition is sensed by the
status detector 205 which commands through the circuit 212
that the balanced termination 219 be momentarily connected
across the line. This artificially signals to the central
office 103 that the called station has answered and ringing
is discontinued.
~ig. 2 also shows a dinger control 217 which
provides a tinkle ring automatically. When the station 101
goes OFF HOOK and the line 102 is determined as BUSY, the
line condition is detected by the sensor 201 which functions
with the other described circuits to activate the relay 204
keeping the station 101 in the ~CTIVE state. At this time,
the dinger control 217 is set. When after the station 101
goes ON HOOK and the line 102 becomes BUSY the condition is
detected by the sensor 201 enabling -the detector 205, the
control logic 207 and the dinger control 217 which applies
a pulse of rinying current throuyh the swi-tch 203 across the
~ L3 -
ringer ~not shown) of the station 101. This produces a sinyle
ding that alerts the -telephone sub3criber to the fact -tha-t
the line is now available.
A remote disconnec-t and remo-te test fea-ture are
provided by the connect switch 211 and the termination relay
218 which are selectively enabled by the circuit 212. As
will be described in greater detail in -the followin(3 de-
scription, the RID 100 is responsive to a predetermined
signal received from the central office to remove local loop
current from the station 101 when the s-tatiorl goe~ Or~'F HOOK.
This disconnects the station 101 from the line 102 while
retaining the voltage sensor 201 and certain other circuits
in an.,enabled state for subsequent reconnection. As indicated
in Fig. 2, the line termination circuitry 202 is shown in a
disconnected position in which -the RID 100 is operational
and may xespond to changing conditions and signals appearing
on the line 102. In its BALA~CED state, the circuitry 202
is switched by the relay 218 to terminate the line 102 with
a matched impedance while maintaining active the sensor 201.
In the BALANCED and TEST TONE state a 1000 Hz test tone `
of controlled frequency and amplitude is placed on the line
102 across a termination 219.
On occasion a first calling s-tation will have
ini.tiated a ringing signal on the line 102 just prior to a
second calling station going OFF HOOK. The RID 100 is able
to handle this situation since in the instance of the second
calling station the ringing code is not recognized and the
local tone generator 208 therein is enabled to genera-te
simulated ringing for the second calling sta-tion for as long
as line rin~ing is present. When the called station responds
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7 /~ C~
to its ring and goes OFE~ HOO~, this condi-tion on the line 102
is detected by the voltage sensor 201 which enables the local
tone generator 208 of the second calling station to produce
a BUSY tone and to alert the caller to the fact that the
line 102 is now BUSY.
Fig. 3 is another block diagram of the RID 100
showing the various circuit blocks of Fig. 2 in greater
detail. A more complete description of the RID 100 will now
follow having regard to Fig. 3 and to Figs. 4~7 which illus-
trate in schematic diagram form the circuit ~tructure of theblocks shown in Fig. 3.
All signals appearing across the line 102 are
detected by the voltage sensor 201 which monitors the line
voltage in order to determine a busy condition. Connected
across ~he line 102 in Fig. 4 there is shown a voltage divider
comprising the battery 216 serially connected with a zener
diode 401, a zener diode 402, a current limiting resistor
403, and an optical coupler 405 including a diode bridge 405'.
The diode bridge 405' will permit the optical
coupler to be activated for either normal polarity with the
tip line positlve with respect to the ring line or reversed
polarity wi-th the tip line negative with respect to the
ring line. With normal polarity on the line 102, the sum of
the voltage of the battery 216 plus the reverse breakdown
voltage of the diode 402 sets the voltage threshold to de-
termine when the line 102 becomes busy after a station 101
is connected to the line. With reverse polarity on the
line 102, the difference between the reverse breakdown voltage
of the diode 401 and the battery 216 sets the voltage thresh-
old to determine when the line 102 becomes busy. In each case,
- 15 -
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the voltage threshold so established w:ill differ from the
line voltage by the sums o~ three forward bias voltages of
three diodes in the diode bridge 405'. The BUSY condition
on the line 102 will normally be less than 20 volts. Only
when the line voltage is above -the threshold will a battery
charging current, with magnitude established by t~le resistor
403, flow in the voltage divider.
When signals appear across the line 102 current
` flows through the coupler 405. Emitted llight from the
diode portion energizes a transistor por-tion which results in
an output signal labelled VOLTAGE. It will be observed that
the signal :is squared by inverters 406 and 407 to produce an
output signal compatible with logic levels. Between the
inverters there is connected a RC filter 408 in the form of
a simple integrator that prevents line noise, detected by
the coupler 405, from operating the RID 100.
~ inging voltage on the line 102 is detected by
the polarity sensitive ring detector221shown in Figs. 2, 3 and 4.
The design of the RID 100 will se1ectiv~lypermit ringing
voltage to be detected between the tip conduc-tor 104 and
metallic ground or the ring conductor 105 and metallic ground
or in a balanced state between tip and ring. Ringing current
applied from the central office 103 will be passed from the
tip or ring conductors through a D.C. blocking capaci-tor 461,
through a large resistance 462 and through an optical coupler
460. A diode 463 assures that a large reverse bi~s voltage
is not applied to a light emitting diode of the coupler ~60.
When ringing is applied batween tip or ring and metallic
ground, the other side of the line 102 , ring or tip respect-
ively, is connected by the central office 103 to metallic
- L6 -
~t^~'7 i~
ground. When ringing is applied in a balanced configura-tion,
both ring and-tip conductors are disconnected from ground.
It is obvious that the op-tical coupler 460 will detect
ringing in any possible mode and provi.de a logic level BAL RING
on each half cycle of ringing voltage.
Should the RID 100 be required to recognize its
own ringing code on one side of the line 102l tip or ring
to ground, this selection is made by a switch 450. With the
switch 450 in position "a" ringing current is pas~ed from .
the tip conductor 104 through the optical coupler 453 to
ground. In position "c" the switch ~50 will direct ringing
current from the ring conductor 105l denoted ~ , to ground.
In each of these positions, the impedance to ground offered
by a DC blocking capacitor 451' and a large isolating re-
si~tance 452' is balanced on the opposite side of the line
by equal value components 451 and 452 respectively. In
position "b" of the switch 450, the ring detector wi.ll detect
balanced ringing only. When ri.nging is detected on the
side of the line established by the switch 450, a logic level
POL RING will be presented at the output of the optica.l
coupler 4 5 3 .
Current detection in the current sensor 206 is
obtained by the station 101 going off hook and allowing a current
to flow from the battery 216, throuah a transistor 414 of the
connect switch 211, through the station 101, through the
switch 203, and through a resistor 206' to the circuit
ground at the negative terminal of the battery 216. ~he
voltage developed across the resistor 206', due to the
current therethrough, forward biases a -transistor 206" into
a conducting state. A current path i5 thws established
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~2~P2~
~rom the Vcc -terminal of -the battery 2:L6 through a resistor
428 and th.rough the transistor 206l' to the circuit ground
of the battery 216. A ground voltaye will be developed at
the collector of the transistor 206" and simultaneously at
the input of a NAND gate 430. Thus due to the current
through the s-tation 101, the gate 430 will be turned off
resulting in a high voltaye or CURRRNT level at -the output
of the gate. It is to be noted that a CURRENT signal will
appear at the output of the gate 430 irrespective of current
direction through the sensor 206, Should current be from
the negative side of the battery 216 and through the resistor
206' and station 101, such as when normal voltage polarity
exists on the li.ne 102 arld the switch 203 is in the ACI'IVE
position, the transistor 206 will be biased on causing a
positive CUR~NT output from the NAND gate 430.
The relay 204 and its swi-tch 203 are shown in a
normal position in which the RID 100 is in the ACTIVE state.
It will be observed that the relay 204 in fact comprises a
pair of field windings 204l and 204" and that each winding
is operated by a separate two ~tage transistor amplifier 412
and 413, respective].y. Reference to Figs. 3 and 5will show that
drive signals for each amplifier are obtained from an active
analyzer 301 that forms part of the control logic 207
circuitry.
Referring next -to the circuit 202 of Fig. 2, a DPDT
swi-tch having two sets of contacts 404 and 404' is shown
with the contacts 404' disconnected from the line termina-tion
219. This is required when the RID 100 is in the normal
opera~iollal mode. The switch contacts of the c:ircuit 202
form part of t.he relay 218 which is operably responsive to
an output from the circuit 212. This circui-t arrangement
provides a remote -testing Eeature to remo-tely connect or
lf3
7~
disconnect a termination and test tone across the line 102
and will ke described in greater detai:L in the circuit
description to follow.
Having described briefly the switchable .inpu-t
connections to the RID 100, remotely controllable from the
central office, the voltage and current sensors of the device,
and lts ACTIVE, ACTIVE modes, a more thorough understanding
of the RID 100 will be developed through a detailed discussion
of its operation.
Considering firstly the operation of the RID 100
when originating a call, it will be remembered that certain
c~nditions must be met before the relay 204 will set the
switch 203 to the ACTIVE position. Not only must the line
102 be available to establish a communication path with the
called station, as determined ~y the voltage sensor ~01,
the current sensor 206 must also provide an output to the
detector 205 which is shown in Fig.3 as an off/on hook sense
circuit 303 and a hook ~lash sense circuit 304~
The signal VOLTAGE from the sensor 201 is applied
to a busy detector 302 producing a BUSY output that is applied
to one input of the analyzer 301. It will be evident from
Fig. 3 that the output from the analyzer 301 is ACTIVE since
the inputs thereto indicate that the line 10~ is B[JSY and
that the station 101 is OFF HOOK. The ACTIVE output from the
analyzer 301 is then applied to the input o~ the amplifier 412
which drives the field winding 204' to set the relay switch
203 which serially connects the station 101 through the
sensor 206 and across the ].ine 102.
In Fig. 4 it will be seen -that the swi-tch 211
comprises a pair of transistors 414 which provide an operating
_ l9 _
~ ~Z7'~
current for the station 101, the current being supplied through
a diode 415. In this way, ~hen the station 101 goes OFF HOOK
while the RID 100 .is ACTIVE, current for the station 101 is
taken from the cathode of -the diode 415. When the RID 100
goes ACTIVE and switches the station 101 across the line 102,
the current supply is then obtained from the talking ba-ttery
of the central office 103.
It has been described that the analyzer 301
decides when the RID 100 connects the station 101 to the line
102. There are four conditions that will cause the RID 100
to go ACTIVE and connect the station across the line:
RING DECODED~ON ~OOK-BUSY Ibeing called)
OFF HOOK-RTU-REV-BUSY (going off hook to initiate
a call)
BUSY-BRE.AK-IN (break in)
BUSY-REV RES (revertive call)
In order to go ACTIVE the following conditions
are required:
ON HOOK-RTU (on hook)
REVREV R~S (revertive call)
It should be understood that when the RID 100 has
been remotely disconnected, a drive signal CONNECT for the
switch 211 is low thereby turning off the switch and preventing
current in the local loop when the subscriber 101 goes OFF HOOK.
Without this local current the RID 100 is inoperable. When
current is detected, as in the case when the RID 100 is in
normal operation and the station 101 is OE'F HOOK, the output
signal CURRENT from the sensor 206 enables a binary coun-ter 601.
After 250 ms an on/off hook flip-~lop 602 is set. An OFF HOOK
output from the flip-flop 602 goes high and the coun-te:r is
gated off. Going back ON HOOK resets the counter 601.
In the event that the line 102 is BUSY and -the
~ 20 -
~z~z~
station 101 goes OFF HOOK, there will be heard in the receiver
of the station a busy tone generated by the RID 100. In keeping
with the objectives of the present :invention, the station 101 will
not be given access to the line since the RID will rernain ACTIVE.
The BUSY condition is sensed by the sensor 201 if
the voltage drops below i-ts threshold typically of 20 volts
for a minimum of typically 10 ms. Under this conditiorl~ a
VOLTAGE signal appears at the output of the inverter 407. As
a conse~uence,a latch 603 in the busy detector 302 is enabled
and,after 250 ms, sets a busy flip-flop 604 so that its
output signal susY goes high. The BUSY signal is then applied
to one input of a two-input NAND gaté 605. The second input
of the gate is low, determined by the revertive reset output
REV RESET of a flip-Elop 606. Accordingly, the output of
the gate 605 is high which is coupled through the logic
circuitry of the analyzer 301 and appears.as an input high,
together with a low ACTIVE signal, on respective inputs of a
NOR gate 607. An output ON LINE of the gate 607 is low and
is applied to -the set input of a flip-flop 608. The reset
input OFF LINE of the flip-flop 608 is concurrently high and
is applied to the amplifier 413, operating -the relay 204
and setting the switch 203 to the ACTIVE position.
The BUSY high signal is also coupled to the local
tone genera-tor 208 which is shown in greater detail in Fig. 4.
Specifically, the BUSY high is input to a three input NAND
gate 416. It will be observed that the other -two inputs
comprise an ACTIVE input and a 1 Hz signal from a master
timer 318 shown in Fig. 7 as a binary counte:r 701. The
resulting switching output from the gate 416 is applied to
one input of a two input NAND gate 417 together with an output
- 21 -
'P2~ ~
high from a NOR gate 418. A swltching output from the
gate 417 is then gated throuyh a three input NAND ga-te 419
O /-
~together with an OFF HOOK input signal and a ~-2~ Hz signal
6~a
producing a ~ Hz BUSY tone output that is interrupted at
a 1 Hz rate. The BUSY Outpllt signal is inverted by an
inverter 420 and is coupled through a variable attenuator 421
and a capacitor 422 to the ~CrrIVE contact of the switch 203.
The station 101 remains ~herefore disconnected from the line
and receives the BUSY si.gnal for as long as the line 102
is in use.
When the line 102 becomes availa~le once again,
the output VOLTAGE from -the sensor 201 goes high thereby
enabling a latch 609 which, after 1600 ms of continuous
signal, resets -the flip-flop 604. The BUSY output -therefrom
goes low which disahles the local tone generator 208. Con-
currently, the BUSY output goes high which is coupled through
the logic circuitry of the analyzer 301 producing an ON LINE
high at the output of the gate 607 which drives the arnplifier
412. Consequently, the field winding 204' is energized,
setting the swi-tch 203 to the ACTIVE position and connecting
the station 101 across the line.
Emergency access to the line 102 is provided by
the RID 100 merely by initiating a hook flash as earlier
described. It has been previously stated that -the hook
1ash disables the busy tone and the s-tation 101 breaks the
privacy of the line. In so doing, the loss of privacy is
indicated by an intrusion tone produced by the C.O. tone
generator215.
Looking now to the hook flash sense circui-t 304
of Fig. 6, it will be observed that with a CURRENT output
high from the sensor 206, together with an OFF' ~IOOK high
from the flip-flop 602, both of which are applied to
22 -
1~2~
respective inputs of a -two i.nput NAND gate 610, an output low
is produced which is applied to the input of an inverter 611.
The resultiny output high from the inverter 611 i~ applied
to a reset input of a binary counter 612 which is -thus held
on raset and is gated off. At the beginning of a hook flash,
CURRENT goes low and the counter 612 begins to count. After
0.6 sec, an output OHF from a N~ND gate 629 goes low and a
hook flash flip-flop 613 is set thereby to produce an output
HFT high. This output signifies the start of the hook flash
time (0.6 to 1.6 sec). If the CURRENT signal remains low
for greater than 1.6 sec, an output taken from the counter
612 and inverted by an inverter 614 to produce HTO, goes low.
The flip-flop 613 is then reset by HTO and the counter 612 is
gated off to indicate an invalid hook flash. If the CURRENT
signal goes high in less than 1~6 sec, the HFT signal output
Erom the flip-flop 613 remains high and the counter 612 is
held on raset to signify a valid hook flash.
The conditionsrequired to generate a break-in are:
a) the line 102 is BUSY;
b) the RID100 is ACTIVE;
c) a revertive call is not being placed; and
d) a hook flash is generated~
Reference to a break-in sen~e circuit 305 of Fig. 3 r and
to the more detailed schematic diagram of the circuit in
Fig. 6 shows that three inputs thereto are connected to a
thrae-input NAND gate 630. In a break-in operation an output
low from the gate 630 is applled to an inverter 615 which
generates an output high that is applied 9 together with a
BUSY high, to respective inputs of a two-input NAND ga-te 616.
The resulting output low therefrom is connected to a set input
of a break-in flip-flop 617 that produces an output high
-- 23 --
~Z~ 7~
BREAK IN signal which ls coupled -to an input of the tone
generator 215. Going back ON HOOK resets the flip-flop 617.
The BREAK IN high from the flip-flop 617 is shown
iTI Fig. 4 being coupled to one input o:E a two-input NAND
gate 423. The second input to the gate 423 is taken from the
output of a four-input NAND gate 702 which is shown in Fig. 3
AS a br~ak-in gate 322~ The ga-te 702 ou-tput i~ derived ~rom
the master timer 701 and is used to gate the break-in tone
onto the line 102~ A 0.5 sec gate is produced every 8 sec.
An output low from the gate 423 is gated through another
NAND gate 424 and thereafter is ga-ted through a two-input
6 ~d
NAND gate 425 together with a ~ Hz signal. The outp~t of
the gate 425 is coupled through an attenuator 426 and a
coupling capacitor 427 which is connected to the ACTIVE
terminal of the switch 203. Intrusion signals therefore
announce that privacy has been lost by means of a break-in
6 ~
"beep" tone comprising 0.5 sec of the ~ Hz tone which is
placed on the line 102 every 8 sec.
User privacy is ensured through ring coding which
uni~uely identi~ies a predetermined RID 100. Since the RID
100 responds only to its ring code, a ringer (not shown)
o~ a station 100 i~ therefore only actua-ted after the
corresponding RID 100 identifies its ringiny code.~\All of
the other stations on the M2S line will rernain silerYt. It
takes one complete ring cycle in order for the RID 100 to
decode the ringing signal. Responding to the proper code,
the RID 100 will go ACTIVE and allow the ringing voltage to
be applied across the ringer of the station. In the event
that a second station goes off hook while the line is ringing
the called party, the second station will receive a simulated
- 2~ -
ringing signal which is generated by the RID 100. Under
these condi.tions, the second s-~ation canno-t gain access to
the line 102.
Ring detection is initia-ted by the output signals
BAh RING and POL RING from the ring detector 221. Referring
now to Fig. 3, observe that the BAL RING signal is applied
to an input of a differentiator 306 and output therefrom to
a ring detector 307~ Specific circuit details may be seen
in Fig. 6.
A buffer 631 acts as a squaring circu:it 325 to con-
dition the BAL RING signal. The differentiator 306 Eollows
the buffer and comprises merely a coupling capacitor 618
and a discharge resistor 619. On the leading edge of the
BAL RING signal a pulse is derived from the differentiator
306 to reset a counter 621 via an OR gate 621'. After counting
for 25 ms or 16 cycles of the 640 Hz input clock, the counter
621 sets an input flip flop 620. A counter 622 is enabled
when the input flip-flop 620 is set. Because of the 25 ms
delay, the counter 620 acts as a lo~ pass filter to prevent
ringing frequencies greater than 40 Hz from being detected.
The counter 621 also operates as a high pass ilter such that
if a second output pulse from the differentiator 306 is
obtained within 75 ms (13.3 Hz), the counter 622 remains
enabled while a counter 623 is disabled and the counter 621
is reset.
Should it be that the BAL RING inpu-t signals to
the di~ferentiator 306 appear at a rate grea-ter than 13.3 Hz,
the counter 622 will set a ring flip-flop 624 150 ms after
the first such BAL RING sig,nal. Conversely, if the BAL RING
signal input is not present or appears at a rate less than
. 25 ~
13.3 Hz or appears at a ra-te greater than 40 ~Iz the coun-ter
623 is enabled and a~ter a Aelay of 150 ms the flip-flop 624
is reset. Consequently, the output from the flip-Elop 624
corresponds to the envelope of the input ringing voltage wi~h
a delay of 150 ms.
Turning next to Fig. 3, it will be observed that
the RING ou-tput of the detector 307 i.s employed -to enable
several circuits. One such circuit is a long ring detector
308 to which the RING signal i6 input. If the RING signal
is high for 4 sec, as established by the half period of the
0.25 H~ clock input to a NAND gate 704' a long ring flip-flop
703 in the detector 308 is se-t and the output therefrom, shown
as an output LR, goes high. The long ring following the
appropriate coded ring is thus used to place the RID 100
into one of its four operating modes. Ilhe flip-flop 733
is reset by a signal ON HOOK RESET which is coupled to the
input of an inverter 704, the output of which is connected
to the reset input of the flip-flop 703. This reset occurs
when the station 101 is ON HOOK and a ring timed out signal
RTU from a corresponding circuit 309 is present.
The LR output from the detector 308 is input to
a ring/command decoded circuit 310. A more detailed view of
the structure of the circuit 310 may be seen in Fig. 7.
It will be observed therein that a ring decoded flip-flop 705
is set by a RING DECO~ED pulse. The flip-flop 705 is reset
12 sec after -the last RING RESET pulse with the 12 sec being
logically derived from the master timer 318 and a NAND ga-te
705'. This allows the circuit 310 to remember being decoded
between successive rings. An output from the flip-flop 705
is gated through a three-input NAND gate 706 together wi-th
- 26 -
~tj~
~ LONG RING and RING input pul~es -to produce an output
COMMAND pulse. The COMMAND pulse occurs when RING DECO~ED,
LONG ~ING and RING are all high.
The COMMAND pulse from the circuit 310 is input
to a four stage command circuit 311. It will be seen in
Fig. 5 that the circuit 311 comprises a ripple counter 501
that initiates an appropriate action for the RID 100. In i-ts
initial state, the ~ID 100 is adapted to be connected across
the line 102 and unbalanced :Eor normal operation. This
condition is set up by the first co~nand from the circuit 311.
The CONNECT output signal from the circuit 311 enables the
switch 211 which supplies the station 101 with direct current
as previously discussed. In addition, the CONNECT signal
is applied, through an EXCLUSIVE OR gate 501', to an input of
a transistor driver 502 through a differentiating circuit 517.
The resulting current pulse through the driver 50~ energizes
a field winding of the relay 218 to switch the line terminating
circuit 202, shown in Fig. 4, to an unterminated line con-
dition as required when the RID 100 operates in its normal
mode.
With a second COMMAND pulse, the CONNECT signal is
removed from the connect switch 211, thus eliminating Local
loop current through the station 101. In this mode the RID
100 is remotely disconnected and inoperati~e.
With the third COMMAND pul5e, a BALANCE.signal is used
as input to a second driver 503 which operates a second field
winding of the relay 218. When so energized, the relay 218
dxops the termination 219 in the form of a res:istive and inductive
load across the line 102. Simultaneous with connectillg
the termination 21~ across the line, the switch contact 404
connects a low value resistor 462' in -the .ring detec-tor 221
between the tip and ring linesl 104 and 105 respectively.
~ 27 -
This increases the sensitivity of the balanced ring detector
221 such -that the RID 100 may be commanded to remove the
termination.
On the fourth command CAL TON:E, the circult 311
enables a NAND gate 505 which in turn enahles a reference
tone generator 323. A 1000 Hz reference tone the.re:Erom
is gated on by a transistor switch 599. The constan-t output
amplitude of a Eeedback circuit ormed by operational amp-
lifiers 597 and 598 is maintained by a light emitting diode
and light sensitive resistor element 596. It will be seen
in Figs. 4 and 5 that the output signal from the amplifier
598 is capacitively coupled to a resistor 431 where it is
available to be switched onto the l.ine 102 when the line is
terminated as described. Following receipt of the next
command, the RID is re-initialized, which is to say that it
is once again connected and unbalanced.
In order to respond appropriately to a ring c~de
appearing across ~he line 102, the RID 100 includes a ring
analyzer 312 that analyzes the RING output from the detector
307. It will be recalled that the RING output follows the
envelope of th0 ringing siynal with a delay of 150 ms and
in a format that can be readily compared with the code that
uniquely identifies a particular RID 100.
Figs~ 3 and 7 show that a 4 Hz output from -the
timer 318 is coupled to a long/short timer 320 to provide
a plurality of timed outputs that include .25, .75, and 1.25
sec~ each of which is applied to a predetermined input of
the analyze.r 312. The three timed outputs of the timer 320
are coupled to irst inputs of corresponding -two-input NAND
gates 708 that are shown in Fig. 7. A second input for the
gates 708' and 708" is the RING output pulse. The second
- 2~ ~
7~
input for the gate 708"' is a RING pu] se .
An output low of the firs-t gate 708' occurs .25 sec
after the start of the .RING pulse. This indlcates -th~t khe
RIN~ signal is at least as short (ALAS) and sets a long/short
flip-flop 707. If the RING signal is still present after
.75 sec, then the signal is -taken to be a long signal. The
output of the second ~ate 708" then goes low and resets the
flip-flop 707. This procedure con~inues until the RING code
is completed.
If a RING pulse is not present for 1.25 sec, then
the output o~ the third gate 708l"! generates a sLANK output
pul~e which indicates that the code is complete. When the
BLANK pulse is high, an exclusive OR gate 709 to which the
pulse is coupled acts as an inverter, Conversely, when
the ~L~NK pulse is low, the gate 709 acts merely ~s a trans-
m.ission gate.
The output from the gate 709 appears as ~ATA pulses
which are coupled to a code comparator 313 where the ringing
code is compared with the par-ticular code identifying the
RID 100. To assure that the input data identifying the length
of the rin~s are entered into the comparator 313 anly after
they have been obtained, a data cl'ock signal is required and
is derived from a NOR ga-te 712 0O25 sec past the end oE each
ringing burst in the ring cycle. Information as to the length
of the last ring i5 s~robed in at the inputs of shift regis-
ters 711 and 715.
The fourth input of the ring analyze.r 312 comprises
the dual inputs of the NOR gate 712 to which are fed the RING
signal and the .25 sec inverted output from the timer 320
Since the timer 320 is reset and begins to recoun-t on the
leading and trailiny edges of the ring envelope -the output
signal from the gate 712 consis-ts of -the high level that
- 29 -
~p~
occurs .25 sec after the trailing edge of the RING signal.
This output signal is instrumental in generating DATA CLOCK
pulses that are used to strobe the DATA output of the gate
709 into the code comparator 313, since -the ou-tput of khe
flip-flop 707 is valid only after the RING signal is low
which indicates that a ring burst is complete.
Because code length i5 a variable quantity, the
RID 100 is required to detect -the end of the code and to
5trobe into the code comparator 313 an appropria-te numher
of extra bits ~o make a total of four bits. These extra
bits must ~e in the opposite s-tate of the last cocle bit.
When the ring code is complete, as detected by the ou*put
signal BLANK going low from the third gate 70B"', the last
state of the flip-flop 707 is inverted through the gate 709
and the required extra bits with this opposite sign are put
onto the DATA output of the gate 709.
Note that the BLANK~ pulse gates the 640 Hz signal
into a DATA CI.OCK bit stream by means of a gate 724. When
the code comparator 313 has filled its registers, a DATA LATCH
output signal is returned to a data latch circuit 713 which
stops the flow of extra bits and then initiates a DAllA RESET
signal at the output of a NAND gate 714 a-t th~ beginnincJ of
the next ring buxst as determined by ring reset circui-try 720,721,
and 722, later described. ShiEt registers 711 and 715 are-then reset.
The register 711 keeps track that four shifts
have been made since its input is held high during each shift,
and generates a DATA LATCH output pulse. On the other hand,
the register 715 contains the actual ring code information.
The outputs of the register 715 are compared wi-th preset code
switches, shown in Fig. 7 as comprising four SPST switches 717
that operate as a store for decode da-ta.
_ 3() w
12f?Z7~
~utputs Ql ~ Q4 from the register 715 are connected to the
corresponding inputs of two-input exclusive OR gates 716. A
second input to each gate has connec-ted thereto a prese-t
code switch 717 as shown. In -this way, the outputs of the
register 715 are compared to the preset code swi-tches 717 by
means of the gates 716. When the codes are the same, a VERIFY
output from a NAND ga-te 718 goes low. When the code is
completed the DATA LATCH output from the register 711 goes
high and a ring decode signall RING DEC, from a NAND gate 719
goes low and ring decoded flip-flop 705 is set.
At the start of another ring signal on the line
102, the DATA RESET output of the gate 714 reinitiates the
comparator circuit 313 by resetting the shift registers 711
and 715. The comparator 313 is now set to decode the next
ring signal.
Referring ~ack now to the active analyzer 301,
it will be recalled that one set of input conditions therein
that cause the RID 100 to go ACTIVE and connect the station
101 to the line 102, requires that the ringing signal be
decoded, the station 101 be ON HOOK, and that the line be
BUSY. Since these con~i-tions are now met, with khe ringing
signal fully decoded, ~he ON LINE output from the gate 607
is coupled to the base input of the amplifier 412 which
energizes the field winding 2D4' setting the switch 203 to
the ACTIVE position.
When a called station is on the same party line
as the calliny station, this is referred -to herein as placing
a revertive call. To initiate such a call, -the salling
station 101 goes OFF HOOK and if the line is BUSY -the
~ 3:1 ~
~S~7~9~
RID 100 becomes active. The calliny number is -then dialed
which is followed by a hook flash a-t the calling station.
The RID 100 recognizes -these events as ~ revertive call
which causes the RID to go ACTI~E. [n -this way,
the station 101 leaves the line without going ON HOOK in order
to free the line ~o permit rinying the called station. When
the called station responds by going OFE~ HOOK, the line 102
then goes BUSY. This condition is sensed by the RID 100,
causing it to go ACTIVE and thus completing the call.
`A rever~ive sense circuit 314 is shown generally
in block diagram form in Fig. 3 and in schema-tic diagram
form in Fig. 6. Referring now to these figures, it will be
recalled that the conditions to make a revertive call require
that the calling station 101 be OFF HOOK, ACTIVE, not in
a break-in mode and that the station has genera-ted a hook
flash. These conditions set a revertive flip flop 625
producing a low output, REV that is input to a NOR gate 632.
A second input to the gate 632 comprises the BUSY level ou-tput
from the fl.ip-flop 604. The resulting output ~rom the
gate 632 is applied to the flip~flop 606 to produce the
REV RESET output therefrom which is high when the line 102
is BUSY. The RID 100 unit will thus remain ACTIVE.
The REV RESET output of the flip-flop 606 remembers
that the RID 100 is in the revertive mode. This output is
then applied to one input of the ga-te 605 toge~her with the
_ _
BUSY pulse producing a BUSY-~REV~RES ou-tput therefrom that is
processed by the logic circuitry of the analyzer 301 to
produce an ON LINE output at the gate 607. The RID 100
therefore goes ACTIVE when the Eield winding 204' is energized
by the amplifier 412. The station 101 is -thus placed back
- 32 -
t7~
on the line 102 when the called par-ty answers.
Should ringing be applied to the line 102 during
a revertive call and should the callecl party fail to yo off
hook, a ring exterminator circuit 32~ of Fig. 3, which is
detailed in Fig. 7, will cause a simulated off hook condition
and terminatesthe ringing voltage from -the central office
103. When the station 101 goes ACTIVE to place a call, an
output RTU from a flip-flop 723 is high and an exterminator flip-
flop 727 is set enabling a NOR gate 728. The OFF HOOK level
applied to the NOR gate 728 resets a decade counter 729.
When the station 101 goes on hook, the counter 729 is
clocked up by a 2 Hz clock input. After 2.5 sec, a counter
output BAL ON is applied through an exclusive OR gate 501t' to
the driver 503 to actuate the relay 218 and connect the ter~
mination 219 across the line 102. rrhe central office 103
will ~ense the line current flowing through the termination
219 and removes ringing voltage. After 1.5 sec, the relay
218 will similarly again be actuated with the signal BAL OFF
applied through the exclusive OR gate 501' to the driver 502,
thus disconnecting the balanced termination 219 from the line
and restoring the RID 100 to its normal operational stateO
A ring reset circuit 315 is responsive to the
ring output from the detector 307 and produces a narrow pulse
for each transition of the RING signal. Looking now to
Fig. 7, it will be seen that the RING signal is applied
directly to one input of an exclusive OR gate 720. The same
signal with its txansition time lengthened by capaci-tor 730
is inverted by an inverter 721 and applied to the second input
of the gate 720. The outpu~ from the ga~e is applied as a
reset ~ignal for a divider circuit 316. The signal is also
-- 33 --
inverted by an inverter 722 to produce a RING RESET signal
used to reset the timer 318. ~n this way, the reset circuit
315 is used to time the length of a ring signal, or spaces
therebetween, in the ring detector 307.
An input to the divider 316 is provided by a
clock 317 which is shown in Fig. 7 as compxising a pair of
serially connec-ted NAND gates 731 having a feedback circuit
interconnec~ing the input and output thereof. The clock 317
runs at a frequency of 640 H2 and is divided by 80 -to provide
an 8 Hz output ~o the master timer 318.
The ring -timed out circuit 309 is rese-t with each
RING ~ESET pulse which is applied to a reset input of the
flip-~lop 723. The flip-flop 723 is set 12 sec after the
last ring pulse producing the output RTU and indica-tes that
ringing has stopped.
The RTU output is connected to the inverter 627,
the output of which is coupled to the input of a NOR gate 628
toget~er with an OFF HOOK signal to produce an ON HOOK RESET
output signal. This latter signal is input to -the inverter 704
which produces an output in response thereto to reset the
flip-flop 703 and the flip-flop 608 thereby se-tting the RID 100
into an ACTIVE state and disconnec~ing -the station 101 and
its ringer (not shown) from the line 102.
A convenient feature of -the RID 100 provides an
audible indication when the line 102, which previously was
BUS~, becomes available. Thus, lf the station 101 goes
O~F HOOK when the line is BUSY and then returns to the ON HOOK
mode without breaking into the communication path, the ringer
of the stakion 101 will be energized -to produce a single
audible ding when the l:ine becomes available. A dinger
- 3~ -
control circuit 319 ~enerates an output DING signal au-tomatically
which is input to a voltage inverter 321 and output therefrom
a~ a ding voltage that is connected to -the ringer of the
station 101.
When the line 102 is BUSY and the station yoes
OFF HOOK, corresponding signal inputs to a NAND gate 516
produce an output that sets a flip-flop 508. If the station
101 then goes ON HOOK and the line la-ter becomes available or
BUSY a timer, shown as a shift register 509, is started.
The register 509 produces an output that sets a flip-flop 510
which turns on a transistor switch 511. The switch applies
an operating voltage to a transistor oscillator 512 that
generates an output in the form of pulses of about 300 volts
peak. These pulses are coupled through a step-up trans~ormer
513 and are rectified by a diode 514 to produce a DC potential
that charges up a capacitor (not shown) in the station 101.
It will be noted that the charging voltage i5 in fact connected
to the ACTIVE c.ontact of t~e switch 203 and i5 therefore
applied to the station 101 only when the station is disconnected
from the line 102.
Following a predetermined period of time set by
the operation of the shift register 509, the DING output
signal is produced which resets the flip-flop 510 and is
capacitively coupled to the base of a transistor swi-tch 515.
The switch closes, shorting out the station 101 and discharging
the station capacitor to momentarily energize the ringer.
A remote disconnect feature of the RID 100 is pro-
vided in the aforedescribed four stage command circuit 311.
By means thereof each station 101 can be remo-tely disconnected
from service to allow a remote functional tes-t of any
RID unit and to test the status of the line 102 from the
- 35 -
~2~ 4~
central office 103 to each station 101. To operate the remote
disconnect feature, the ring code that uniquely identifies
a predetermined RIV 100 is generated at -the central office
and, following the first ring cycle, an extra long riny of
greater than 4 seconds duration is applied to the line. Upon
receipt of the extra long ring, the connect switch 211 removes
local loop current, preventing the relay 204 from closing
and thus removing the automatic number identifica-tion ~ANI)
network 220, the busy tone to line interface, the on/off
hook detection circuitry and the ringer (not shown). The re~
maining circuitry of the RID 100 remains active while awaiting
a reconnect or other test commands.
It was previously de~cribed tha-t when the RING
signal is high for 4 seconds the flip-flop 703 is set and the
long ring output LR therefrom goes hiyh. The signal LR is
then gated through the gate 706 with the RING DECODED and
RING signals to produce the COMMAND signal that is input to
the command circuit 311. The ripple counter 501 is enabled
thereby and a BALANCE output signal is produced which triggers
the driver 503. Actuation of the relay 218 results and the
line 10~ is switched across the termination 219. Concurren-tly,
sensitivity of the ring detector 221 is increased.
It will be observed that when the RID 100 is in khe
CAL TONE mode the counter 501 will reset itself to normal
operation at the onset of the next long ring signal. Alter-
nately, the operation of a switch 595 will set the RID 100
into its normal CONNECT state.
Following the foregoing disconnect procedure,
an output signal CONNECT ~rom the counter 501 will go low
causing a disconnect verification tone of 640 Hz interrupted
- 36 -
2~
at 2 ~z to be passed through a NAND gate 499. The duration
of this signal is typica]ly 6 sec as determined by RTU going
high and gating off a NAND yate 498. The veri~ication tone
is capacitively coupled by the capacitor 427 onto the line 102.
The tone level is set by the resistor 426. This indicates
to the central office 103 that a disconnect has occurred~
Since this tone can be heard only if the station 101 is dis~
connected, personnel at the central office are assured that
the remote disconnect has in fact taken place.
As the voltage sensor 201 and other RID circuitry
have a low power consumption, the RID 100 remains energized
during the disconnec-t period. Accordingly, a remote connect
operation may be performed by repeating the disconnect
sequence, i.e., generating the ringing code followed by a
long ring. In this instance, the counter 501 produces the
CONN~CT output which reapplies current to the connect switch
211 and restores the RID to normal operation. A similar con-
nect verification to~e of 6 sec duration, 640 Hz but at an
interruption rate of 1 Hz occurs when a remote connect is
effected and CONNECT goes high.
The four party automatic number identifica-tion (ANI~
circuitry 220 shown in Fig. 2 allows the central office 103
to determine automatically which subscriber is ACTIVE. This
determination is required for example in billing long distance
tolls. To effect an automatic number identification the
central office 103 connects together both the tip and ring
conductors of the line 102. The central office then transmits
a bipolar pulse typically 100 ms in length and of -~50 volts
typically in amplitude with respect to metallic ground. Each
RID unit will pass the simplexed current to ground in a
- 37
manner dependent upon the electrical component connecting
tip and ring conductors to ground. :[n this way, the current
developed during each polarity swiny of the bipolar ~NI pulse
will identify one of four possible subscribers being ACTIVF~.
The detailed embodiment of the ANI function is
shown in Fig. 4. At the time an automatic number identifica~
tion is performed the sta-tion 101 is oEf hook and the RID 100
is ACTIVE. The OFF ~IOOK signal is applied to one input
of a three input N~ND gate 480. ~t the time the tip and
ring conductors 104 and 105, respectively, are connected
together, at the central o~fice, the voltage difference be-
tween them is reduced typically to less than 1 volt. The
reduction of this voltage turns off a PMOS FET (P channel
metal oxide semiconductor field effect t.ransistor) device
481 producing a positive voltage level output that is applied
to the second input of the NAND gate 480. Due to the con-
nection togethe.r of the tip and ring conductors at the
central off.ice 103, loop current is prevented during the
automatic number ldentification. The absence o~ loop
current is detected by the current sensor 206 and a positive
voltage is applied to the third input of -the NAND gate 480.
The gate 480 is thus enabled, turning on a transis-tor 482
which biases on light emitting diodes 483' and 484' in
photoresistors 483" and 484" thus reducing their resis-tances
from typically several megohms to several tens of ohms. The
back to back zener diodes 485 and 4~5'.shunt ringing current
around the photoresistors 483" and 4B4".
Wit~ the photoresistors 483" and 484" act.ivated,
the ANI circuit elements 486 and 488 are c.onnectable -to pass
bipolar current during the ANI pulse subject to the setting
-- 38 -
~z~p~
of a four pole single throw switch 487. Each of four RID
units are uniquely identiEied by the closure of one switch
at the time the RID unit is installed. With the switch 487 (1)
closed there is no connection to me-tallic ground and no cur-
rent will flow due to the bipolar pulse. Closure of the
switch 487 (2~ c~nnects a conducting strap 488' to ground
thus allowing current to flow on each half of the bipolar
ANI pulse. The switch 487 (3) connects a diode 488" to
ground such that current will flow to ground only during the
positive portion of the bipolar pulse. Switch position 487 (4)
connects a diode 488"' to ground such that current flows only
during the negative portion of the bipolar ANI pulse. Each
current condition is detected at the central office 103,
thereby identifying which station is ACTIVE and connec~ed
to the line 102.
It is seen from -this description that the ANI
circuit 220 has the unique feature of connectin~ the ANI
circuit elements 486 and 488, already known in the art, to
the line 102 only when an automatic nu~er identification is
being made~ This feature thus allows the use of four
party ANI withou~ significant insertion loss or reduced
longitudinal balance.
Having regard to the description and illustrations
of the embodiment of the present invention, it will be apparent
to those skilled in the art that variations thereof within
the scope of the invention are readily feasible. Accordingly,
the disclosed and illustrated embodiment herein should be
considered as exemplary rather than restrictive o~ the in-
vention which is defined in the accompanying claims.
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