CA1202740A - Rural interface device - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Individual line service features on a multi-party two-wire telephone network are provided by apparatus that interfaces individual subscriber stations with the network.
The apparatus includes a current sensor for detecting on hook and off hook modes of its station and a voltage sensor for determining the availability of the network to establish a bidirectional communication path with the station. Control logic circuitry responsive to the sensors sets the apparatus to an active state when the station is off hook and the network is available and to a nonactive state when the network is not available. Signal generators that are operably responsive to the control logic produce tone signals that indicate such states. A relay operated by the control logic circuitry in response to the status of the apparatus then connects its station across the network in the active state and disconnects the station in the nonactive state. Ringing signals on the network are detected and within one complete ring cycle are decoded. This ensures privacy since each apparatus is respon-sive only to its own code. Ringing voltage is then applied across the ringer of the called station. A break-in feature permits intruding on a conversation in case of an emergency, but surreptitious eavesdropping is prevented by an intrusion signal that is placed on the network for all stations to hear.
Revertive calls between calling and called stations on the same network are performed by dialling the required number followed by a hook flash at the calling station which temporarily places the calling station in the nonactive state to free the network for ringing the called station until it goes off hook. Should the network be determined as being busy, the station is returned to its on hook mode and a previously charged capacitor is dis-charged through its ringer to provide an audible alarm when the network later becomes available for use.

Description

'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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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, 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 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 z~

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 - :L4 -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, z~

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 ~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 ~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 '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 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 ~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 ~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 ~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 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 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 ~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 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 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 ~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.

- 39 ~

Claims (65)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED
AS FOLLOWS:

1. Apparatus for interfacing a subscriber station with a two-wire telephone network having predetermined operating conditions, comprising:
means for detecting an on hook and off hook mode of said station irrespective of the network conditions;
means for determining the availability of the network to establish a bidirectional communication path with the station irrespective of the station mode;
conditioning means responsive jointly to the detected mode of said station and the determined availability of the network for selectively conditioning the apparatus to an active or nonactive state; and means responsive to the status of said apparatus for connecting the station across the network in the active state and for disconnecting the station therefrom in the nonactive state.

2. Apparatus as claimed in Claim 1 wherein the conditioning means are adapted to set the apparatus to the active state when the station is off hook and the net-work is available and to the nonactive state when the net-work is not available.

3. Apparatus as claimed in Claim 2, further comprising:
means for detecting network ringing code signals;
means for decoding a predetermined network ringing code signal that uniquely identifies said station; and means responsive jointly to the decoded ringing code signal and the on hook mode of the station for setting the apparatus to the active state.

4. Apparatus as claimed in Claim 3, further comprising:
tone generator means responsive jointly to the conditioned state of the apparatus and to predetermined ones of detected network signals for generating predetermined tone signals and selectively injecting the tone signals into at least one of, the network and said station.

5. Apparatus as claimed in Claim 4 wherein the tone generator means are adapted to generate a busy tone which is applied to the station when the network is not available and the station is in the off hook mode.

6. Apparatus as claimed in Claim 5, further comprising:
revertive circuit means for manually resetting said apparatus from the active state to the nonactive state; and circuit means responsive jointly to a predeter-mined network signal and the nonactive state of the apparatus for restoring the apparatus to the active state.

7. Apparatus as claimed in Claim 6 wherein the revertive circuit means include means actively responsive to a hook flash signal for resetting the apparatus from the active to the nonactive state after the station originates a call to a called station on said network and a central office terminal outputs a busy tone on the network in response thereto, means for maintaining the nonactive state until the called station goes off hook, and means for effecting the active state when the called station goes off hook.

8. Apparatus as claimed in Claim 7 wherein the means for determining the availability of the network to establish a bidirectional communication path include a voltage sensor responsive to a network voltage fluctuation produced by the off hook mode of the called station, said sensor being adapted to operate with either polarity of a talking battery connected across the network and said sensor generating a signal in response to the fluctuation that enables said circuit means for restoring the apparatus to its active state and establishing the communication path between the calling and called stations.

9. Apparatus as claimed in Claim 8, further comprising break-in circuit means for manually resetting said apparatus from the nonactive to the active state in response to a hook flash signal when the revertive circuit means are inactive and the network is not available to establish the communication path with the calling station.

10. Apparatus as claimed in Claim 9 wherein said tone generator means include means responsive jointly to the nonactive state of the apparatus, the inactive revertive.
circuit means and the hook flash signal, for generating intrusion signals and injecting same into at least said network during an interval when the apparatus has been reset from the nonactive to the active state and the network is not available for establishing said bidirectional communi-cation path.

11. Apparatus as claimed in Claim 10, further comprising:
circuit means responsive to a predetermined network signal originating at the central office terminal for selectively disconnecting the apparatus from and reconnecting said apparatus to the station; and means responsive jointly to the central office signal and the status of the apparatus for enabling the -tone generator means to apply a verification tone to the network indicating that the subscriber station has been disconnected or reconnected, respectively.

12. Apparatus as claimed in Claim 11, further comprising:
circuit means responsive to said means for decoding the ringing code signal when said station is not identified and coacting with the tone generator means when the station is in the off hook mode to enable the generator means for generating said busy tone.

13. Apparatus as claimed in Claim 12, further comprising:
means providing a source of direct current for charging a ringer capacitor of said station;
control means responsive jointly to the nonactive state of the apparatus, the inactive revertive circuit means, the off hook mode of the station and the network not being available, for enabling the charging means to charge said capacitor for a predetermined interval; and switch means adapted to discharge the capacitor through its ringer to produce an audible alarm in response to a gate signal generated after the station is returned to its on hook mode and when the network becomes available to establish said communication path.

14. Apparatus for interfacing a subscriber station with a two-wire telephone network having predetermined operating conditions, including means responsive to the status of said apparatus for connecting the station across the network in an active state and for disconnecting the sta-tion therefrom in a nonactive state, the apparatus comprising:
means for detecting an on hook and off hook mode of said station irrespective of the network conditions;

means for determining the availability of the network to establish a bidirectional communication path with the station irrespective of the station mode; and conditioning means responsive jointly to the de-tected mode of said station and the determined availability of the network for selectively conditioning the apparatus to the active or nonactive state.

15. Apparatus as claimed in Claim 13 wherein the means for maintaining the nonactive state maintains said state while the central office terminal outputs a ringing signal corresponding to the called station, the tone genera-tor means further comprising circuit means for generating a ring-back tone in step with the ringing signal and feeding said ring-back tone to the calling station until the called station goes off hook.

16. Apparatus as claimed in Claim 15, further comprising:
a termination relay having a first switch contact connected to one conductor of the network;
a line termination connected between a second con-tact of the switch and the other conductor of the network; and circuit means responsive to the station going on hook before the revertive call has been completed for tuating the relay and connecting the termination momentarily across the network to simulate connection of the called station and thereby signalling the central office terminal to discontinue the ringing signal.

17. Apparatus as claimed in Claim 16 wherein said means for connecting and disconnecting the station includes an activate relay having a first switch contact connected to a tip conductor of the network and a common contact connected to a cor.respondincJ conduct(~r o:E the station fox selective:ly enga~ing the f:irst contclct when t,he ap~cl:raL:u~ :i,s a~ti.ve, ~,he~
sta-tion be~ng ~eri.ally connected w:i-th ~he sw:i,tch conLact,s -to isola~e a ringer of said stcltiorl :Erom rin~incJ si~n~L1s on the ne-twork when the apparatuS is nonact:i.ve and -the swi.tch con--tacts are disengaged.

18. Apparatus as claimed in Claim 17 wherein the means for detecting the on hook and off hook modes of the sta-tion comprise a current sensor serially connected with a ring concductor of the ne-twork and a corresponcdincJ collductor of the s-tation, said curren-t sensor beiny adap-ted to detect a flow of ope.ra-ting current in -the sta-tion.

19~ Apparatus as claimed in Claim 18 whe:rein the ac-tivate relay lncludes a second switch con-tact for selec~
tively enya.ging -the cornmon Gcntac-t when the appara-~us is non--active, the apparatus further comprising:
a source of opera-ting current for the s-ta-tioni a connect switch serially connected with the source of current; and a diode serially connected with the connect sw:itch and the second contact of the activate relay, -the diode being poled to supply current through the .second contac-t when the station goes off hook and the apparatus i~ nonactive.

20. Apparatus as claimed in Claim 19, furt,her coln-prising an automa-tic number iden-tification ci.:rcuit corlrlec-.
table acros.s the network and to a ground return, -the iden--tification circuit being responsive to a predeterm:ined bi polar'pulse ou-tput :Erom the central office -terminal and in-cludin~ means fo:r predetermining current flow between the net-work and the ground return for identlyin() incl:ivicLua,l ~Icti.ve vnes of a plurality oE the apparatus when cor~espol~cl::i.rlcl one~s of the s-ta-tions are o:Ef hook.

~ 45-

21. Apparatus as claimed in Claim 20, further comprising circuit means operably responsive to a pre-determined minimum voltage across the ring and tip conductors of the network for disabling said identification circuit and disconnecting same from the ne~work when the bipolar pulse is not being sent.

22. Apparatus as claimed in Claim 21 wherein the voltage sensor is connected across the tip and ring conductors of the network and comprises:
a rechargeable battery having its negative elect-rode connected to the ring conductor;
a pair of zener diodes serially connected anode to anode with the cathode of the first diode connected to the positive terminal of the battery;
a current limiting resistor having one end connec-ted to the cathode of the second diode; and an optical coupler including a diode bridge ser-ially connected with the free end of the resistor and the tip conductor.

23. Apparatus as claimed in Claim 22 wherein:
the diode bridge permits the optical coupler to be activated ~or either normal network polarity with the tip conductor positive with respect to the ring conductor, or reversed polarity with the tip conductor negative with re-spect to the ring conductor, the sum of the battery voltage and the reverse breakdown voltage of the second diode,setting a voltage threshold to determine when the network is busy, after the station is connected thereacross ~Inder normal pol~r-ity, and the difference between the reverse breakdown voltage, of the first diode and the battery volta~e settiny a voltage threshold to determine when the network is busy after the station is conn~cted thereacross under reverse polarity.

24. Apparatus as claimed in Claim 23 wherein the voltage threshold in each case differs from a voltage across the network by the sums of the forward bias voltages of three diodes in the bridge and wherein a battery charging current is produced only when the network voltage exceeds the threshold.

25. Apparatus as claimed in Claim 24 wherein said network includes a ground return in respect of which the tip and ring conductors are unbalanced, the means for detecting network ringing code signals comprising:
a first optical coupler capacitively coupled to the tip conductor and connected to said ring conductor for detecting balanced ringing code signals and producing in re-sponse thereto a corresponding output signal; and switch means connected between said conductors and adapted to switch the conductors alternately to a line balance circuit and a second optical coupler, the second optical coupler being capacitively coupled to the switch means and connected to the ground return for detecting polar-ity sensitive ringing code signals occurring between predeter-mined ones of said conductors and the ground return and pro-ducing in response thereto a corresponding output signal, said line balance circuit having an impedance substantially the same as the second optical coupler and being connected between the switch means and the ground return to maintain the network in a balanced state.

26. Apparatus as claimed in Claim 25 wherein the means for decoding the ringing code signal that uniquely identifies said station comprise:
ring detector means for generating a ring output signal that follows the envelope of said ringing code signals in a comparable format and with a predetermined delay in response to the outputs of said means for detecting network ringing code signals;
clock means;
timer means coupled to the clock means and respon-sive to clock pulses therefrom for generating a plurality of predetermined timed outputs;
ring analyzer means coupled to the ring detector means, the timer means, and said clock means for generating predetermined control pulses and a stream of data pulses corresponding to short and long ringing code signals and completion of the ringing code signal; and code comparator means, including a plurality of preset code switches, responsive to the data pulses and said control pulses for generating a ring decoded output signal corresponding to the code signal that uniquely identifies said station.

27. Apparatus as claimed in Claim 26 wherein the means for selectively disconnecting the apparatus from and recon-necting the apparatus to the station and the means for enabling the tone generator means include the termination relay, said line termination, the active relay and said connect switch, and comprise:
a long ring detector responsive to the ring output signal and a predetermined timed output of said timer means for generating a long ring pulse when the ring output signal is present over at least a half period of the timed output;
a ring/command decoded circuit coupled to the long ring detector, said code comparator means and the ring de-tector means and responsive to the output signals therefrom for generating command pulses;

a ripple counter operably responsive to the command pulses for generating sequentially individual ones of control signals at a plurality of corresponding outputs;
a first driver coupled to a first control signal output for actuating a first field winding of the termination relay and disconnecting the termination from the network for the duration of the first control signal;
circuit means connecting the first control signal output to the connect switch and enabling same for connecting the source of operating current to said station for the duration of the first control signal;
circuit means connecting the first control signal output to said tone generator means during the interval of the second control signal, a second output of the counter being disconnected and the first driver and the connect switch being rendered inoperative by the first control signal going low for the duration of the second control signal whereby said station is remotely disconnected and inoperative, the tone generator means being operably responsive to the low control signal for generating a timed disconnect verification tone and coupling the verification tone to the network;
a second driver coupled to a third control signal output for actuating a second field winding of the termination relay and connecting the termination across the network for the duration of the third control signal; and circuit means coupling a fourth control signal out-put to a reference tone generator, the generator being enabled by the fourth control signal to generate a timed reference tone accurate in frequency and amplitude that is coupled to said termination where the tone is available to be switched across the network when said network is terminated.

28. Apparatus as claimed in Claim 27 which is adapted to interface any type of standard subscriber station equipment to the two-wire telephone network, including rotary, Touch-Tone (Trade Mark) or decorator telephone sets.

29. Apparatus for decoding a ringing code signal that uniquely identifies a subscriber station interfacing a telephone network, including means for sensing bursts of network ringing code signals, comprising:
ring detector means for generating a ring output signal that follows the envelope of said ringing code signals in a comparable format and with a predetermined delay in response to an output of the sensing means;
timer means, including clock means, for generating a plurality of predetermined timed outputs;
ring analyzer means coupled to the ring detector means, and the timer means for generating predetermined control pulses and a stream of data pulses corresponding to short and long ringing code signals in each burst and com-pletion of each burst of ringing code signals; and code comparator means, including storage means containing decode data, responsive to the data pulses and said control pulses for comparing the data pulses with the stored data and generating a ring decoded output signal corres-ponding to the code signal that uniquely identifies said station.

30. A method for decoding a ringing code signal that uniquely identifies a subscriber station interfacing a telephone network and including means for sensing bursts of network ringing code signals, the method comprising the steps of:
generating a ring output signal that follows the envelope of said ringing code signals in a comparable format and with a predetermined delay in response to an output of the sensing means;
generating a stream of data pulses corresponding to short and long ringing code signals in each burst and the completion of each burst of ringing code signals in the delayed ring output signal; and comparing the data pulses in each burst of ringing code signals with stored decode data and identifying said station when the data pulses and decode data correspond.

31. Apparatus for identifying a plurality of sub-scriber stations of which individual ones are off hook and operatively interface a two-wire telephone network having a ground return and tip and ring conductors that are connected together at a central office terminal during the identifica-tion procedure, the apparatus comprising:
circuit means having an intermediate tap and in-cluding switching means adapted to operatively enable said circuit means only when the interfacing station is off hook, for conducting current between either conductor and the ground return via the tap in response to a predetermined bipolar pulse sent out by the central office terminal;
shunt means connected between each of the tip and ring conductors and the tap to shunt a network ringing current around the circuit means;
a plurality of circuit elements for selectively predetermining the current flow in the circuit means; and switch means serially connecting one of said elements between the tap and ground return to establish a predetermined measurable current flow at said terminal which identifies the interfacing station.

32. A method for identifying a plurality of sub-scriber stations of which individual ones are off hook and operatively interface a two-wire telephone network having a ground return and tip and ring conductors that are connected together at a central office terminal during the identifica-tion procedure, the method comprising the steps of:
sending out a predetermined bipolar pulse from the terminal onto the network to induce a current flow in circuit means,including switching means adapted to operatively enable said circuit means only when the interfacing station is off hook, connected between the conductors at the interfacing subscriber station, the current being conducted between either conductor and an intermediate tap in said means;
connecting a current determining element serially with the intermediate tap and the ground return; and measuring at the terminal a current predetermined by the saidelement to identify the interfacing station.

33. A station set interface circuit for connection to tip and ring leads of a multi-party telephone line at a location remote from a standard subscriber's set comprising:
(a) pair of first terminal means for connection to the multi-party telephone line;
(b) second pair of terminal means for connection to a pair of leads which are connected to the standard subscriber's set;
(c) means connected to the first and second terminal means for detecting the idle or busy status of the multi-party line and in response to a request for service from the subscriber's set for automatically connecting a talking path between the first and second terminal means in the event the status of said line is idle and for causing an open circuit to be maintained between the first and second terminal means in the event the status of the multi-party line is busy; and (d) means connected to the second terminal means for applying a busy signal to the second terminal means for transmission to the subscriber's set upon the subscriber's set going offhook when the detecting means detects said busy status of the telephone line.

34. A station set interface circuit as defined in Claim 33, in which the means for detecting is comprised of a line voltage detector connected between the first pair of terminal means having voltage threshold means for indicating the line is idle when the detected voltage is above a pre-determined threshold and busy when the detected voltage is below said threshold.

35. A station set interface circuit as defined in Claim 34 in which the busy signal is a busy tone, and further including interrupt means for terminating the busy tone and for automatically providing said talking path between the first and second terminal means upon reception of a predetermined signal at the second terminal means from the station set.

36. A station set interface circuit comprising:
(a) pair of first terminal means for connection to a multi-party telephone line;
(b) second pair of terminal means for connection to a subscriber's set;
(c) means connected to the first and second terminal means for detecting the idle or busy status of the multi-party line and in response to a request for service from the subscriber's set for automatically con-necting a talking path between the first and second terminal means in the event the status of said line is idle and for causing an open circuit to be maintained between the first and second terminal means in the event the status of the multi-party line is busy; and (d) interrupt means for automatically providing said talking path between the first and second terminal means upon reception of a predetermined signal at the second terminal means from the station set.

37. A station set interface circuit as defined in Claim 33 further including a status circuit having an active and an inactive state, means for sensing a request for service at the first or second set of terminals and means for automatically connecting a talking path between the first and second set of terminals upon reception of the request for service only in the event the status circuit is in its active state and the telephone line is idle.

38. A station set interface circuit as defined in Claim 37, in which the means for sensing a request for service is comprised of means for detecting an off-hook condition from the subscriber's set at the pair of second terminal means.

39. A station set interface circuit as defined in Claim 37 in which the means for sensing a request for service is comprised of means for detecting ringing signals at the first terminal means, for determining if the ringing is designative of said subscriber's set, and for auto-matically connecting said talking path only in the event said ringing signals have been determined as being designative of said subscriber's set.

40. A station set interface circuit as defined in Claim 37 in which the means for sensing a request for service is comprised of a ringing voltage detector connected to said first terminal means for detecting ringing signals, means connected to the ringing voltage detector for decoding bursts of the ringing signals, for matching the decoded bursts with a predetermined code, and for enabling connection of the conductive path only in the event the decoded bursts match the predetermined code, and upon enabling by the decoding means.

41. A station set interface circuit as defined in Claim 33, further including status memory means for storing an indication of the active or inactive states of the inter-face circuit relating to responsiveness of the interface circuit to requests for service, means for sensing a disconnect signal from a central office at the first terminal means, means for determining the address of the disconnect signal and for enabling the status memory means to change the active or inactive state of the interface circuit in response to the sensing of both the disconnect signal and said address, the latter being designative of said inter-face circuit, and means for prohibiting the connection of a talking path between the first and second terminal means in the event the status of the interface circuit is inactive.

42. A station set interface circuit as defined in Claim 33, 37 or 41 further including means after establish-ment of the talking path between the first and second terminal means and dialing of the digits by said subscriber's set designative of another subscriber's set connected to said multi-party telephone line, for sensing a hookswitch flash at said second terminal means and in response for temporarily open-circuiting said talking path and applying ringing tone to said second terminal means, and for reclosing said talking path upon said another subscriber going off-hook.

43. A station set interface circuit as defined in Claim 38 further including means connected to the second terminal means for applying a busy tone to the second terminal means for transmission to the subscriber's set upon the subscriber's set going off-hook while the detecting means detects a busy status of the telephone line.

44. A station set interface circuit as defined in Claim 43 further including status memory means, means for sensing a disconnect signal from a central office at the first terminal means, means for determining the address of the disconnect signal and for enabling the status memory means to change the active or inactive state of the interface circuit relating to the responsiveness of the interface circuit to requests for service in response to the sensing of both the disconnect signal and said address, the latter designative of said interface circuit, and means for prohibiting the connection of a talking path between the first and second terminal means in the event the status of the interface circuit is inactive.

45. A station set interface circuit as defined in Claim 38, further including a ringing voltage detector connected to said first terminal means for detecting ringing signals, means connected to the ringing voltage detector for decoding bursts of the ringing signals, for matching the decoded bursts with a predetermined code, and for enabling connection of said talking path in the event the decoded bursts match the predetermined code, a status circuit having an active and an inactive state, and means for connection of said talking path only in the event the status circuit is in its active state and upon enabling by the decoding means.

46. A station set interface circuit as defined in Claim 45 in which the means for sensing a request for service is comprised of means for detecting an off-hook condition from the subscriber's set at the pair of second terminal means.

47. A circuit for disconnecting of a station set which is connected to a subscriber's line from a remote location, comprising means for receiving a disconnect signal of a predetermined nature from the subscriber's line, means for translating the disconnect signal into a status set signal, means for receiving a second signal designative of the station set from the subscriber's line, and for translating the second signal into an enabling signal, and means for causing disconnection of the station set from the subscriber's line upon reception thereby of both the enabling signal and the status set signal.

48. A circuit for disconnection of a station set connected to a subscriber's line from a remote location as defined in Claim 47 in which the means for translating the second signal is comprised of means for translating a ringing signal applied to the subscriber's line for ringing the station set.

49. A circuit for disconnection of a station set connected to a subscriber's line from a remote location as defined in Claims 47 or 48 in which the means for trans-lating the disconnect signal is comprised of means for translating a ring burst of a predetermined minimum length of time.

50. A circuit for disconnection of a station set connected to a subscriber's line from a remote location as defined in Claim 47 in which the means for translating the disconnect signal is comprised of means for translating a ring burst of a predetermined length of time, and the means for translating the second signal is comprised of means for decoding a ringing signal designative of the station set to which the ringing signal is directed and in response to the decoding, for generating said enabling signal.

51. A station set interface circuit as defined in Claim 33, further comprising interrupt means for auto-matically providing said talking path between the first and second terminal means upon reception of a predetermined signal at the second terminal means from the station set, and for removing said busy signal while said talking path is provided.

52. A station set interface circuit as defined in Claim 33, further including means for detecting ringing signals at the first terminal means and for determining if the ringing is designative of said subscriber's set, and for automatically connecting said talking path in the event said ringing signals have been determined as being designative of said subscriber's set to allow further ringing signals to be applied to said subscriber's set.

53. A station set interface circuit as defined in Claim 34, further including means for detecting ringing signals at the first terminal means and for determining if the ringing is designative of said subscriber's set, and for automatically connecting said talking path in the event said ringing signals have been determined as being designative of said subscriber's set to allow further ringing signals to be applied to said subscriber's set.

54. A station set interface circuit as defined in Claim 34, further including a ringing voltage detector connected to said first terminal means for detecting ringing signals, and means connected to the ringing voltage detector for decoding bursts of the ringing signals, for matching the decoded bursts with a predetermined code, and for enabling connection of the talking path in the event the decoded bursts match the predetermined code.

55. A station set interface circuit as defined in Claim 35, further including a ringing voltage detector connected to said first terminal means for detecting ringing signals, and means connected to the ringing voltage detector for decoding bursts of the ringing signals, for matching the decoded bursts with a predetermined code, and for enabling connection of the talking path in the event the decoded bursts match the predetermined code.

56. A station set interface circuit as defined in Claim 33, further including a ringing voltage detector connected to said first terminal means for detecting ringing signals,means connected to the ringing voltage detector for decoding bursts of the ringing signals, for matching the decoded bursts with a predetermined code, and for enabling connection of the talking path in the event the decoded bursts match the predetermined code; a status circuit having an active and an inactive state, and means for connection of said talking path only in the event the status circuit is in its active state and upon enabling by the decoding means.

57. A station set interface circuit as defined in Claim 34, further including a ringing voltage detector connected to said first terminal means for detecting ringing signals,means connected to the ringing voltage detector for decoding bursts of the ringing signals, for matching the decoded bursts with a predetermined code, and for enabling connection of the talking path in the event the decoded bursts match the predetermined code; a status circuit having an active and an inactive state, and means for connection of said talking path only in the event the status circuit is in its active state and upon enabling by the decoding means.

58. A station set interface circuit as defined in Claim 37, further including means for receiving a disconnect or reconnect signal accompanying ringing signals from a central office at the first terminal and for providing a status change signal to the status circuit whereby the status circuit is placed into its inactive state upon receipt of the disconnect signal with the ringing signal designative of said subscriber's set, and into its active state upon receipt of the reconnect signal with the ringing signal designative of said subscriber's set.

59. A station set interface circuit as defined in Claim 40, further including means for receiving a disconnect or reconnect signal accompanying ringing signals from a central office at the first terminal and for providing a status change signal to the status circuit whereby the status circuit is placed into its inactive state upon receipt of the disconnect signal with the ringing signal designative of said subscriber's set, and into its active state upon receipt of the reconnect signal with the ringing signal designative of said subscriber's set.

60. A station set interface circuit as defined in Claim 45, further including means, after establishment of the talking path between the first and second terminal means and dialing of the digits by said subscriber's set of another subscriber's set connected to said multi-party telephone line, forsensing a hook-switch flash at said second terminal means, and in response to said flash, for temporarily open circuiting said talking path and applying ringing tone to said second terminal means, and for reclosing said talking path upon said another subscriber going off-hook.

61. A station set interface circuit as defined in Claim 46, further including means, after establishment of the talking path between the first and second terminal means and dialing of the digits by said subscriber's set of another subscriber's set connected to said multi-party telephone line, for sensing a hook-switch flash at said second terminal means, and in response to said flash, for temporarily open circuiting said talking path and applying ringing tone to said second terminal means, and for reclosing said talking path upon said another subscriber going off-hook.

62. A control circuit adapted for use with a station of a multiple party telephone line to provide single line circuit features for that line with said circuit being coupled to that station, said circuit including means for detecting ringing signals applied to the line from the office to which the line is coupled, logic means, means for enabling said logic means in response to the detection by said detecting means of ring signals, said logic means including programmable means for storing data indicative of coded ringing signals, and operative when enabled to receive and analyze subsequent coded ringing signals applied to the line by comparison of the code of subsequent ringing signals against said stored data to determine whether the station is to be signaled, said stored data being derived from a preset indicator of ringing signal codes, and means responsive to a successful comparison for forwarding ringing signals to said station.

63. A control circuit as claimed in Claim 62 in which said logic means is responsive to special signals from said office and in which there are means responsive to the receipt by said logic means of said special signals to remotely disconnect said station from said line.

64. On a multiple telephone line having a station representing each party, a plurality of control circuits, each adapted to be permanently associated with and coupled to a respective one of said stations to provide single line features for the stations of that line, each said circuit including means for receiving ringing signals applied to the line from the office to which the line is coupled, each circuit including logic means and memory means, means for enabling the logic means. and memory means of each circuit in response to detection by said receiving means of coded ring signals, each said logic means operative when enabled to receive subsequent coded ringing signals applied to the line and analyze said subsequent coded ringing signals by comparing their code with respective preset code indicators of ringing signals to determine if the associated station is being signaled, and means in each of said circuits responsive to a successful comparison within its logic for forwarding ringing signals to its associated station.

65. A control circuit as claimed in Claim 62 in which said programmable means comprises data processing means and in which said indicator comprises a plurality of settable switches for signalling said data processing means.