Electronic Key Telephone Systems

The invention which is the subject matter of the present application is advantageously employed with the inventions disclosed and claimed in U.S. patent application Ser. No. 870,225 entitled COMPUTER CONTROLLED KEY TELEPHONE STATION SET filed on Jan. 17, 1978, in the name of Frank A. Coviello and in U.S. patent application Ser. No. 803,243 entitled COMMUNICATION CIRCUIT filed on June 3, 1977, in the names of Alan Bloch, Frank A. Coviello, Ira Guzik and Candido Puebla, and the disclosures of both of said applications are incorporated herein by reference.

In various known key telephone systems, the implementation of the many available services or features required additional circuits in each station set and additional conductors running from each station set to the key service unit (KSU). It is not uncommon to have as many as 50 pairs of conductors connecting each station set to the KSU. Consequently, the time, effort and cost of installing, modifying or moving such key telephone sets has become excessive.

Some attempts have been made to overcome these problems by implementing key telephone service features by means of signal multiplexing techniques, thus achieving substantial reductions in the number of conductor pairs required for connection of each key telephone set to the KSU. Although this approach has alleviated the cost and space problems somewhat, it has been only moderately successful. As before, the addition of telephone services or features required circuit modifications in the KSU and in the station sets. Also, such signal multiplexing systems frequently employ a common control circuit to supervise signalling and control for all of the station sets. Thus, failure of the common control circuit causes the entire system to fail.

The present invention is embodied in and carried out by an electronic key telephone system in which telephone services or features are implemented by switching in the key service unit in response to signals received from the station sets, and signals received by the station sets from the key service unit control energization of visual indicators and connection of internal circuitry in the station sets. The key service unit includes a distributed space-division matrix with active and passive ports, and has distributed time-division multiplexed control with interleaved duplex signalling between the key service unit and the station sets. All system options are effected in the key service unit. Each station set is connected to the key service unit by only four conductors, which form two voice communication channels. The duplex signalling and the power transmission to each station set are both accomplished over a phantom pair incorporating the four conductors which form the voice communication channels.

The present invention may be better understood by reading the written description thereof with reference to the accompanying drawing, of which:

FIG. 1 is a block diagram of a first preferred embodiment of applicants' electronic key telephone system;

FIG. 2 is a block diagram of a second preferred embodiment of applicants' electronic key telephone system;

FIG. 3 is a block diagram of the station card circuit which is a component of applicants' electronic key telephone system;

FIG. 4 is a schematic wiring diagram of a section of the station card circuit shown in FIG. 3;

FIG. 5 is a block diagram of the line card circuit which is a component of applicants' electronic key telephone system;

FIG. 6 is a block diagram of the tone generator card circuit which is a component of applicants' electronic key telephone systems;

FIG. 7 is a block diagram of the link card circuit which is a component of applicants' electronic key telephone system as shown in FIG. 2;

FIG. 8 is a block diagram of the register card circuit which is a component of applicants' electronic key telephone system as shown in FIG. 2;

FIG. 9 is a partially schematic, partially block diagram of the page card circuit which is an optional component of applicants' electronic key telephone systems; and

FIG. 10 is a schematic diagram of the internal conference card circuit which is an optional component of applicants' electronic key telephone system as shown in FIG. 1.

Applicants' electronic key telephone system (EKTS) incorporates the novel concept of a distributed space-division matrix formed by active and passive ports. An active port is defined as one which has the ability to seize a passive port, but cannot be seized by any port. A passive port is defined as one which has the ability to be seized by an active port, but cannot itself seize any port. In applicants' system, any active port can seize any passive port to which it is connected. Any active port may be connected to any passive port by means of backplane wiring in the cabinet of the key service unit. The sequence of events involved in the seizing of a passive port by an active port are as follows: (1) interrogating the passive port by the active port to determine its status (seized or not seized); if the interrogated passive port is not seized, (2) closing a talk/signal path from the active port to the passive port; (3) signalling from the active port to the passive port that it has been seized; and (4) signalling from the passive port to all other active ports to which it is connected that it has been seized. If, upon interrogation of the passive port by the active port, it is found that the passive port has been seized, the interrogating active port will not be able to seize the passive port except in the special case of an active port which is capable of pre-emptively seizing the passive port, i.e., in spite of the passive port having already been seized by another active port.

Referring specifically to the first preferred embodiment of applicants' EKTS shown in FIG. 1, the system includes a variable number of electronic key telephone sets 100-1, . . . , 100-N the details of which are set forth in cross-referenced application Ser. No. 870,225, the disclosure of which is incorporated herein by reference. The key service unit KSU-1 is formed by a variable number of station cards 200-1, . . . , 200-N equal to the number of station sets; a variable number of line cards 300-1, . . . , 300-K; an optional page card 400, a tone generator card 500, and an optional conference card 600. Power is provided to KSU-1 by a power supply (not shown) generating outputs of +25 volts DC, -25 volts DC, and -5 volts DC, and power is provided to each station set 100-1, . . . , 100-N through its associated station card 200-1, . . . , 200-N via the quad-wire connection, which is described in detail in cross-referenced application Ser. No. 803,243, the disclosure of which is incorporated herein by reference. Each station card 200 has an array of active ports A and a number of passive ports P. In this embodiment, each station card active port A is connected either to a passive port P in another station card 200, or to the passive port P in a line card 300, or to a passive port P in the page card 400, or to the passive port P in the internal conference card 600. Each page card zone output is connected to the page inputs of one of four groups of station cards 200. Tone generator card 500 provides the station cards 200 with audible and visual indicator signals for transmission to the station sets 100. In this embodiment, all internal calling is accomplished without dialing by actuation of a single station set pushbutton or key to establish a direct connection through the associated station card and the backplane wiring to another station card and its associated station set. This embodiment employs a non-blocking matrix arrangement that assures connection to the desired internal station set regardless of the amount of traffic on the system (if that station set is not busy).

Referring specifically to the second preferred embodiment of applicants' EKTS shown in FIG. 2, the system includes a variable number of electronic key telephone sets 100-1, . . . , 100-N as in the system of FIG. 1. The key service unit KSU-2 is formed by a variable number of station cards 200, a variable number of line cards 300, a variable number of register cards 600, a variable number of link cards 700, a tone generator card 500, and (optionally) a page card 400. System power is provided as in the embodiment of FIG. 1. Each station card 200 has an array of active ports A and a number of passive ports P. In this embodiment, each station card active port A is connected either to the passive port P in a line card 300, or to the passive port P in a link card 700, or to a passive port P of one other station card 200, or to a passive port P in the page card 400. Each link card active port A is connected either to the passive port P of a line card 300, or to a passive port P in a station card 200, or to a passive port P in the page card 400. Each page card zone output is connected to the page inputs of one of four groups of station cards 200. Tone generator card 500 provides audible and visual indicator signals to the station cards 200, the line cards 300, and the link cards 700 for transmission to the station sets 100 and to the C.O. line, as may be appropriate. In this embodiment, most internal calling is accomplished by dialing from a station set 100 through the associated station card 200 and a link card 700 to another station card 200 and its associated station set. However, each station card does have a hotline connection from one active port A through the backplane wiring to a passive port P in one other station card. It is also possible to seize a C.O. line which does not have its line card's passive port P connected to an active port A of a particular station card by having that station card seize a link card and then seize that C.O. line via the link card.

In both embodiments, there is an absolute minimum of common equipment, and so the systems are never burdened with more circuitry or hardware than is actually needed for any given installation. Each system also has the capability of allowing two groups of users to be separated by lines and stations so that one group has C.O. lines that do not appear at stations in the other group. This configuration has application when two or more departments or business entities in the same location wish to be serviced by the same key telephone system. System installation requires a minimum of time and expense. Because all system options are accomplished in the KSU, rather than in the station sets, the installer is not required to open the station sets during installation or when implementing new services or features. The wiring from the KSU to the station sets is accomplished with standard 4-wire non-twisted 22 AWG wire, which may be run for distances over 3,000 feet, and is connected by modular plugs and jacks. All of these advantages are yielded by the use of a solid state, distributed space division matrix with time-division distributed control. Specifically, all of the logic functions for a given station are performed by the station set circuit and the associated station card circuit. Thus, a logic failure in one station card circuit will affect only the associated station, and repair or replacement of the failed station card circuit may be effected without interrupting or interfering with service on any other line.

Referring specifically to FIG. 3, the station card circuit 200 shown there includes transformers 202 and 204, which are connected to the A path (conductors 206 and 208) and the B path (conductors 210 and 212), respectively. The phantom path is connected from the center-tapped windings of transformers 202 and 204 to the data transceiver 214, which is fully described in co-pending patent applications Ser. No. 870,225 and Ser. No. 803,243, the disclosures of which are incorporated herein by reference. The data received from the associated station set circuit 100 via the phantom pair is fed in a continuous stream from the data transceiver 214 to the microcomputer 216, which decodes the data. The microcomputer 216 is preferably the Part No. R6500/1 single-chip microcomputer system manufactured by Rockwell International Corporation. When the data received from the associated station set 100 includes a word having a bit which is indicative that a particular button has been depressed by the user of the station set, the microcomputer 216 generates the outputs necessary to implement the service or feature associated with that button. For example, if a C.O. line is to be seized, an appropriate output is fed from output terminal A5 of microcomputer 216 to the current generator 218, which is a digital-to-analog converter formed by four operational amplifiers with their output terminals tied so as to sum their respective outputs. In response, the current generator 218 generates a predetermined DC output current (1.6 ma) which is superimposed upon any signal received from the station set on the A path fed through transformer 202 and DC blocking capacitance 220. Both the output of current generator 218 and the signal received via the A path are transmitted to the crosspoint array 222, which consists of a number of MOSFETs, each having its source electrode connected either to an associated line card circuit, or to a link card circuit, or to a page card circuit, or to another station card circuit, or to a conference card circuit, all of the drain electrodes being connected in common to the outputs of current generator 218 and transformer 202. Each of the gate electrodes of the MOSFETs is connected to an associated crosspoint latch in the array of crosspoint latches 224, which consists of demultiplexers formed by three eight-bit latches (CD4099BE). The array 224 receives three continuous data streams from microcomputer 216; latch command data, which is issued to all of the crosspoint latches and consists of a one bit command (0 or 1); write enable strobe data, which identifies which of the eight-bit latches is to receive the latch command data; and address data which identifies with a three-bit address that latch in each package which is to receive the latch command data. When the proper conjunction of inputs causes one of the crosspoint latches 224 to close, the associated MOSFET crosspoint in array 222 is biased conductive to close the audio path from A-pair 206, 208 through transformer 202, DC blocking capacitance 220 and the drain and source electrodes of the conductive MOSFET in array 222. A dial disable circuit 226 formed by a saturating operational amplifier receives inputs from only those crosspoint latches 224 associated with crosspoints which are connected to line cards; those inputs are indicative of the status of those latches, and when any such latch is closed, circuit 226 responds by terminating the dial disable output signal which is normally fed to negative impedance circuit 228 and to function multiplexer 230. Removal of the dial disable signal activates the negative impedance 228 from an open circuit condition to a first level of -600 ohms so as to match the impedance in the station card 200 to the impedance to which connection has been made via the conductive MOSFET crosspoint. The function multiplexer 230, formed by a tree of analog gates (a single CD4051BE) which receives a four-bit address from the microcomputer 216, also receives a number of simultaneous inputs including the presence or absence of a dial disable signal, the presence or absence of a hands-free disable signal, and slow-flashing and quick-flashing visual indicator signals. These simultaneous inputs are time-division multiplexed to form the input to terminal B7 of microcomputer 216, and are thus available for inclusion in the data stream output at terminal D3 to the data transceiver 214. The array of digital crosspoints 232 are, like the function multiplexer 230, formed by a tree of analog gates (also a single CD4051BE) which receives a four-bit address from the microcomputer 216, and also receives a series of simultaneous inputs in the form of voltages indicative of the status (seizable, not seizable, etc.) of the passive ports to which the crosspoints 222 are connected. Each of the analog crosspoints 222 is thus paired with one of the digital crosspoints 232 to form an active port. The simultaneous inputs to digital crosspoints 232 are time-division multiplexed and output to the voltage discriminator 234 formed by three analog-to-digital converters which provide a three-bit message to the microcomputer 216 at terminals B0,B1 and B6. Viewing the inputs to these terminals at any point in time, if none of the three voltage reference levels of discriminator 234 is exceeded by the signal voltage (+2 v, 0 v, -2 v or -4 v) sampled by digital crosspoint 232, then logic 0 appears at each of the terminals B0, B1 and B6, indicating to the microcomputer 216 that the passive port being interrogated at that point in time is seizable and it associated visual indicators in the station sets are off. If the first voltage reference level is exceeded, a logic 1 appears at B6 and logic 0 appears at B0 and B1, indicating that the passive port being interrogated at that point in time is seizable, but its associated visual indicators are on, indicating that the station card incorporating the interrogated passive port is in use on its A-pair. If the second voltage reference level is exceeded, a logic 1 appears at B6 and B0 and a logic 0 appears at B1, indicating that the passive port being interrogated at that point in time is not seizable, except by an active port with a pre-emptive seize capability, and its associated visual indicators are on. If the third voltage level is exceeded, a logic 1 appears at B0, B1 and B6, indicating that the passive port being interrogated at that point in time is not seizable by any active port, i.e., it has been pre-emptively seized, and its associated visual indicators are on. If the passive port of a line card being interrogated at any point in time indicates that it is either receiving an incoming call on the associated C.O. line, or is on HOLD, i.e., when the voltage signal input to data crosspoints 232 alternate between 0 v and +2 v, this flashing signal will be sampled by the associated digital crosspoint in array 232 and will be passed through voltage discriminator 234 to terminal B6 as a series of logic 1s alternating with a series of logic 0s at the rate and duty cycle of the flashing signal being sampled. All of this information is employed by the microprocessor 216 to determine whether to signal the current generator 218 to send signal current on the audio path via a conductive MOSFET crosspoint to a passive port, and to signal the station sets from terminal D3 via data transceiver 214 as to the status of all the interrogated passive ports.

If and only if a requested telephone service or feature is indicated as being available upon interrogation of the relevant passive port, signal current will be generated by the active port to advise the passive port of its seizure and the desired service. Specifically, the current generator 218 can receive only one of four command signals from microcomputer 216 in response to data received at terminal A0. Absent any command signal, the output from current generator 218 is nil (0 ma). A seize signal may appear at terminal A5, causing a seize current (1.6 ma) to flow from current generator 218 on the audio path and through a conductive MOSFET crosspoint to the passive port to which that crosspoint is connected. That passive port is thus informed that it has been seized, and it responds by generating a voltage signal which is transmitted to all of the active ports to which the passive port is connected to indicate to those active ports that it has been seized. A conference signal may appear at terminal A6, causing a double seize current (3.2 ma) to flow from current generator 218 to signal seizure of two passive ports, e.g., conferencing two incoming C.O. lines. The seized passive ports respond by each generating a voltage signal indicative of seizure. The conference signal from terminal A6 of microcomputer 216 is also input to negative impedance 228 to adjust its value to -200 ohms so as to match the impedance of the two conferenced lines to which the station card has been connected through the seized passive ports. A flash signal may appear at terminal A7 of microcomputer 216, causing a flash current (6.4 ma) to flow from current generator 218 and break the loop current on a seized C.O. line while continuing to hold that line. Hold and privacy release signals may appear at terminal A4 of microcomputer 216, causing either a hold current pulse (14 ma) of brief duration to be generated, or a privacy release current pulse (14 ma) to be generated as long as the privacy release button in the associated station set is depressed. In response to the aforementioned signals currents from current generator 218, the KSU circuit incorporating the receiving passive port implements the requested service indicated by the signal current level, and the receiving passive port generates a voltage signal indicative of its seizure.

Each station card 200 includes one or more passive ports, viz., a hotline B-port 238, a link B-port 240, or a page input to B-port switching circuit 246. The term "B-port" refers only to station card passive ports, which provide access to the B-pair communication channel. In the system shown in FIG. 1, the station cards require no link B-ports 240, since there are no links. If the optional paging and background music features are eliminated, the PAGE and BGM input circuits in B-port switching circuit 246 may be eliminated. There are multiple connections to the hotline B-port from one active port in each of the other station cards in this system. In the system shown in FIG. 2, each station card has a link B-port which may be seized by a link card circuit. The hotline B-port is connected to one active port in only one other station card 200; if a system is desired in which all internal calling is to be accomplished through the link cards, the hotline B-port may be eliminated. As in the system of FIG. 1, if the optional paging and background music features are eliminated, the PAGE and BGM input circuits in B-port switching circuit 246 may be eliminated. These B-port circuits are shown in detail in FIG. 4. The hotline B-port 238 comprises a transformer 250 for isolating the DC current signals, which are received from the active ports to which transformer 250 is connected via the HLB ANALOG conductor, from the VOICE or AUDIO PATH. A resistor 252 is connected across the DC signal-current-receiving winding of transformer 250 for the purpose of providing a shunt path past the winding so as to minimize transients induced in the other winding by the DC current signals, and an impedance-matching resistance 254 of equal value is connected across the other winding. A signal current sensor circuit comprises transistor 256, which is biased by -9 v. applied through resistor 260 to its base, which is also connected to the anode of diode 258, the cathode of which is grounded. The collector is connected through parallel resistor 262 and capacitor 264 to -9 v., and the emitter is connected through resistor 252 and its parallel winding of transformer 250 to the HLB ANALOG conductor. A seize signal current (1.6 ma) is detected as an increase in the voltage across resistor 262, which causes voltage comparator 266 to generate a HL SEIZE 2 signal which is transmitted to input terminal B5 of microcomputer 216. A latch command is then generated in the data stream from terminal D5, causing the pertinent one of function latches 236 (FIG. 3) to close and thereby issue the HL CONTROL 2 signal (logic 1) to digital-to-analog converter 261 in hotline B-port 238. In response, the D/A converter 261 generated an output which is applied through resistor 263 to voltage follower 269, which generates a DC signal voltage indicating seizure (-2 v.) which is transmitted to the digital crosspoint(s) of the active ports to which hotline B-port 238 is connected. The microcomputer 216 further responds to the HL SEIZE 2 or 3 signals by simultaneously generating a SPLASH TONE ENABLE signal at terminal A1, thereby triggering splash tone control circuit 294 which then turns on JCT FET 293 for about 1 second to pass the steady SPLASH TONE signal received from tone generator circuit 500 to the AUDIO PATH, thereby signalling the called party that there is an incoming call. The SPLASH TONE ENABLE signal is also applied as an input to inverting OR gate 290, which responds by turning off JCT FET 289 to block any PAGE or BGM transmission. The microcomputer 216 responds to the depression of the appropriate buttons at the called party's station set by generating the HL B ENABLE signal at terminal D1, which turns on MOSFET 268 in seize control circuit 242 and is also applied as an input to inverting OR gates 290 and 286 in B-port switching circuit 246, thereby turning off both JCT FET 289 and MOSFET 287 to block PAGE and BGM transmissions, and causing inverter 285 to turn on MOSFET 284 in the VOICE or AUDIO PATH. Thus, a path is closed for the transmission of voice signals via transformer 250, MOSFET 268, attenuator 244 formed by a resistive pi network and MOSFET 284 through DC blocking capacitor 248 and transformer 204 to the B-pair communication channel comprising conductors 210 and 212.

With a seized DC signal voltage (-2 v.) at the output of voltage follower 269, the hotline B-port 238 may be pre-emptively seized by an active port having that ability. If a pre-emptive seize current signal (3.2 ma) is received by hotline B-port 238, voltage comparator 263 generates a HL SEIZE 3 signal which is fed to function multiplexer 230. A latch command is then generated in the data stream from terminal D5, causing the pertinent one of the array of function latches 236 (FIG. 3) to issue the HL CONTROL 3 signal to digital-to-analog converter 257. In response, the D/A converter 257 generates an output which is applied through resistor 259 to voltage follower 269, which generates a DC signal voltage indicating pre-emptive seizure (-4 v.). If the user of the associated station set 100 is utilizing the A-pair communication channel, the microcomputer will generate a HL CONTROL 1 signal (logic 1) at terminal DO, thereby causing digital-to-analog converters 265 to generate an output through resistor 267 which causes voltage follower 269 to generate a DC signal voltage (0 v.) indicating that the hotline B-port 238 is seizable and that the associated visual indicator in the calling party's station set is to be lit. If each of the signals HL CONTROL 1,2,3 is a logic 0, the voltage follower 269 will generate a DC signal voltage (+2 v.) indicating that the hotline B-port 238 is seizable and that the associated visual indicator in the calling party's station set is not to be lit.

The link B-port 240 comprises an isolating transformer 270 with DC shunt resistor 272 connected across the DC signal current-receiving winding, and an impedance-matching resistor 274 connected across the other winding. A combination current sensor and voltage sender circuit is formed by transistor 275, operational amplifier 276, and the parallel-connected resistor 277 and capacitor 278. A DC current signal received on the LINK conductor from an active port is sensed as an increase in voltage across resistor 277, which in turn causes comparator 279 to generate a LINK SEIZE signal which is applied to terminal B2 of microcomputer 216. If the call is received handsfree at the called speakerphone, the microcomputer 216 also generates a SPLASH TONE ENABLE signal at terminal A1 in response to the LINK SEIZE signal, thereby triggering splash tone control circuit 294 which then turns on JCT FET 293 for about 1 second to pass the steady SPLASH TONE signal to the AUDIO PATH. The SPLASH TONE ENABLE signal is also applied as an input to inverting OR gate 290, which responds by turning off JCT FET 289 to block any PAGE or BGM transmission. If the seizing link sends DC signal current (3.2 ma) indicating an intercom ringing call, i.e., one which is to be answered via the handset hybrid circuit instead of the speakerphone at the called station, then voltage comparator 280 will also generate an output, which is fed to terminal B3 of microcomputer 216 as an ICM RING CALL signal. In response, the microcomputer 216 generates a latch command at terminal D5 instead of a SPLASH TONE ENABLE signal at terminal A1. The latch command is fed to the function latches 236 (FIG. 3) along with address data and write enable strobe data. Function latches 236 are formed by one eight-bit latch (CD4099BE) which acts as a demultiplexer of the three streams of received data in the same manner as crosspoint latches 224. In response to the aforementioned latch command, an ICM RING TONE ENABLE signal is generated and fed to B-port switching circuit 246 (1) to turn on JCT FETs 288 and 296 to allow passage of ICM RING TONE to the called party through normally-conductive MOSFET 287 and to the calling party through transformer 270 in the link B-port 240, and (2) to turn off JCT FET 289 through inverting OR gate 290 to block any PAGE or BGM transmission.

Upon termination of either the SPLASH TONE ENABLE signal or the ICM RING TONE ENABLE signal, a LINK ENABLE signal is generated at terminal D2 of microcomputer 216 to turn on MOSFET 283 in seize control circuit 242. The LINK ENABLE signal is also applied as an input to inverting OR gate 286 in B-port switching circuit 246, thereby turning on MOSFET 287 and causing inverter 285 to turn on MOSFET 284. Thus, a path is closed for the transmission of voice signals via transformer 270, MOSFET 283, attenuator 244, and MOSFET 284 to the B-pair communication channel. The LINK SEIZE signal is also applied to the positive input terminal of operational amplifier 276 connected as a voltage follower with its negative input and its output tied to the emitter and base, respectively, of transistor causing its emitter to 275 become more positive. Thus, a DC voltage signal (-2 v.) indicating seizure is generated at the emitter of transistor 275 and is applied through resistor 272 and its parallel winding of transformer 270 to the LINK conductor for transmission to the link.

If a call coming in to link B-port 240 is answered at an alternate station set by dialing the called station set extension number with a prefix code number, the alternate station set is connected to the seizing link through the link B-port 240 of the called station set. Under these conditions, the seizing link will generate a call steal DC current signal (3.2 ma) which causes comparator 281 to generate on output which is fed to terminal B4 as a CALL STEAL signal in response to which the microcomputer 216 prevents user access to the B-pair communication channel of the called station set. The CALL STEAL signal is also applied to the call steal timer circuit 282 to reset it. The call steal timer circuit starts to run for a predetermined period of time (e.g., 30 seconds) whenever it receives a LINK SEIZE signal, and while it is running it biases the positive input terminal of operational amplifier 276 so as to allow the generation of a DC voltage signal at the emitter of transistor 275 indicating seizure of the link B-port, thereby enabling the call from the link to be "stolen" by pre-emptively seizing the link B-port 240 via an active port in an alternate station set. The call steal timer circuit is also reset by the LINK ENABLE signal from terminal D2 of the microcomputer 216 so as to prevent an answered call from being "stolen", thereby preserving the called user' s privacy.

The PAGE and BGM signals are received from the page and tone cards, respectively, and are controlled by JCT FETS 291 and 292, respectively, which are normally conductive when PAGE ENABLE and BGM ENABLE signals are applied to their respective gate electrodes. These enabling signals are generated by function latches array 236 in response to latch commands issued at terminal D5 of microcomputer 216 in response to data received at terminal A0 from the associated station set 100. A combination on/off/volume switch in the station set 100 controls the generation of the BGM ENABLE signal by the associated station circuit 200; when the switch is off, the signal will not be generated, and when the switch is on, the signal will be generated. The PAGE ENABLE signal will normally be generated in each station circuit 200, but will be terminated upon seizure of the page card through a link card or directly from the station card in response to requests from the station set.

CO AUDIBLE is a signal which is continuously generated within the EKTS on the tone card 500 (FIG. 6) for transmission to the internal speaker in the station set upon receipt of a C.O. AUDIBLE ENABLE signal at the gate electrode of JCT FET 295. These enabling signals are also received from the tone card 500 in response to INCOMING CALL ENABLE signals generated by the various line cards 300 (FIG. 5) in response to a ringing signal from the Central Office.

The program listing which follows on pages 21-44 is the sequence of instructions stored in microcomputer 216 of each station card 200 (FIG. 3). Each ROM address (LOC) with machine operation code (CODE) followed by the assembly language instruction of the program is included in the listing. The assembly language instructions are explained in detail in the manufacturer's Data Sheet entitled "Part No. R6500/1 Microcomputer System", Document No. 29000D51, Revision 2, dated October 1978 and published by the Rockwell International Corporation. The Symbol Table on pp. 45-46 includes four columns of program symbols on each page, with the value of each symbol listed at the right side of each symbol. ##SPC1## ##SPC2## ##SPC3## ##SPC4##

Referring specifically to the circuit shown in FIG. 5, each line card 300 includes a single passive port which is connected to one active port of each of a variable number of station cards 200 in the system of FIG. 1, and also to an active port in each of the link cards in the system of FIG. 2. The passive port receives DC signal currents from the connected active ports through the shunt resistor 304 and through the DC signal current-receiving winding of transformer 302, and current sensing circuits 306 generate either a SEIZE, FLASH, or HOLD/PRRL (privacy release) signals depending upon whether the DC signal current is 1.6 ma, 6.4 ma, or 7-14 ma. The SEIZE signal causes the seize relay control circuit 310 to energize the winding of seize relay 312, causing the ganged armatures to disconnect from load resistor 314 and establish connections to the tip T and ring R of the CO line. The SEIZE signal simultaneously triggers timer circuit 319 which activates a clamping circuit 321 for about 1 second in order to limit the noise and ringing signal coming in from the CO line. With the line seized, dialing may be accomplished at the station set by either dual-tone, multi-frequency (DTMF) signal generators or by an outpulse generator. DTMF dial signals are transmitted through transformer 302, music-on-hold interface circuit 322, bi-directional amplifier 320, transformer 318, and seize relay 312 to the CO line. Outpulse dialing is accomplished by pulse-modulating the SEIZE signal so as to cause the seize relay 312 to modulate the loop current as a function of the pulses generated at the station set.

The SEIZE signal is also fed to the voltage sender 324, which responds by generating a DC signal voltage (-2 v.) indicating seizure of the line card passive port. If the seized CO line is put on hold by transmission of a 14 ma DC current pulse from the seizing port, a current sensing circuit 306 generates a HOLD signal pulse which causes hold condition sensing circuit 308 to generate a constant output which activates the MOH interface circuit 322 to pass the MOH SOURCE signal received via the tone card 500 to the held CO line. Because the analog crosspoint in the previously seizing active port is now open, no signal current will flow to current sensing circuits 306, and the MOH SOURCE will not be heard at the associated station set. The output of the hold condition sensing circuit 308 is also fed to an input of AND gate 322, which receives a second input from inverter 328 in response to the termination of the SEIZE signal, and a third input F-FLASH which is received from the tone card 500. The output of AND gate 332 consequently fluctuates in synchronism with the F-FLASH SIGNAL, as does the output of voltage sender 324, thereby signalling the digital crosspoints of every connected active port that the CO line is on hold and may be seized by any connected active port. A PRRL signal pulse from one of the current sensing circuits 306 in response to a 7-14 ma DC signal current pulse from the seizing active port will cause hold condition sensing circuit 308 to generate a constant output, so that the MOH interface circuit 322 will be activated and one input will be provided to AND circuit 332. The SEIZE signal is simultaneously terminated by de-activation of the DC current signal generator in the seizing active port, causing inverter 328 to provide another input to AND circuit 332. Thus, voltage sender 324 ceases to generate a seized voltage signal (-2 v.) and begins to generate an output which fluctuates between 0 v. and -2 v. at the F-FLASH rate. Therefore, other station sets 190 in the system are informed that they may also make connection to the CO line.

When a FLASH signal is generated by one of current sensing circuits 306 in response to a flash DC current signal (6.4 ma), the flash switch control circuit 326 responds by de-activating seize relay control circuit 310 for about 1 second, thereby breaking the connection of seize relay 312 to the CO line for that period of time. The SEIZE signal is not interrupted, however. Thus, loop current is broken to disconnect the outside party's circuit from the CO line while retaining the line for another call. If the EKTS is employed behind a PBX which requires it, this line card circuit is modified by breaking the controlling connection from flash switch control circuit 326 to the seize relay control circuit 310, and by adding a relay 316 which has its armature connected directly to the upper armature of seize relay 312. In this revised circuit, the generation of the FLASH signal will not affect the seize relay control circuit 310, leaving the seize relay 312 energized to connect transformer 318 to the PBX line, but the flash switch control circuit 326 will cause relay 316 to connect the conductor T to circuit ground during FLASH. As in the first embodiment of this circuit, the SEIZE signal is not terminated. Thus, loop current is broken to disconnect the other party's circuit from the PBX line while retaining the line in order to place another call.

In the case of an incoming call, the ringing signal from the central office will cause ring detection circuit 328 to activate incoming ring indicator circuit 330, which then generates an INCOMING CALL ENABLE signal which is fed to tone card 500, and a logic 1 input signal to AND gate 330. Since there is no SEIZE signal, the output of inverter 328 is also logic 1. Thus, the S-FLASH signal will pass through AND gate 330 to modulate the output of voltage sender 324 to provide an indication of the incoming call to the digital crosspoints of the connected active ports, any one of which may seize the passive port of the line card circuit and thereby seize the associated CO line.

Referring specifically to the circuit shown in FIG. 6, tone generator card 500 comprises a 64 hz oscillator circuit 502 which provides a square wave to the countdown combiner and driver circuits 504, and these in turn generate a Q-FLASH (I-hold) signal, an F-FLASH (hold) signal, and an S-FLASH (incoming call) signal. The Q-FLASH and S-FLASH signals are fed to the function multiplexer 230 of station card circuit 200 (FIG. 3) to be sampled, the samples being serially transmitted to microcomputer 216 for possible inclusion in the data transmitted to the associated station set 100. The S-FLASH and F-FLASH signals are fed to line card 300. The countdown combiner and driver circuits 504 also provide driver signals to the driver circuits 506, 508, 510 and 512, which also receive the output of combiner circuit 514, which mixes the outputs of the 400 hz oscillator circuit 516 and the 470 hz oscillator circuit 518. These oscillator outputs are also fed to driver circuits 520 and 522, respectively, which generate DIAL TONE and SPLASH TONE, respectively. DIAL TONE is fed only to the register card circuit(s) 800, and SPLASH TONE is fed only to the station cards 200.

Driver circuit 506 produces CO AUDIBLE tone at the rate of 1 second on, 1 second off, 1 second on, 3 seconds off, and is fed to the station cards 200 and page card 400. Driver circuit 508 produces ICM RING TONE at two isolated outputs at the rate of 1 second on, 3 seconds off, and is fed only to the station cards 200. Driver circuit 510 produces BUSY TONE at the rate of 0.5 second on, 0.5 second off, and is fed to the link card circuit(s) 700. Driver circuit 512 produces REORDER TONE at the rate of 0.125 second on, 0.125 second off. The outputs of driver circuits 508, 510, 512 and 520 are all employed in internal link functions, and may be eliminated in a system which has no link circuits 700.

The split ringing and gating circuit 524 receives the INCOMING CALL ENABLE signals which the line cards 300 generate when an incoming call is detected, and in response generates several CO AUDIBLE ENABLE signals, any one or more of which can be transmitted through backplane wiring to any station card 200 or group of station cards 200. For example, if an INCOMING CALL ENABLE signal is received from line card 1, 2 or 3, a CO AUDIBLE ENABLE signal is sent from output COEGPA via the backplane to a predetermined group of station cards 200. The following outputs COEGPB, C, D, E, F and G are similarly associated with line cards 4-6, 7-9, 10-12, 13-15, 16-18 and 19-21, respectively. Output COE 1-14 is associated with line cards 1-14, and a CO AUDIBLE ENABLE signal will be provided via the backplane to all of the station cards 200 connected to this output. Output COE 15-18/19 is similarly associated with line cards 15-18/19, and COE 15-19/21 is similarly associated with line cards 15-19/21. This arrangement provides great flexibility in the distribution of the CO AUDIBLE tone, e.g., enabling direct signalling to predetermined small groups of station sets behind PBX that there is an incoming call from one of a small group of CO lines, and facilitating the prevention of such direct signalling on those incoming calls that are received at an attendant's station for forwarding to the called party.

A station set 100 designated by backplane wiring controls the night ring control circuit 526. As in all station sets, the BGM ENABLE signal is generated in response to turn-on of the BGM control switch. Due to the connection of the BGM ENABLE signal output from the station card 200 associated with the designated station set 100 to the night ring control circuit 526, turn-on of the BGM control switch at the station set enables generation of the NR ENABLE signal upon receipt of a logic level signal from the split ringing and gating circuit 524 in response to any INCOMING CALL ENABLE signal. The NR ENABLE signal is fed to page card 400 to broadcast CO AUDIBLE TONE as an all-page transmission. The optional MOH interface circuit 528 distributes the MOH SOURCE signal to all line cards selected to receive it through the backplane wiring. The optional BGM interface circuit 530 distributes the BGM SOURCE signal to all station sets selected to receive it through the backplane wiring, except when it receives a BGM DISABLE signal from the page card 400.

Referring specifically to FIG. 7, the link card circuit shown there includes a single passive port and multiple active ports. The passive port comprises current detector circuit 702, which includes a DC current signal blocking transformer in the ANALOG LINK path and is constructed in a manner similar to the current detector circuit in hotline B-port 238 in station card 200 (FIG. 4). The sensed DC signal current from an active port of a seizing station card 200 will cause the current detector 702 to generate an output voltage which is fed to a series of voltage comparators 722, 724, 726, and 728 which will be successively triggered in response to sensed DC signal currents of 1.6 ma, 3.2 ma, 6.4 ma, and a 14 ma pulse (50 milliseconds), respectively, which cause the voltage output of current sensor 702 to increase to successively higher levels. In response to a DC signal current indicating seizure (1.6 ma) received from a station card active port on the ANALOG LINK input to current sensor 702, a LINK DXP DC voltage signal (-2 v.) will be generated by comparator 722 and sent to all station cards 200 which are also connected to the ANALOG LINK input to current sensor 702. Simultaneously, comparator 722 applies a SEIZE signal to request logic circuit 744, which in turn generates a LINK REQUEST signal which is sent to the register card(s) 600. If the register or registers are in use, no LINK CONNECT signal will be returned. In a system with more than one register card 600, only one can be in the ready status, and the other(s) will all be either in use, i.e., connected to a link circuit 700, or in standby status. The ready register becomes connected in response to the LINK REQUEST signal, and sends a LINK CONNECT signal to connect logic circuit 742, and a LINK DATA ENABLE signal and a LOAD ADDRESS signal to AND gate 740, which responds by enabling the tens latches 748 and the units latches 752. The connecting register will then send back an 8-bit address, four bits to tens latches 748 and four bits to units latches 752, representing either a decimal 98 or 99 which identifies the connected register (in a two-register system) to which a register crosspoint REG 1 or REG 2 is to be closed. The binary-to-decimal converter circuits 712 and 714 decode the binary outputs of level shifters 750 and 746, respectively, so that B/D converter 714 provides an input to AND logic circuit 710 representative of the first register-identifying digit (9) and B/D converter 712 provides an input to AND logic circuit 710 representative of the second register-identifying digit (8 or 9). These two inputs will cause AND logic circuit 710 to generate either a REG 1 GATE signal or a REG 2 GATE signal to close either the REG 1 crosspoint to a first register or the REG 2 crosspoint to a second register. Dial tone is now passed from tone card 500 via the connected register 600 through the closed REG crosspoint and over the ANALOG LINK BYPASS to indicate to the caller that he may now send dial signals to identify that passive port which he wishes to seize. Either DTMF or outpulse dial signals will be transmitted via the ANALOG LINK BYPASS around current sensor 702, and current sender 704 through the closed REG crosspoint to the connected register 600, which decodes the dial signals, puts them in binary form, and decides whether or not the number is valid. If the number is not valid, e.g., there is no passive port to which the dialed number has been assigned, the REORDER TONE from tone card 500 is sent via the connected register 600 over the ANALOG LINK path through a designated internal crosspoint 706, which is closed in response to a reorder crosspoint address (97) sent by the register 600 to the latches 748, 752, which have been re-enabled by AND gate 740 to receive the reorder crosspoint address, which replaces the invalid address. This new address displaces the register-identifying address (97 or 98) and thereby causes the REG crosspoint to the register to re-open. Also, a REORDER signal is sent to start kick-off logic circuit 716, which generates a 20 millisecond pulse signal TKO after a predetermined period of time (e.g., about 8 seconds), in response to which the current detection circuit 702 causes the LINK DXP output of comparator 722 to go to +2 v. for 20 milliseconds, which causes the seizing active port to cease transmission of its DC seize current signal. As a result, the LINK DXP signal goes to +2 v. to indicate that the link is idle, and the SEIZE signal to request logic circuit 744 is terminated, causing termination of the LINK REQUEST signal. The caller may release and reseize the link before kick-off occurs.

If the called number is valid, the register 600 generates a LOAD MODE signal in addition to the previously-generated LINK DATA ENABLE and LOAD ADDRESS signals to cause AND gate 738 to enable the mode latches 736 and the current mode latches 754. The register is preferably designed to recognize the following number groups as valid and to issue the oppositely-listed current mode signals and address data:

Thus, a dialled number 56 will cause a logic level signal representative of a decimal 0 generated in the register to be sent by the register to the first input to the current mode latches 754 via the first link data units line, thereby providing a SEIZE signal to mode control circuit 730. The absence of a prefix number indicates that an internal call is being made and can be answered handsfree. The first address digit is a binary-coded 5 which is fed to tens latches 748, and the second address digit is a binary-coded 6 which is fed to the units latches 752. As another example, a dialled number 148 will cause the register to send a logic level signal representative of a decimal 1 generated in the register to the second input to the current mode latches 754 via the second link data units line, thereby providing an ICM RING signal to mode control circuit 730. The prefix number 1 indicates that an internal call is being made to the called handset and must be answered via the called party's handset. The first and second address digits are a binary-coded 4 and binary-coded 8, which are fed to the tens latches 748 and units latches 752, respectively. A third example is the case of a dialled number 265, which will cause the register to send a logic level signal representative of a decimal 2 generated in the register to the third input to the current mode latches 754 via the third link data units line, thereby providing a STEAL signal to mode control circuit 730. The prefix number 2 indicates that the caller is effecting a "call steal", i.e., answering a call that is coming in at another station set, sometimes referred to as alternate-point answering. In this case, the address data (binary-coded 6 and binary-coded 5) identifies the station set to which the incoming call is directed. The prefix number 9, when dialled before the numbers 17 through 30, indicates that the caller wishes to seize a CO line through one of the active ports comprising the paired CO crosspoints labelled LINES 17-30 ANLG and LINES 17-30 DXP, and causes the register to send a logic level signal representative of a decimal 9 generated in the register to the fourth input to the current mode latches 754 via the fourth link data units line, causing a CO signal to be sent to mode control circuit 730. The prefix 9, when dialled before the numbers 91 through 96, indicates that the caller wishes to seize one of the passive ports in the page card 400 via one of the group of active ports comprising the single-line internal crosspoints 706 labelled PAGE 1, 2, 3, 4 and 5 and ALL PAGE. As before, the register sends a logic level signal representative of a decimal 9 generated in the register to the fourth input to the current mode latches 754, which in turn provide a PAGE signal to mode control circuit 730. There may also be a special single-digit number such as 0 which is frequently dialled, e.g., to reach the operator at an attended station in the system. The register(s) will interpret the 0 as requesting transmission of a two-digit address (90) which will cause the link to close the internal crosspoint labelled OPR to the operator's station.

The mode latches 736 receive a single input signal pulse from the register at their second input simultaneously with the logic level input to current mode latches 754, causing a constant, inverted output T2 to be fed to called station response timer 734 which in turn activates busy test logic circuit 732 for a predetermined period of time (e.g., 2.5 seconds). During this time period, if no CALL BUSY signal is received from the DXP detection circuit 718, busy test logic circuit 732 will generate no output, but at the end of the time period it will generate a CSSE (called station send enable) signal, thus enabling current sender (mode) control logic circuit 730 to generate a control voltage output to current sender 704. Then, in response to inputs either from the voltage comparators 724, 726 or 728 or from current mode latches 754, mode control logic circuit 730 will alter its normal DC output (+8 v.). In response to a logic level SEIZE, ICM RING, CO/PAGE or STEAL input signal from current mode latches 754, mode control logic circuit 730 will generate a DC voltage output of +7.2 v., + 6.5 v., +7.2 v., or +6.5 v., respectively, which will in turn cause current sender 704 to generate a DC output current of 1.6 ma, 3.2 ma, 1.6 ma, or 3.2 ma, respectively, which is transmitted via a closed crosspoint in arrays 706, 708 to the passive port to be seized. If the passive port is in the page card 400, or is the link B-port 240 of a station card 200, the DC voltage signal indicating seizure (-2 v.) or, in the case of the station card, pre-emptive seizure (-4 v.), will be sent back to the link 700 over the single conductor on which the DC signal current effecting seizure (1.6 ma) is sent to the page card or link B-port, and will pass through the closed crosspoint (one of the group from PAGE 1, 2, 3, 4, 5, ALL PAGE, and STA 30 through STA 89) on the STA COMMON DRAIN conductor to DXP detection circuit 718 during the time period in which the busy test logic circuit 732 is activated. In response to either -2 v. or -4 v., which indicate a seized or pre-emptively seized passive port, respectively, the CALL BUSY signal will be generated by the DXP detection circuit 718, in response to which the reset logic circuit 720 clears the address latches 748, 752 and the busy test logic circuit 732 generates a BUSY TONE ENABLE signal, which starts the kick-off logic circuit 716 and is fed as an input to AND logic circuit 710. The address latches 748, 752 now hold the 00 address, which is the address of the BUSY TONE crosspoint. In response to this address and the BUSY TONE ENABLE signal, the AND logic circuit closes the BUSY TONE crosspoint to pass the busy signal from the tone card via the ANALOG LINK path to the caller's station card 200 and thence to associated station set 100. The caller must then either release the link before generation of a TKO signal pulse and then reseize it or another link, or he will be kicked off the link upon generation of the TKO signal by logic circuit 716. Upon either release or kick-off, the SEIZE signal to request and connect logic circuits 744 and 742 will be terminated, causing initiation of the LINK RESET signal to reset logic circuit 720, which responds by issuing both MODE REG RESET and ADDR REG RESET signals to latches 736, 754 and 748, 752 respectively. The address latches 748, 752 have already been cleared in response to the CALL BUSY signal to the reset logic circuit 720.

If a line card 300 is seized through a pair of crosspoints 708, e.g., the pair labelled LINE 17 ANLG and LINE 17 DXP, the DC voltage signal from the line card passive port is received via the latter (DXP) crosspoint and fed via the COMMON DRAIN LINE DXP connected from all the drain electrodes of the digital crosspoints labelled LINES 17-30 DXP to the DXP detection circuit 718, which responds in the same manner as it does to the DC voltage signals received from the link B-port of a station card over the STA COMMON DRAIN conductor, as previously described.

If the passive port seized by link 700 is in a line card 300, it is possible for the calling party to effect multi-line conferencing (seizing of a second line card 300), flash signalling (breaking loop current long enough to drop the called party while retaining the line), placing the called party on hold, and privacy release, all in response to DC current signals to the current detector 702 which will respond thereto by generating a voltage output to voltage comparators 724, 726 and 728 as described earlier herein. Those voltage comparators provide their outputs to mode control logic circuit 730, which responds when enabled by the CSSE signal to cause current sender 704 to generate a DC signal current at the same level as the DC signal current received by current detector 702.

Upon termination of a completed call, internal or external, the calling party's station card 200 ceases to send DC signal current to the link 700, causing the output of current sensor to drop below the triggering threshold of voltage comparator 722. Thus the LINK DXP signal goes to +2 v., indicating that the link is again seizable, and the SEIZE signal to request logic circuit 744 is terminated. The register has previously been disconnected in response to signal T2 from mode latches 736, which overrides the SEIZE signal from comparator 722. The LINK RESET signal is now generated to cause reset logic circuit 720 to generate MODE REG RESET and ADDR REG RESET to clear all of the latches 736, 748, 752, and 754.

Referring specifically to FIG. 8, the register circuit 600 shown there may be connected to any number of link circuits 700, e.g., four, as shown, any one of which can send a LINK REQUEST signal to link search circuit 628, which responds to that signal by sending a COMMON LINK REQUEST signal to register control timing circuits 626. If it is not in the ready status, circuit 626 will not send SEARCH PULSES, which have the same width (2 milliseconds) and frequency (250 hz) as the output of clock generator 610, to circuit 628, and the register's tristate buffers 630, 632, 634 and 636 will not be enabled to pass data to the link. If it is in the ready status, circuit 626 will send SEARCH PULSES to the link search circuit 628, which responds by sending a constant logic level ENABLE signal to tri-state buffer 630 and by transmitting a logic level LINK CONNECT signal to the specific link identified by the data output from link search circuit 628 to the buffer 630. A LINK FOUND signal is also sent to timing circuit 626, which responds by sending an ENABLE signal to tri-state buffers 632, 634 and 636. Buffer 632 then provides a logic level LINK DATA ENABLE signal to the specific link identified by the data output from link search circuit 628, and buffer 634 sends a LOAD ADDR signal to all the link circuits in the system, to which its two outputs are commonly connected. Timing circuit 626 then sends a SEND REG ADDR signal to output logic circuit 622, causing it to feed the register's stored two-digit address, (97 or 98) to the output tri-state buffer 636, the first digit being sent in four-bit binary code to the link data tens inputs, and the second digit being sent in the same code to the link data units inputs. The timing circuit 626 then sends the SEND REG MODE signal to output logic 622 and subsequently terminates the ENABLE signal to buffers 632, 634 and 636 and sends (1) the DIAL TONE & DIALING signal to the register reset and time-out circuit 606 to enable it to generate an output and (2) the inverse of that signal to LED 642 to energize it. Meanwhile, the link 700 closes the REG crosspoint, through which its ANALOG LINK BYPASS conductor is connected to the analog decoupling and current detect circuit 602, which normally receives the DIAL TONE ENABLE signal from register reset and time out circuit 606, and consequently allows its DIAL TONE input from tone card 500 to pass through the link 700 to the seizing station card 200 and its associated station set 100. Dial signals may now be received by the register 600 from the station set 100 via the station card 200 and link card 700. Both dual-tone multi-frequency (DTMF) dial signals and dial pulse train signals are received by analog decoupling and current detection circuit 602, which incorporates a transformer with one winding connected to the link circuit(s) 700 in the system, and the other winding connected to the tone filtering and decoding circuit 612. The current detector may be constructed in a manner similar to the current detector in the hotline B-port of station card 200 (FIG. 4). DTMF signals are converted into four-bit binary tone data by the decoder 612, and that data is fed to an up counter 614 which, in response to a PRESET signal generated by control logic 608 in response to the TDV (Tone Data Valid) signal from decoder 612, is enabled to hold each dialed digit for transmission to the binary-to-decimal decoder 616 in the same sequence in which the binary-coded digits were received. The digit assignment control logic 608 is normally in a waiting state, and is enabled either by the TDV signal from decoder 612 when DTMF dial signals are received, or by the B signal (interdigit time signal) from outpulse filter 604 when dial pulse train signals are received. Then, the logic circuit 608, under the timing control of clock 610, causes the first digit to be received by the B/D decoder 616 from counter 614. While it is held in the B/D decoder 616, the first digit is tested for validity by the control logic 608. If the first digit is a 0, 1, 2 or 9 (prefix code numbers), it is not passed to invalid dialing trap circuit 620, but is held in function latch 618. If the first digit is not a 0, 1, 2 or 9, it is passed to trap circuit 620 in the form of a logic level signal on one of ten conductors representing decimal numbers 0 through 9 in response to a control logic 608 signal to trap circuit 620, which in turn generates the SEND REORDER ADDR to output logic 622. This command is executed by sending the reorder address (97) stored is hardwired memory 624 through buffer 636, which has again received an ENABLE signal from control timer 626 in response to the SEND REORDER ADDRESS signal, to the link address latches 748, 752 in response to the SEND MODE and SEND ADDR signals sent in sequence to the output logic 622. The control timer then resets itself, and sends the RESET IDLE signal to link search circuit 628. As the control timer 626 goes through these steps, it causes status indicator LED 644 to be momentarily energized, followed by constant energization of LED 638 until the register status is changed by data received from another register in the system.

The control logic 608 will cause each digit in a dialed number to be passed in sequence to B/D decoder 616 for validity testing, and the sequence of events described above will be carried out in response to any invalid digit.

If the dialing number is valid, trap 620 sends a DIALING DONE signal to register control timing circuit 626, which responds by again sending an ENABLE signal to buffers 632, 634 and 636, and then providing the SEND MODE signal to output logic 622 to cause transmission of the logic level output representative of a 0, 1, 2, or 9 from function latch 618 via the appropriate one of the link data units paths through buffer 636 to the connected links' inputs to current mode latches 754. Simultaneously, a pulse is sent by output logic 622 via a predetermined link data tone path through buffer 636 to the connected links' inputs to mode latches 736. Register control timing circuit then transmits a SEND ADDR signal to output logic 622, causing the address held in counter 614 to be sent through output logic 622 and buffer 636 to the link address latches 748, 752. The register control timer 626 then sends a RESET IDLE signal to link search circuit 628, which in turn terminates the ENABLE signal to buffer 630, which responds by terminating the LINK CONNECT signal to link connect logic 742.

When the dialed signals received through the link are current pulse trains, the current detector 602 generates corresponding digital logic level voltage pulses which are passed through pulse filter 604 to counter 614, which counts the pulses and holds the digits represented thereby in four-bit binary code for transmission to the B/D decoder 616 in the same sequence in which the digits' respective pulse trains were received, and in the same manner as described above in the case of DTMF dial signals.

The pulse filter circuit 604 generates four logic level voltage outputs A, B, C and D which are fed to reset and time out circuit 606 along with the TDV (tone data valid) signal from tone decoder 612. The DIAL TONE ENABLE signal is generated only when the A, B, D and TDV signals are logic 0s and C is logic 1. Signal A is the filtered off-hook signal, which goes high after receipt of the first dialed pulse and then goes low during each pulse. Signal B is the inter-digit time signal, which is high between each pulse train and low during each pulse train. Signal C is the inverse of signal A, i.e., it is a voltage duplicate of the dialed current pulse trains. Signal D is the on-hook terminate signal, which is a logic 0 generated when the caller's station set is on-hook, and thereby resets the register. The TDV signal is normally low, and goes high during each valid DTMF signal received. The signals A, B and TDV are all utilized to trigger reset and time-out circuit 606 after it has been enabled to issue an output by the DIAL TONE & DIALING signal from control timer 626. Specifically, signal A triggers the timer 606 which measures the time from its receipt of the SEND REG MODE signal from control timer 626 until the receipt of the first dialed signal (DTMF or pulse trains). If this time period exceeds a predetermined maximum (e.g., 10 seconds), the circuit 606 will cause the control logic circuit 608 to send a signal to the trap circuit 620, which responds by issuing the SEND REORDER ADDR signal. The sequence of events described earlier will ensue. The B signal triggers the timer 606 which measures the period between dialed pulse trains. If that period exceeds a predetermined maximum (e.g., 10 seconds), the circuit 606 causes generation of the SEND REORDER ADDR in the manner described above. Signal C has only the function of combining with signals A, B, D and TDV to cause the circuit 606 to generate the DIAL TONE ENABLE signal. Signal D also causes the termination of the SEND REORDER ADDR signal by causing circuit 606 to terminate the signal it sent to control logic 608 in response to signal A. The TDV signal triggers the timer 606 which measures the period between the termination of each DTMF signal and the initiation of the following DTMF signal. If that period exceeds a predetermined maximum (e.g., 2 seconds), the circuit 606 generation of the SEND REORDER ADDR signal as described above.

The register control timer 626 is connected to at least one other register, with which it exchanges data indicating register status. Specifically, if the register shown in the drawing is in the ready status, the SEARCH ENABLE OUT signal is present, and the SEARCH ENABLE IN signal from the other register is absent, indicating that it is in the standby status. The inverse is also true, i.e., if the register shown is in the standby status, the SEARCH ENABLE OUT signal is absent, and the SEARCH ENABLE IN signal from the other register is present, indicating that it is in the ready status. If the register shown is sending mode or address data to a link, the DATA LINK ENABLE OUT signal is present and will prevent the other register from sending mode or address data. If the other register is sending mode or address data to a link, the DATA LINK ENABLE IN signal is present, and will prevent the register shown in the drawing from sending mode or address data. This mutual exclusion feature is necessary because of the common data output connections of all registers to all links. This complete signalling arrangement causes the registers to share their workload approximately equally, since the register which is idle while the other is functioning and will handle the next link request.

Referring specifically to FIG. 9, the page card circuit shown there includes five passive ports, one in each of zone 1, zone 2, zone 3 and zone 4 circuits and one in the all-call circuit. In the several zone circuits, each of which is identical to the zone 1 circuit shown in detail, a DC current signal-blocking transformer 402 has a shunt resistor 404 across one winding, both being connected to current sensor circuit 408, which is constructed in the same manner as the combination current sensor-voltage sender in the link B-port (FIG. 4), and responds to a DC seize current (1.6 ma) received from an active port of a station card 200 or link card 700 to generate two voltages, one to the associated crosspoint(s) in the link card(s) 700, and another as an input to voltage sender 410, which in turn generates a DC voltage signal (-2 v.) indicating seizure to all of the station cards 200 to which the output of voltage sender 410 is connected. A paging message can then be transmitted from the seizing station card 200 or link card 700 through transformer 402, volume-controlling variable resistor 406, resistor 412, normally-conductive MOSFET 414, and amplifier 416 to the group of station cards selected to form zone 1, each of which receives the paging message as the PAGE input to the B-port switching circuit 246. OR gate 422 also has received the voltage output of current sensor circuit 408 as an input which causes generation of the BGM DISABLE signal, which is fed to the BGM interface circuit 530 on the tone card 500 (FIG. 6) to interrupt the transmission of background music until the page card passive port is no longer seized. In the station card 200 associated with the station set 100 from which paging is initiated, the PAGE ENABLE signal to JCT FET 291 in the B-port switching circuit 246 (FIG. 4) is terminated for as long as the page button is depressed in the station set 100, so as to block transmission of the paging message to the originator's station set.

When the all-call passive port is seized by a DC current signal (1.6 ma) from an active port of a station card 200, current sensor 430 (identical to 408) generates one output voltage to the associated crosspoint in the link 700 cards, and another voltage which causes generation of a DC voltage signal (-2 v.) by voltage sender 432, both voltages indicating seizure of the all-call circuit to all the station cards 200 and link cards 700 to which it is connected, and also causes generation of the BGM DISABLE signal by OR gate 422. The output of voltage sender 432 is fed as an input to OR gate 440, causing it to render MOSFET 414 (and its counterparts in zone circuits 444, 446, and 448) non-conductive. Thus, an all-call page will pre-empt a page to any single zone. The all-call page message is received from the seizing station card 200 and transmitted through transformer 424, normally-conductive MOSFET 434, and resistor 438 to all of the zone circuits, which receive and transmit the message to the station cards 200 in their respective zones, e.g., in the zone 1 circuit through resistor 420 to the input of amplifier 416, the output of which is fed to the station cards 200 in zone 1.

When the NR ENABLE signal is received from the night ring control circuit 526 on tone card 500 (FIG. 6), it is fed as an input to OR gate 440, to inverter 436, and to JCT FET 442. Consequently, any paging message is blocked because MOSFET 414 and its counterparts are all driven non-conductive by the output of OR gate 440, and the MOSFET 434 in the all-call circuit is driven non-conductive by the output of inverter 436. The CO AUDIBLE TONE is passed through JCT FET 442 and resistor 438 to all of the zone circuits (1) until the incoming call is answered, thereby terminating the NR ENABLE signal as a result of termination of the INCOMING CALL ENABLE to the tone card 500 because of termination of ringing detection in the line card circuit 300, or (2) until the caller terminates the call before it is answered, thereby causing the same sequence of signal termination as in (1) above.

Referring specifically to FIG. 10, the conference card circuit 600 shown there consists of a passive port which is of essentially the same construction as the hotline B-port 238 (FIG. 4), the differences being that it does not provide an outgoing communication path and, when seized, causes the DC output of voltage comparator 618 to go from +2 v. to 0 v. Thus, the conference card circuit 600 acts only as a meeting point for internal conferences, i.e., it is a passive port which enables connection of all active ports which are connected to it to each other. Once this circuit is seized by a first caller, who has previously requested (e.g., by paging or by intercom call) several other system users whose station sets 100 and station cards 200 have this feature to meet him by actuating the conference button or key in each of their station sets, the visual indicators associated with the conference buttons will be energized in response to the 0 v. DC output voltage from voltage comparator 618, indicating that the conference card circuit is in use. Thus, anyone whose station set 100 and station card 200 has this feature, but who has not been requested to join the conference, will be aware that the conference card circuit is in use and that he should not use this feature. However, any such uninvited system user can join in a conference, since any number of station cards 200 can seize the conference card circuit simultaneously. There is no provision for exclusion, except by selection of the station sets 100 and station cards 200 which are to have this feature.

In summary, the electronic key telephone systems and the various component circuits thereof disclosed herein provide a high degree of reliability owing to the distribution of control on a station-by-station basis and implementation of the control of each station by means of a stored-program digital microcomputer with non-volatile program storage. A high degree of flexibility is also provided by the removable microcomputer, which may be either replaced by another microcomputer with a different program, or reprogrammed and re-inserted into the station card circuit 200, in order to implement different or additional telephone services. The program set forth herein on pages 21 through 44 will enable the station card 200, in cooperation with the various other system circuits 100-700, to provide the various telephone services and features listed hereunder:

1. Handsfree talkback on intercom calls to another station set: the called party can answer from across his desk or across the room without touching the telephone. A volume switch is provided to control the volume of transmissions received handsfree without affecting the volume setting for background music.

2. Handsfree origination for both internal and Central Office line calls: by depressing a handsfree key, and then depressing either the key for the desired internal station, or the key for a C.O. line, the caller can both dial a call and then conduct a handsfree conversation by means of a speakerphone built into the station set without ever lifting the handset.

3. Multi-line conferencing: the caller can establish a first C.O. line call, put it on hold, establish a second C.O. line call, put it on hold, and then depress both C.O. line keys to establish a conference between the caller and the two outside parties.

4. Busy station display: provided at all station sets to allow a caller to know what stations, C.O. lines, etc. are busy before placing a call.

5. Music-on-hold: provided to any outside party on a held C.O. line.

6. Background music: provided to system users at their station sets, which have an on/off/volume control switch to enable a user to control the level of background music without affecting the volume setting for the handsfree feature.

7. Simultaneous call handling: the systems allow two simultaneous calls to be on any station set at the same time, one on the handset and one on the speakerphone, without having to place either call on hold.

8. Automatic privacy: no action needs to be taken by a station set user to ensure privacy, but he may release the privacy feature to allow other station set users to have access to the called C.O. line.

9. Paging: the systems provide for paging to as many as five zones, plus all-call paging to all zones, each accessed by different dial codes. Also, each station set has a paging key that may be used to access any one of the zones or all-call directly, without dialing.

10. Meet-me conference: internal group conferences may be held by having each conferee depress a conference key on his station set, which connects each of the conferees' station sets to every other conferee's station set.

11. I-hold signalling: the visual indicator on the line(s) which a user has put on hold flashes in a distinctive manner to identify those lines.

12. Hotlines(s): each station set can reach one or more other station sets directly, without dialing, by depressing a hotline key. Access to a hotline-connected station set is guaranteed, even if it is in use.

13. Single key intercom operation: an intercom key on the station set may be depressed to seize the first available link and register, by means of which a caller may dial an internal call. Intercom calling may be conducted handsfree by the caller and the called party, and may continue for unlimited periods of time without tying up registers. For privacy, or if in an environment with a high ambient noise level, the caller or the called party may switch the call to his handset by picking up his handset.

14. Access to lines that do not appear at the telephone: in a non-square system, or a system having private lines, a user may dial into a line that does not appear at his station set by depressing the intercom key and dialing a pre-determined number.

15. Handsfree mute: by depressing a mute key in the station set, the user can disable the microphone which is normally operative in the handsfree mode. Thus, handsfree calls can be placed in a monitor-only mode. A caller may also establish privacy to talk to someone at his location without the called party overhearing. A second depression of the mute key reenables the microphone.

16. Do not disturb: by depressing a DND key in the station set, it is made to appear busy to all internal calls. A subsequent depression of this key restores the station set to normal availability.

17. Alternate point answering (call steal): an internal call may be answered from another telephone using the intercom key to seize a link and register, and then dialing 2 followed by the number of the telephone to which the call is directed. If the call is a handsfree voice call, it may be intercepted in this manner only within about 30 seconds after it has been received. If the call is an ICM RING call, it may be intercepted in this manner for as long as the caller is ringing.

The advantages of the present invention, as well as certain changes and modifications to the disclosed embodiment s thereof, will be readily apparent to those skilled in the art. It is the applicant's intention to cover all those changes and modifications which could be made to the embodiment s of the invention herein chosen for the purposes of the disclosure without departing from the spirit and scope of the invention.