CA1282506C - Multiplexed data channel controlled telephone system - Google Patents

Abstract

Abstract of the Disclosure A multiplexed data channel telephone system comprising a master station and one or more remote stations is disclosed. Each station is bridged, in a parallel electrical connection, onto a wire-pair that extends from the stations to a telephone switching system. Each station includes a voiceband line interface for connecting to the wire-pair at voiceband frequencies, and a radio frequency line interface for connecting to the wire-pair at radio frequencies. Each station further includes multiplexer and demultiplexer equipment for frequency shifting a data channel between baseband and rf. Amplitude modulation and half-duplex transmission are used in the data channel. Telephone status and control information, including ringing and answer control, are exchanged between processors via the data channel. Remote stations minimize the amount of equipment required by using the data channel to control common equipment at the master station that interacts with incoming telephone lines.

Description

T~LEPHONE SYSTEM

Field of the Invention The ~resent invention relates to telePhone systems havina a master and one or more remote stations and more particularly to a system in which the master station per~orms telephone control functions under the control of a remote station.
Ba~karou~d Qf ~he Invention It has become incre~singly desirable in our communications-oriented society to uP~rade and imProve business and residential tele~hone e~ui~ment by addinq telephones having improved functionalitv and increasinq the number of incoming telephone lines at minimal cost.
However, sinc~ one cannot expect to find more than a single wire-pair at any given location, it is frequently necessary to su~ply additional wirinq be~ond the single wire-~ir used or basic telephone service in order to increase the number of communication channels. Costs associated with addin~ or relocatinq wires can be significant. Furthermore, many commercially available systems that are used to increase information distribution caPacity are incompatible with existinq telephone equipment, so a user's investment is immediately eroded.
From a convenience standpoint, it is desirable for a customer to merely purchase a telephone Product, take it home, and Pluq it into an existinq telePhone jack. Many systems that utilize the local telePhone wire-pair at a user's premises require that a master station be placed in a series connection with the incomin~ wire-pair from the tele~hone central office.
Series connections require that internal wirinq be 2S~
--~ 2 modified which usually means that a telephone installer or an electrician needs to be hired.
A problem, however, with having multiple telephone stations share the same wire-pair is that each one presents a parallel 600 ohm AC impedance in its off-hook state. As a result, both the transmit and receive signal levels are reduced so that users frequently strain to hear each other when more than two are involved in the conversation.
It is therefore an object of the present invention to provide telephone apparatus having improved functionality and is compatible with existing telephone extensions that share a common wire-pair.
It is another object of the invention to provide substantially constant transmit and receive signal levels, regardless of the number of stations at one premises, in a multiple party conferencing call.
Summary of the Invention In accordance with one aspect of the invention there is provided telephone apparatus for installation a-t a subscriber's location and interconnection with a wire-pair that extends from the subscriber's location to a telephone switching system, the telephone apparatus including a master station and one or more remote stations bridged onto the wire-pair in a parallel electrical connection, each station including a hybrid circuit for interconnecting electrical signals, at baseband, between the wire-pair and a microphone and loudspeaker, CHARACTERIZED IN THAT the master station comprises: a processor for controlling the operation of the master station in accordance with the stored program, said processor exchanging signaling information with the telephone switching system over the wire-pair at baseband, and exchanging status and command information with the remote stations over the wire-pair at frequencies above baseband; a data multiplexer for frequency shifting status information, generated by the processor into the frequency band above baseband; means interconnecting the data multiplexer with the wire-pair; a data demultiplexer for frequency shifting ~82~6 2a multiplexed command information, generated by the remote station, to baseband; means interconnecting the data demultiplexer with the processor; and the remote station comprises: a station housing including visual display devices and one or more manually operated switches for generating command information: a data multiplexer for frequency shifting command in~ormation generated by the manually operated switches into the frequency band above baseband; means interconnecting the data multiplexer with the wire-pair; a data demultiplexer for frequency shifting multiplexed status information, generated by the processor at the master station, to baseband; and means interconnecting the baseband status information to the visual display device~
In accordance with another aspect of the invention there is provided improved apparatus for installation at a subscriber's location and interconnection with a wire-pair that extends from the subscriber's location to a telephone switching location, the apparatus including a master station and at least one remote station bridged onto the wire-pair in a parallel electrical connection, each station including an interface circuit for interconnecting electrical signals, in the audio frequency band, between the wire-pair and a microphone and loudspeaker, CHAR~CTERIZED IN THAT the master station comprises: means interconnected with the wire-pair for detecting the presence of ringing signals thereon and for generating a digital signal indicating the presence or absence of ringing signals; a data multiplexer having input and output ports, responsive to said digital signal at its input port for frequency multiplexing same into a frequency band above the audio frequency band at its output port; means interconnecting the output port of the data multiplexer with the wire-pair;
and the remote station comprises: a data demultiplexer, responsive to the multiplexed digital signal, indicating the presence or absence of ringing, for converting same into an alerting control signal at its output port; and means responsive to said alerting control signal for generating an audible alerting signal.

.~
~s~

~3ZS06 2 b A Multiplexed Data Channel Controlled Telephone System comprises a master telephone station and one or more remote telephone stations that are bridged onto a single wire-pair at a user's business or residential locatian in a parallel electrical connection. These customer installable stations are merely plugged into any telephone jack and share a telephone wire-pair with existing telephone extensions without interference. Both master and remote stations operate similar to conventional telephone extensions.
The term "extension", as used herein, denotes a standard telephone set that can only access an audio frequency telephone channel (typically below 4kHz). The term "station", as used herein, denotes a telephone set according to the invention that includes frequency multiplexing and demultiplexing equipment used to create a data channel at frequencies that do not interfere with telephone extensions already present on the wire-pair.
The data channel is used, for example, to communicate -1~325~

station control information such as line status, rin~in~
and hold.
The master station contains electronic circuitry that interacts with a telephone line switchinq system te~g., central office, PBX, or keY e~uiPment) for dialing, ring detection, answerin~, and impedance matching. It exchanges control si~nals with each remote station via the frequency multi~lexed data channel.
Each remote station includes a line interface 19 circuit havinq a relatively hi~h impedance. Thus when a pluralit~ of remote stations wish to ~articiPate in a conferencing arrangement, transmitted and received signal levels are not significantly attenuated.
It is a feature of the ~resent invention that the circuits used for dialinq, rin~ de~ection, and DC
sei~ure of the telephone line reside onl~ in the master station and ma~ be controlled by an~ remote station via the multiPlexed data channel.
It is another feature of the invention that the remote stations do not appreciably load the rec~ived siqnal since the AC terminatin~ impedance is Primaril~
su~plied by the master station.
These and other objects and features of the Multipl~x~d Data Channel TeleDhone Svstem are discussed in qreater detail in the detailed descriPtion o~ the invention, and will be understood more full~ when reference is made to the accomPanying drawin~s.
~ief Desc~ ion of tbLe Dr3~i~
FIG. 1 discloses a block dia~ram of the multiplexed data channel telephone system in accordance with the invention;
FIG. 2 is a dePiction of a customer's Pr~mises showing how the Present telePhone line carrier system may be installed;
FIG. 3 is a drawin~ of the base for a master or remotR station illustratin~ various features and functions that are available;

~X~32S~6 FIG. 4-5 is an enhanced block diaqram of the master s~ation according to the invention FIG. 6 is a schematic dia~ram of a data transceiver used in the frequency modulation and demodulation of di~ital data exchanged between master and remote stationss FIG. 7 is a schematic diaqram of an RF
interface used to interconnect frequencY multi~lexed signals with the tele~hone wire-Pair;
FIGo 8 is an enhanced block diagram of the remote station accordin~ to the invention, FIG. 9 illustrates the frequency allocation of the radio frequency channels used in the present invention~
FIG. 10 illustrates various waveforms associated with data transmission usinq a Pulse width encoded - non return to zero format and on/off carrier keying;
FIG~ 11 discloses the tele~hone line carrier system message packet for the d~ta communications channel; and FIG. 12 ~hows the manner in which FIG. 4 and 5 interconnect.
~3iled ~ iE~
The subject Multiplexed Data Channel TelePhone System is a ~user friendly" enhancement to a residence or business communication system. The System com~rises a master station and one or more remote stations that ~luq into any available wall tele~hone jack and share the intraPremises telephone ~ire-Pair with existinq tele~hone extensions without interference. Each station requires Power from a standard 115 volt source. The master and remote stations Provide a number of frequency multi~lexed channels above the audio frequency channel (below 4kHz) used for standard telephone service. A
rudimentary system havin~ one master and one remote station enhances existin~ telephone service by Providing 1~325~)6 two more telePhone sets that can access the existinq telephone line Plus add an intercom channel for internal communications between the newly added stations. Of ~reater significance, however, are the possibilities offered by such a system. Heretofore, individual telephone sets have not been ~iven control over the combination of one or more voice channels and a data channel; and, it is the combination of voice and data channels that yields immense benefits.
Frequency multiPlexed voice channels are used to distribute si~nals from incomin~ tele~hone lines as well as music and intercom signals. The data channel con~rols access to the various voice channels and delivers line status information to all stations.
Functions such as ener~y management, alarm rePorting and a~liance control are accommodated b~ the data channel and made available over the existin~ wire-pair.
FIG. 2 provides a broad view of the System after installation at business or residential Premises 200. The System is shown accommodating two independent telephone lines 211, 212 from the telePhone line switchin~ system. These lines terminate in protector block 210 which Prevents hi~h voltages caused, for example, b~ htnin~ from harmin~ people or telePhone equipment at Premises 200. Prior to the installation o~ a second telePhone line, telephones 231, 232 were the only sets sharing wire pair 201. In accordance with the ~resent invention, master station 221 and remote station 222 are customer installable telePhone sets that brid~e onto existinq telephone wire pair 201. AC Power is required at all stations. Stations 221, 222 ~rovide access to Line 1, intercom service between the stations, and a data channel for distributin~ telephone control in~ormation such a.s line status, key~ad activation status, and alertin~.

~Z82506 A second line 212 from the central office is connected directly to master station 221 over internal wire-pair 202. Master station 221 terminates line 2 and provides direct access to it at the master station, or frequency multiplexes it onto line 1 for access by remote station 222.
The block diagram description of the Telephone Line Carrier System, shown in ~IG. 1, is hereinafter discussed.
Master station 221 bridges onto existing telephone wire-pair 201 in a parallel electrical connection with conventional telephone plugs and jacks. Power to operate the master station is provided by power supply 190 which derives its energy from a 115V AC source. Line 1 interface 110 includes a ringing detector 111 (see FIG. 4) in parallel with wire-pair 201 and a coupling transformer. Components are selected for operation in the audio frequency ranye where standard telephone service ls offered. Line 1 interface 110 communicates with handset/speakerphone unit 140 through telephone hybrid 120 to deliver incoming speech to a loudspeaker and outgoing speech from a microphone to wire-pair 201. A user may optionally select the handset or speakerphone mode of operation. Master station 221 also converts audio frequency signals associated with handset/speakerphone unit 140 to Radio Frequency (RF) signals for distribution over wire-pair 201. In this regard, frequency multiplexer/demultiplexer 500 uses frequency modulation for the voice channels and amplitude modulation for the data channel. RF interface 700 is a hybrid circuit whose components are selected for operation at radio frequencies.
It operates to interface a four-wire circuit with two-wire telephone line 201. Handset/speakerphone 140 connects to wire-pair 201 through line interface 110 at audio frequencies, or through RF interface 700 at radio frequencies.

,, , ~

325~6 In situations where a second telePhone line is needed, line interface 170 terminates wire-pair 202 (Line 2~, and is functionally equivalent to line interface 110.

~AST~R STATION
Referring now to FIG. 4 and 5, wire-~air 201 (Line 1) connects to the master station via line interface 110. Rin~ detect circuit 111 is responsive to incoming ringin~ signals and su~lies an indication of same to controller 160. ~elephone rin~in~ detector circuits are well known and t~pically com~rise a Pair of series connected avalanche diodes whose anodes (or cathodes) are tied together. These diodes are in series with a ca~acitor to eliminate DC. Current flow in ~he series path is detected b~ An opto-isolator circuit whose outPut is electrically remote Erom hiqh vol~a~e line signals. An example of a commercially available ringing detector circuit is the TCM 1501. Activation of relav contact LRl completes a DC path on Line 1 such a5 required for answerin~. Resistor 113 Provides a 600 ohm termination for proper impedance matchin~.
Switch Ll is shown in its normal "make" state, thus connecting signals from Line 1 to telePhone hybrid 120. Resistor 126 is large compared to 600 ohms so as not to a~preciably chan~e the termination impedance reqardless of the state of Ll. Incominq signals to h~brid 120 ~ass throu~h resistor 126 and DC
isolation caPacitor 127. Out~oin~ si~nals are buffered by transistor 124 and resistor 125 which Provide impedance matchinq to the line and is functionally similar to an emitter-follower circuit.
Line interface 170, tele~hone hybrid 180, and components 18~-187 are associated with wire-Pair 202 (Line 2). These circuits operate in the same manner as the correspondin~r above-described circuits that service Line 1. Rel~y contact L2, when operated, interconnects ~28251~6 si~nals from Line 2 to telePhone h~brid 120 and hence to handset/speakerPhone 140 thereby allowin~ the master station user to converse over Line 2. Line 1 and Line 2 are interconnected when relay L2 is ener~ized but relay Ll is not. Conferencing is thereby achieved.
Touch-tone ~enerator 167 includes circuitr~
for simultaneousl~ ~enerating a pair of tones such as used in conventional touch-tone si~nalinq. Leads 168 from controller 160 select the particular tone Pair to be generated in accordance with the particular key de~ressed in keyPad/switch 166. Dual tone multifre~uencies si~nal ~enerators, such as the TP 5088 used in touch-tone si~nalinq, are available in integrated circuit ~orm from a number of manufacturers.
An associated crystal operatinq, for examPl~, at 3.58 MHæ provides frequency stabilit~. Siqnals from touch-tone generator 167 are selectably a~plied to Line 1 or Line 2 via transfer switch TTO. To Prevent these strong si~nals from annoying the user, controller 160 operates relay switch HTR durin~ touch-tone dialin~.
Transmission between the various stations in the TLC system Proceeds at radio frequencies according to the frequenc~ spectrum allocation set forth in FIG. 9. Data communication takes place in channel D
which ls centered at 455 kHz. All stations transmit and receive at this ~requenc~ usin~ amplitude modulation.
Voice communication takes Place in channel IA, IB, 2A, 2B and 3A, 3B when, for example, a third telephone line is used. Stations transmit and receive at the frequencies indicated in FIG~ 6 usin~ frequency modulation.

VOI OE COMMUNI CATION
In the ~referred embodiment of FIG. 4 the master station includes two Pairs of FM transmitters and receivers for communicatin~ with remote stations and - g ~X~3Z5~6 bridgin~ them in a conference connection. Each of the FM transmitters and receivers of FIG. 4 o~erate on fixed pre-assigned channels. Remote stations, on the other hand, have only one FM transmitter and receiver whose o~erating frequencies are switched between the various ones available de~ending u~on whether the intercom or Line 2 switch is activated, and whether the call is being oriqin~ted or answered at the remote station. The selection of ~requencies is discussed in connection with FIG. 8 and is controlled by the state of control leads IRC, IOC and L2C from controll~r 360 at the remote station.
FM transmitters 561 and 571 oPerate with a center frequency of 347 kHz and 517 kHz resPectively.
These transmitters are enabled when signals TXEN2 and TXENI are in their low state. Audio si~nals Presented to the FM transmitters are limited at 0.5V ~eak to peak and include a pre-emPhasis sta~e, a modulator such as the LM 567, and a buffer sta~e for interconnection to RF
interface 700. Each o~ the FM receivers 562, 572 includes a buffer amPlifi~r, channel select filter, demodulator such as the MC 3357, and a frequency de-em~hasis circuit. Becaus~ o~ the ~roximit~ in frequency between channel IA and channel D, ~ 455 kHz trap is used in the FM receiver of channel IA. A more im~ortant s~eciication however is the modulation index - equal to the ratio of the frequency deviation to the hi~hest modulatinq frequency. FM transmitters 561, 571 o~erate with a modulation index of 3.3. Since the FM
transmitters are directly coupled to the tele~hone line, the maximum allowable power is set bv Part 68 of the FCC
Rules. Ener~y in the ~requency band 270 k~z - 6 MHz must be less than -15 dBv when avera~ed over a 2 microsecond interval. ~he desiqn of such FM
transmitters and receivers is well known and, qiven the specifications Provided herein, may be readily constructed usinq conventional circuit desiqn techniques.
~A

~2~32S~6 DATA COMMUNICATION
The Data Communication channel is desiqned to be an inexPensive technique for reliable communications in a potentially nois~ environment. The communications protocol is implemented in firmware usin~ the controller of the master ætation and the remote station. The particular controller has inteqrated har~7are for a dual-edge detection vectored interrupt and an event timer with vectored interrupt on overflow. The data channel uses a half-duplex broadcastin~ scheme with each station having access to the same channel. To facilitate orderlv access to the data channel, all stations look for incoming data before transmittinq.
The channel must be idle for 12 ms before attemptin~ to transmit a new messa~e.
The data transmitter is reæ~onsive to a binary digital signal for turnin~ a 455 kHz carrier off and on.
First, however, the binary diqital siqnal is encoded into a series of pulses havin~ alternatinq ~olarity in which a "1" has a duration o~ 1 ms and a "0" has a duration of 2 ms. Accordingly, if "0" and "1" are e~ually likely, the nominal 6~gnaling rate for the data channel is 667 bits per second. This si~nalinq scheme is known as the Pulse Width Encoded - Non Return to zero (PWE-NRZ) format. The data receiver converts the presence and absence o~ ~55 kHz carrier into discrete levels and thereafter performs a~proPriate decodin~ to recover the binary di~ital si~nal. FIG. 10 discloses an exemplar~ binar~ diqital signal (10110001)~ that is pulse width encoded and thereafter used to am~litude modulate the 455 k~z carrier. Althou~h on/off carrier keying and half-duplex transmission are used in the preferred embodiment of the ~ata Communication channel, it is clear that other forms of modulation and transmission may be used within the æco~e of the invention.

2~325~

FIG. 11 discloses the TLC message ~acket which is defined to be 27 bits long and broken down into the following fields.
Header: 3 bits, '110' to access the channel and announce the forthcomin~ messa~e.

Destination: 4 bits, used to identif~ which receivin~
stations should consider Processing the messaqe.
A message can only be ~rocessed if the destination address matches that of ~he station address or if the destination address is '0000', the ~lobal addressing value.

Source: 4 bits, identifyinq the transmitting station.
Stations without a programmed address should use lO000l.

15 Content: 12 bit~, containin~ the in~ormational ~art of the messa~e. The content ma~ be considered as 3 separate 4 bit fields, Function Code, Data 1, and Data 2.

LRC: 4 bits, resultin~ ~rom a Lon~itudinal Redundanc~
Ch~ck calculated over the destination, source, and contents.

All fields are transmitted msb first (see FIG. 11). A
message ~acket can range from 32 ms to 48 ms in duration. Incoming messages are examined if there has been no transition on the data channel for ~ ms. If the correct numb~r of bits has been received and the actual LRC matches the calculated LRC, the next 4 ms interval will bP dedicated to lookinq for a negative acknowledgment (NAR) from any other station. In the absence of a NAK, the message is acce~ted for destination ~creeninq and further ~rocessing. If a NAK
is detected, the message is discarded. A NAK consists ~282~6 of turning carrier ON for a 2 ms interval startin~ 4 ms after the completion of the messa~e. The absence of carrier for 12 ms is deemed to be the idle state. After 12 ms of idle, any station may start transmittinq - so long as another has not. When collision occurs, each of the stations will com~lete its message to enforce the collision. The result, then, is that the messaqe will surely be ~arbled and retransmission will be required.
A transmitting station that has its messa~e NAX'd is allowed to retransmit the mess~e after the channel has been idle ~or 6 ms to 9 ms. The actual time interval is determined b~ calculatin~ a "Pseudo-random" number from 0 to 3 and adding it to the base delay of 6 ms. The need for a random re-transmission time arises from lS messa~e collision considerations when two stations transmit at the same time, both calculatinq idle time from the same event.
The goal of the data channel mess~e is to Provide a waY to reach the correct ~lobal system state wlth no more than one data messaqe for each action taken by a user. Data channel messa~es related to facility use (e.~., status of line 1, line 2 or intercom channel) will utiliæe function code 0000 to indicate a remote station as the source and 0001 if the master station is the source. Followin~ the function code are two data nibbles (4 bits each), the first specifyinq those facilities bein~ released, and the second sPecifyin~
those facilities currently in use. The content portion of FIG. 11 is therefore organized in the followinq manner:

I Function Codel Da~a 1 I Data 2 I (4 bits3 1 (4_bits) 1 (4 bits) ¦ 0000/0001 ¦ h + facility ID¦ c + facility ID¦
where: h indicates facility on hold (1), or hunq up (0);
and, c indicates a chanqe from the previous condition (1), or a reminder of current use ~0).

.

1;~ 5(~

The identification of facilities requires the 3 least si~nificant bits of a nibble.

_ _ I FacilitY I ID I
5 1 _l I
¦ None ¦
I Line 1 1 001l ¦ Line 2 ¦ 010¦
I Line l/line 2 conference I 011l 1 Intercom ¦ 100¦
I Line l/intercom conference l 101l 1 Line 2/intercom conference 1 110¦
I Line 1, line 2, intercom (hold only)~
.

The most si~nificant bit o the Data 1 nibble will indicate how the facilit~ was released, either bY
being ~ut on hold (1) or b~ han~ing uP ~0). Any facility ~ut on hold must have the hold messaqe retransmitted ever~ 15 seconds. Any set not receivinq a hold messa~e for a "they hold" facilit~ in any 30 second interval will idle that facilit~,.
The most si~nificant bit oE the Data 2 nibble will indicate whether this message is a chan~e from the previous glob~l s~stem state or merel~ a reminder messaqe. Reminder messages must be sent ever~ 15 seconds, with an~ "they use" facility bein~ idled out if a reminder has not been received within the la~t 30 seconds. The change bi~'s only important function is to allow each station to maintain a count of how many statlons are of~ hook on intercom.
Example data channel messaqe are shown below.
These messa~es may be generated by the master or the remote station and convey a class of information different from the above described class. A function code 0010 precedes the ~ollowing messa~es:

~28~ 6 . _ ¦ ~ata 1¦ Data 2I Notes I 0000 ¦ dddd ¦ Dial line 1 req 1 0001 I dddd I Dial line 2 req ¦ 0010 1 0000 I Dial line 1 ack I 0011 1 OOQ0 i Dial line 2 ack I I 0010 I Rinq line 1 ¦ I 0011 ¦ Ring line 2 I I 0100 1 Off hook line 1 ack ¦ ¦ 0101 1 Off hook line 2 ack I 1 0110 I Cold restart station ¦ 0100 1 0111 ¦ Warm restart station ¦ I 1010 I Request global status ¦ 1100 ¦ line 1 non-s~stem off hookl I 1 1101 1 line 2 non-system off hookl ¦ I 1110 ¦ line 1 non-system on hook ¦
1 1 1111 1 line 2 non-system on hook I
_ ~ - - t 1000 ¦ addr I Intercom Paqe from master ¦
I 1001 1 addr I Intercom Page from remote I
¦ llaa L bbcc I Global system status where: aa indicates ~lobal intercom status of idle, ringing, holding or in use;
bb indicates global line 1 status of idle, ringing, holding, or off hook;
cc indicates global line 2 status of idle, ringinq, holdinq, or off hook;
addr indicates intercom s~ation address (1010 for a broadcast Page); and dddd indicates dial value DATA TRAN SMI TTE R~ RE CE IVER
FIG. 6 discloses a detailed schematic of data transceiver 510 used in ~ Preferred embodiment of the invention. This transceiver transmits and receives an amplitude modulated 455 k~z carrier wave. A digital data signal, encoded in ~he PWE-NR~ format of FIG. 10 and or~anized according to the transmission order of ~ Z~ 506 FIG. 11, is Present on inPut lead TXD and used to control transistor 541 to ~urn on and turn off a 455 kHz carrier wave Present on output lead 760~ The carrier wave is generated by Phase-locked loop 526, an outPut of which is presented to transistor 544, connected in the emitter follower confi~uration, for drivinq a filter circuit comprisin~ components 545-550, The output of phase-locked loop 526 is a square wave at the 455 kHz rate which is ~iltered to produce a sine wave at outPut 760. The center frequency of the Phase-locked loo~ is deteemined by ca~acitor 527 and resistor 528.
Dual use of component 526 is achieved by "disconnectinq"
input signals wnen carrier is being transmitted (i.e., when TXD = nOn). Transistors 540 and 543 coope~ate to ~round ~he junction between capacitors 517 and 533 and thereb~ disconnect the input siqnal from phase-locked loop 526 when TXD = non.
Data transceiver 510 also operates as a receiver of AmPlitude modulated 455 kHz carriers waves.
Input si~nals are received on lead 730 and amplified b~
a two-sta~e transistor circuit comprisin~
components 511-522 and 533. ComPonent 523 is a commercially available ~our-element ceramic filter having a 25 kHz bandwidth centered at 455 kHz. This filter is broad enou~h to Pass the spectrum of the am~litude modulated si~nal. Components 524 and 525 are used in couplin~ the filtered si~nal to an inPut of Phase-locked loop 526 for demodulation. The demodulated ou~put si~nal aPPears at the junction of resistor 531 and capacitor 532 and is available on outPut lead RXD.
~he comPonents directly associated with comPonent 526 are rePresentative o~ the manufac~urer's specifications for this commercially available device which finds use both as a carrier oscillator and as a tone decoder. An acceptable device is the LM 567 tone decoder.

GENERAL CIRCUITS
Referrinq once again to FIG. 4 and 5, handset/speaker~hone 140 includes circuitry for switching between handset 146 and loudspeakinq tele~hone S components including microphone 144 and louds~eaker 156.
Transfer switches 141 and 147 are shown in their de-activated state interconnecting handset 146 with either (i) telephone hYbrid 120, (ii) telePhone hybrid 180 or (iii) FM transmitter 571 and receiver 572.
Amplifier 148 offers two volume levels throu~h controller 160 in res~onse to a volume control switch.
When the speakerphone mode of operation is sel~cted, controller 160 simultaneously activates transfer switches ~41 and 147 to disconnect handset 146 from an out~oing line, and connect microphone 144 and loudspeaker 156 in its place. In its quiescent state, the loudspeakin~ telePhone set resides neither in the transmit nor the receive mode. The Preserlce of sufficient transmit si~nal ener~y causes the transmit channel ~ain to increase and the receive channel ~ain to decrease. The presence of ~uf~icient receive si~nal energy causes the reverse action. The net looP ~ain of the transmit and receive channels is desiqned to be less than unity ~assumin~ moderate loss between microphone 144 and loudspeaker 156) in order to ~void oscillation. LooP ~ain is the same before and after switching to assure maximum ~ain consistent with stable operation. The transmit channel includes microphone 144, attenuator 143 and amplifier 142. The receive channel princiPally includes attenuator 151, amplifier 152 and loudspeak~r 156. Control circuit 145 is responsive to si~nal enerqy in the transmit channel exceedin~ a ~redetermined threshold. When the threshold is exceeded, control circuit 145 simultaneously increases both ~he ~ain of amplifier 142 and the loss of attenuator 151. Similarly, control circuit 149 is responsive to energy in the receive channel exceedin~ a 2~06 predetermined threshold. When the threshold is exceeded, control circuit 149 simultaneously increases both the qain of amplifier 152 and the loss of at~enua~or 143. In the preferred embodiment, each of the amPlifiers and attenuators are arran~ed to switch 20 dB of ~ain when activated. By switching only 20 dB in each channel (transmitter and receiver) the time required to ~turn around" a speakerphone is minimized and the quality of four-wire transmission is substantially achieved over a two-wire circuit.
Amplifier 153 offers ei~ht different ~ain settings controllable over lines 155 (3 leads) ~rom the controller. LoudsPeaker 156 is also used to Provide a modulated 750 H~ ~or both intercom alertin~ and normal Line 1 or Line 2 rin~inq. Amplifier 157 receives an aler~ing signal from controller 160 which also supplies a signal ~or selectin~ between two rin~er volume levels over line 159. Resistors 154 and 158 are used to buffer the si~nals between amplifiers 153, 157 and louds~eaker 156.
Controller 160 is a 4-bit micro~rocessor havin~ 8K of ~OM, such ~s the NEC 75108, that controls all aspects of the master station in accordance with an internally stored pro~ram. A source of stability for clock signals is supplied by resonator 164 and capacitors 162, 163. Resonator 164 is a color burst crystal, such as used in conventional television receivers, having a nominal resonant frequency of 3.S8 ~Hz.
Li~ht Emittin~ Diodes (LED), collectively desi~nated 165, are illuminated ~nder the control of controller 160. Indications are variously provided to indicate the status of line 1, line 2, intercom, speaker, hands-free answer durin~ intercom (Auto Answer) mode, privacy mode and b~ckuP battery status. The state of the keypad contacts and other switches, collectivel~
desi~nated 166 are ~eriodically sampled by 12~32S06 controller 160. The various LEDs and switch contacts whose state is controlled or determined bY
controller 160 are shown in FIG. 3 and include the handset switch in the earpiece cavity of the station.
Power sup~ly 190 includes a wall-mounted transformer and circuitr~ for providinq a requlated and filtered ~5 volt sup~ly to controller 160 and other using components. Several sources of +5 volts are provided by this su~ply, each havin~ various deqrees of filtering depending on the using equipment. Two of these sources are backed uP with a +9 volt battery and are used in providinq a non-interru~tible source of Power to certain critical areas such as the rePertorY
telephone number memory.
When the station ~oes off-hook, a "battery test enable" siqnal i~ delivered to power suPpl~ 190 over lead 191. In response, ~ower suPply 190 ~rovides an indication to controller 160 that the battery voltaqe exceeds (or is less than) a Predetermined threshold.
When the batter~ is absent or low, controller 160 causes a particular LED to flash - thereb~ warninq the user that stored telephone numbers ~nd basic tele~hone service may be lost in ~he ev~nt of a po~er failure.
Another one o~ leads 192 Provid~s an indication of Power failure. Durin~ power failure, the PFC lead from the controller causes power to be a~Plied to telePhone h~brid 120 only when necessar~ and thus minimizes battery drain. Components used in controllinq the application of power lnclude resistors 121,122 and transistor 123. Durinq normal oPeration the PFC lead is in its low state and power is constantly suPplied to tele~hone hybrid 120.
FIG. 7 discloses a detailed schematic of RF
interface 700 of the master station. It Performs the function of a hybrid circuit at radio frequencies whereby transmit and receive siqnals are shuttled to and from Line 1. RF interface 700 receives sianals from FM
<,~, ~X8ZS(~

transmitter 571 over lead 740, from FM transrnitter 561 over lead 750, and from data transmitter/receiver 510 over lead 760. Each of these RF inputs are additlvely combined in transistor 707 via components 701-706 with 5 DC removed by caPacitors 701, 703 and 705. Feedback resistor 7 09 in combination with resistors 702, 7 04 and 706 set the gain of the input sta~e of interface 700.
The siqnal at the collector of transistor 711 is cou~led to ti-e PrimarY o~ transformer 721 through caPacitor 71~.
10 Transformer 721 is a 1:1 transformer suitable for operation at the RF frequencies of interest.
Inductor 722 and ca~?acitor 723 are selected to resonate at a~pro~imately 455 kHz so that lookin~ into the ~rimary of tran~former 721 the impedance of Line 1 15 (a~?~?roximately 50 ohms) is seen. It is noted that RF
interface 700 operates as a hybrid at radio frequencies.
Inductor 722 and capacitor 723 are tuned for 455 kHz because the data channel transmits and receives at that frequenc~ o it's more important that the h~brid be 20 balanced at that frequenc~ in order to distinquish transmit and receive siqnal ener~
Looking into the secondary of transformer 721 from Line 1, caPacitor 723 is selected to Provide approximatel~ 30 kilohms at 3000 Hz. RF siqnals Present 25 on Line 1 Pass throu~h transformer 721, caPacitor 715, resistor 714, Darlington Pair 717, 718 and finall~r out lead 730. The magnitude o~ resistors 712 and 714 are selected to cancel sidetone b~ combinin~ Portions of the inverted and non-inverted out~oinq si~nals present at 30 the emitter and collector terminals of transistor 711.

RE~OTE ~;TATION
Referrin~ now to FIG. 8, wire-pair 201 (Line 1) connects l:o the remote station throuqh line interface 310 in a Parallel electrical connection. Such 35 connections make the TelePhone Line Carrier s~stem easily installable by a customer usinq existinq .. . . .

2~32~6 telephone jacks at her Premises. CaPacitor 314 ~recludes DC loading when relay LRl is operated. In order for the remote station to answer an incomin~ call on wire pair 201 the following events occur. Rin~in~ is detected at the master station which then broadcasts this information to all remote stations via the data channel on wire pair 201. Data transmitter/receiver 610 responds to information in the 455 kHz frequency ran~e, performs amplitude demodulation and delivers the demodulated data over the RXD lead to controller 360.
Audible alerting at the remote station is provided when controller 360 delivers a modulated 750 Hz tone to am~lifier 357 and hence to loudspeaker 356. One of the grou~ of switches 366 is the switchhook contact, which when activated is sensed by controller 360 . In response, rela~ LRl (part of in~erface 310) is operated.
Operation of LRl, however, does not cause the telephone call to be answered since capacitor 314 Precludes DC
~low through the ~rimary o~ transformer 312. Answerinq is accomPlished at the remote station when controller 360, responsive to the operation o~ the ~switchhook (Part o~ ke~Pad/switches 366), transmits this information over lead TXD to data transmitter/receiver 610 and RF inter~ace 800 to the master station. Upon receivin~ an indication that the remote station is attempting to answer the telePhone call, the master station causes its LRl relay (~art of line interface 110) to operate and draw the necessary DC
current from the telephone line switchin~ s~stem and thereby answer the incoming call. Resistor 113 r shown in FIG. 4, provides the desired 600 ohm AC terminatinq impedance for the incoming telephone line. ThiS is accom~lished in the master station even thouqh it remalns in the on-hook state.
Returning to FIG. 8, once line relay LRl operates, AC terminating im~edance 313 is presented to wire-pair 201. In a ~referred embodiment, resistor 313 ~825(~16 is selected to be 6000 ohms. This value is selected to be large in comparison with the 600 ohm parallel impedance of the master station. As a result, remote stations advantageously do not attenuate the transmit or receive si~nals to any si~nificant extent while in the off-hook state. Multi~le conferees, therefore, each receive a si~nal level equivalent to the non-conference call situation. Furthermore, since only the master station Performs the dialing function, it is important that the remote stations be of hiqh dc imPedance so that dial-pulse dialin~ can be used~ It is noted that both the remote station and the master station are off-hook when the remote s~ation commands the master station to perform dialing.
Telephone hybrid 320 ~erforms two-wire to four-wire conversion in the voice frequency band identical to telephone hybrid 120 of the master station discussed herein~bove. Contacts HTT and HTR are operated by controller 360 to enable voice communication between handset/speakerphone 340 and wire-pair 201 when the remote station wishes to access Line l at voic~band frequencies. Contacts HIT and HIR are o~erated to enable voice communication between handset~speaker~hone 340 and wire-~air 201 when the remote station wishes to access Line l at radio frequencies. Controller 360, responsive to the state of keypad/switches 366 and data received from the data channel on in~ut RXD, implements the selection of radio frequency channels at the remote sta~ion in accordance with the state of outputs IRC, IOC, and L2C. These outputs are delivered ~o voice transmitter/receiver 660 which comprises a ~requency modulator and a frequency demo~ula~or whose operatin~ frequencies are selectable.
The following table sets forth the mutually exclusive states of outPuts IRC, IOC, L2C and the corresPondinq selection of transmit and receive channels:

,S06 TRANSMIT RECEIVE
CONDITIQ~ ~HANNE~ CHANNEL
IXC = n 1 n IB IA
IOC = n 1 n IA IB
L2C = ~1" 2A 2B
IRC = ~1" rePresents the Control signal whereby the remote station Receives a call on the Intercom channel:
IOC = nl" represents the Control siqnal whereby the remote station Originates a call on the Intercom channel and L2C = n 1~ represents the Control siqnal whereb~ the remote station originates or receives a call on Line 2.
Descriptions of other aspects of the remote station shown in FIG. 8 are identical to the corres~ondinq description of the same components in connection with the master station - with the exception that telePhone hybrid 320 does not include ba~tery backup. Thus, in the event of ~ower loss, remote stations are ino~erative.
Both master and remote stations include a microprocessor havin~ a stored pro~ram for controllin~
it:s functions. The ~rogram is interactive with siqnals su~Plied by various hardware items such as rinq detector 111, touch-tone generator 167, keY~ad/switches 166, 366 and the Data Xmtr/Rcvr 510, 610. Shown below, in ~seudo-code, are software control algorithms for the master station to manage ringing, dialing, and switchhook control utilizin~ a data channel to ~rovide full ~hone capabilities to itself as well as remote stations that do not have ring detect, dialer, or switchhook hardware. These alqorithms are coded in the C Programming language ~or which a more detailed explanation is available in the textbook entitled The ~Q~ mln~ Langu~ae, Brian W. Kerni~han and Dennis M.
Ritchie, 1978, Prentice-Hall, Inc.

S(~

#define YES 1 #define LINEl 0 #define MASTER 1 #define NO 0 #define LINE2 1 #define REMOTE 1 #de~ine TRUE 1 #de~ine F~LSE 0 /* hardware inPut requests to subroutine INP~T */
#define TT_DP_SETTING 0 /* return dial mode setting */
#detine LINE_1_RELAY 1 /* return line 1 rela~ Position */
#de~ine LINE_2_ RELAY 2 /* return line 2 relay position */
/* hardware input return values from subroutine INPUT */
#define TT 1 /* dial mode is ~ouch-Tone */
#de~ine DP 0 /* dial mode is dial-pulse */
#define OPEN 1 /* line relay not active, line is idle */
#define CLOSED 0 /* line relay is active, line is in use */
int station_address int ring_l_req;
int rinq_2_req int dialer_busy_l ine_l;
int dialer_busy_line_2;
int dialin~_com~lete_line_l;
int dialing_comPlet Q line_2;
int line_l_master;
int line_2_master;
in~ dP-l-override;
int d~_2_override;
25 int line_l_ack_req;
int line_2_ack_req7 int tX_ty~e /* type of message being transmitted */
/* Allowed values for tx_type are: */
~d~fine UPDATE 0 #define RING_L2 4 #define Ll_DIAL_ACK 1 #define STATUS_CHG 5 ~define L2_DIAL_ACK 2 #define MASTER_DIAL 6 #de~ine RING_Ll 3 int transmitter_busy;
int transmitter_done;
int message_transmitted;
int message_received;
char receiver_buffer[6];
char transmit_buffer[~];
/* the transmit and receiver buffers have the 40 * following message comPonents stored:
* 0 destination address * 1 source address * 2 function code * 3 data 1 * 4 data 2 * 5 longitudinal redundancy check */

~X825~6 main ( ) {
if ( is_rinqing (LINEl ) ) rin~ req = YES:
if(is_rinqing(LINE2)) S rinq_2_req = YES;
if(rinq_l_req && Itransmitter_busy){
ring_l_ req = N9;
message_to_transmit(RING_Ll);
}

if(rinq_2_req && I~ransmitter_busy){
rin~_2_req = NO:
message_to_transmit (RIN~=L2 ) t if(transmitter_done){
transmitter_done = NO
transmitter_busy = NO
switch(tx_type)t case STATU S_ CHG:
if(messaqe_transmitted) /* s*nd messa~e transmitted * si~nal to upper level code.

else if(Imessa~e_received) /* send messa~e undelivered * ~i~nal to upper level cade.

else{ /* process received messaqe first */
transmitter_done = YES;
transmitter_busy = YES
break;
case Ll_DIAL_ACK:
~5 dialer_busy_line_l = NO;
break;
case L2_DIAL_ACK:
dialer_busy_line_2 = NO
break t case RING_Ll:
/* send 1 ine l rinq signal to * upper level code break;
case RING_L2:
/* send line 2 rinq si~nal to * u~per level code break;

1'2~ 6 case MASTER_DIAL:
dial_request(transmit_buffer[4], transmit_ buffer[3], MAS~ER) break;
default:
break;
}

if(messa~e_received){
message_received = NO;
if(receiver_buffer[2] == 0){
/* a data message relatin~ to the use of the * telephone facilities, line 1, line 2, or * intercom, has been received. This could be * a reservation for use, or a release of a * facili~y ~?reviously bein~ used by requestin~ a * hang-u~ or to put the line on hold. Possible * reactions by the upp~r level code miqht be:
* If ~ remote requests use of a line 2~ *
* message_to_transmit(UPDATE, '2', '4', line+'4');
* If a remote hangs u~ a line * hang_up(line);
*
* The master keeps track o all usaqe status so * that the line switches, voice ~aths, and status * indicators can be ~roperly controlled to reflect * the current confi~uration o~ the system.
*/
;
}

else i~(r~ceiver_buffer[2] == 2)l switch(receiver_buffer[3]){
c~se 0: /* line 1 dial request */
dial_request k eceiver_ buffer~4], LINEl, REMOTE);
break;
case 1: /* line 2 dial request */
dial_request(receiver_ buf f er ~ 4], LINE2, REMOTE );
break;
case 9 /* ~a~e received */
/* send intercom paqe si~nal * to uPper level code.
*/
break:
}

5~ }
}

2 ~

if(dialin~_com~lete_line_l){
dialin~com~lete_line_1 = NO;
if tllne_l_master) {
dialer_busy_line_l = NO:
/* send dial comPlete input to upper * level code */
}

else line_l_ack_reg = YES:

if(dialin~_complete_line_2){
dialing_complete_line_2 = NO
if ~ l inP--2--master) {
dialer_bus~_line_2 = NO;
/* send dlal com~le~e input to * u~per level code }
else 1 ine_2_ack_ req = YES, iftline_l_ack_req && Itransmitter_busy){
line_l_ack_req = NO
~5 me~sage_to_transmit( Ll_DIAL_ACK) }

lf~line_2_ack_req && Itransmitter_bus~)~
line_2_ack_req = NO;
messa~e_to_transmit (L2_DIAL_ACK);
}
Other code that is called in the main loop would be key pad scanninq and swltchhook transition detection that would cause this station to qo off hook on a line or dial. Examples of this would be if this station tried to go o~f hook on a currently id~e line 1.
The action would be ~uffered until the transmitter is idle, at which time a line 1 in use reservation messaqe would be sent.
message_to_tx(STATUS_CHG, '1', 'O', '9');
If the successful transmission of the above message is confirmed, then the station will qo off hook.
However, if a data channel messaqe is received while the status chanl~e messa~e was in the ~ransmitter, then the message will be returned to the u~per level code untransmitted~ If the newl~ received message does ~;~825~

nothin~ to make line 1 unavailable, the off hook attempt will be repeated. If this station was off hook on line 1 or 2 and dialing keys were pressed on the keypad, the followinq code would be invoked when the transmitter was idle:
mes~age_to_tx(MASTER_DIAL, 0, line, digit_to_dial);
}

dial_request(diqit_to_dial, line, master) int diqit_to~dial;
int line; /* line to dial on */
int master, /* flag to differentiate master frc~m remote */
/* called from ~eature code in main loop */
{

if(line == LINEl){
if(dialer_busy_line_l) return /* error, all dialing must * wait for ack */
iftINPUT~LINE_l_RELAY) == OPEN) return; /* error, line not beinq used */
if~diqit_to_dial == '*' && INPUT~TT_DP_ SETTING) == DP ~& Id~ l_override){
dp_l_override = YES, digit_to_dial = '0';
~5 }
dialer_busy_line_l = YES;
line_l_master = master;
/* the dial request will be detected in * the dialing interrupt and processed.
*/
else{
i~(dialer_busy_line_2) returnt /* error, all dialinq must wait for ack */
if(INPUT(LINE_2_RELAY) == OPEN) return: /* error */
if(digit_to_di~l == '*' && INPUT(TT_DP_ SETTING) == DP && !dp_2_override){
dp_2_override = YES;
diqit_to_dial - '0';
}
dialer_bus~_line_2 = YES;
line_2_master = masterS
/* the dial request will be detected in the dialinq * interrupt and ~rocessed.
}
}

hang_up(line) int line: /* line beinq hunq uP */
/* called from u~per level code whenever hang-up is detected * either locally or through a data channel message.
*/

if(line == LINEl){
dp~l_override = NO:
/* reset an~ line 1 dialing timers and flaqs */
else{
dP_2_override = NO;
/* re~et any line 2 dialin~ timers and fla~s */
lOi }
dialin~
/* a timer interrupt driven background process that controls the * dialer hardware for both line 1 and line 2. The dial request * is timed as either a dial-pulse or Touch-Tone request, * including the interdi~it time. Interdi~it times for requests * from Remote stations are shortened appropriately to take into * account the data channel transmissio`n delays. When dialinq is * complete, the signal is ~assed back to the foreground process * ~s tollo~s:
*
* if(dialing_is_done(LINEl)) * dlalinq_complete_line_l = YES;
* if ( di al inq,_ is_done ( LINE2 ) ) * dialin~_complete_line_2 = YES;
*
* Since there is onl~ one touch-tone generator, some dial * requests are delayed until the qenerator is available.
* A dial request of 0 can be used force the dialinq code to * si~nal an immediate com~lete. Valid dial re~uests are * 1-10, *, #, pause, and flash.
}
messa~e_to_transmit(mess_type, func_code, data_l, data_2) int mess_tyPe; /* message type to transmit */
int func_code; /* messaqe content */
int data_l;
int ~ata_2;
/* called whenever there is a mess~e to transmit, and the * transmitter is not bus {
int i;
static char messaqe_text[7][5] = {
n OSXYZ ~ t /* status update */
"Os220n, /* line 1 dial ack */
~Os230n, /* line 2 dial ack */
"Os242n, /* rinq line 1 */
"Os243n, /* rinq line 2 */
nOsxyzn, /* status chanqe */
"Os2yz" /* master dial re~uest */
} ;

if(transmitter_bus~) /* error, all calls must return, * check busy fla~ first . */
transmit_buffer[5] = 0; /* initialize lrc */
for(i = 0; i < 5; ~+i){
switch(message_text[mess_type][i]){
case 's':
transmit_buffer[i] = station_addres.s;
break;
case 'x':
transmit_buffer[i] = func_code, break;
case '~':
transmit_buffer[i3 = data_l;
break:
case 'z' transmit_buffer[i] = data_2, break;
default:
transmit_buffer[i~ = messaqQ text[mess_t~pe][i];
break:
transmit_buffer[5] ~= transmit_buffer[i];
}

tx_t~e = mess_t~pe, /* disable receiver */
if~message~received && tx_type == STATUS_CHG){
/* process received messa~e before * transmittin~ this ty~e of message.
*/
messaqe_transmitted = NO
tran~mitter_done = YES;
}

transmitter_busy = YES;
/* enable seceiver */
}

transmitter~){
/* a timer in~erru~t driven background Process that transmits * a messaqe on the data chann~l. The data link protocol is * implemented here. A header is added to the messaqe in ~he * transmit buffer, and when the da~a channel is idle, the * messa~e is transmitted. If transmission was success~ul, * the following signals are returned * if(is_done_transmitter())~
* transmitter_done = YES;
* message_transmitted = YES;
* }
* If a transmission is unsuccessful, retransmission is * attempted. The transmitter also interacts with the * receiver to assure ~roper protocol.
*/
}

~;~8~ 6 receiver~){
/* an edqe trigger interruPt driven backqround process that * decodes messaqes from the data channel. The data link * protocol is enforced here, and siqnals are ~assed to other * processes that need to know about data channel activity.
* If this station is transmittin~, a messaqe corrupted siqnal * is passed to the transmi~ter if necessary so that the * transmitter will resend the messaqe. When a messaqe is * received that passes all the ~rotocol requirements, and the * destlnation a~dress either matches this stations or is * global, a message is moved into the receiver buffer and a * siqnal is Passed back to the foreground process:
* if(is_received_message()){
* messaqe_received = YES;
* if(transmitter_busy && tx_type == STATU5_CHG)~
* transmitter_done = YES;
* message_transmitted = NO;
* }
* }
*

* The receiver also receives timeout siqnals from other * Processes that allow it to Perform the required data * channel protocol timeouts.
}

is_ringin~(line) int line; /* variable desiqnates line to test */
/* This subroutine is a har~ware interface that returns TRUE
* if the line specified by the Passed Parameter is rinqinq, * and FALSE otherwise. Only one TRUE value should be * returned for each rinqinq c~cle.
*/
}
INPUT(input_request) int inPut_requ~st;
/* this subroutine accepts hardware inPut requests and returns * the current state of the reques~ed input.
*/
int return_value:
/* return_value set to state of requested input */
return(return_value);
}

Software control algorithms for the remote station to manage ringing, dialinq, and switchhook control utilizing a data channel to ~rovide full phone capabilities. The remote station depends on the master station to draw dc line current to take a line off hook, to dial, and to inform it when rin~ing is Present on a line. This is accomPlished b~ usinq messaqes over a data channel.
#de~ine YES 1 #define LINEl 0 lO #define NO 0 #define LINE2 l #d~fin~ TRUE l #der ine FALSE O
int station_address:
int tx_type: /* type of message bein~ transmitted */
/* Allowed values for tx_type are: */
#define UPDATE O
#de~ine STATUS_CHG 5 int transmitter_busy;
int transmitter_done;
int messaqe_transmitted;
int message_received;
char receiver_buffer[6];
char transmit_buffer[6];
/* the transmit and receiver buffers have the * following messa~e components stored:
* 0 destination address * l source address * 2 function code * 3 data l * 4 data 2 * 5 lon~itudinal redundancy check */
main(){
if~transmitter_done){
transmitter_done = NO;
transmitter_bus~y e ~0;
switch(tx_ty~e){
case STATUS_CHG:
if(messaqe_transmitted) /* send messa~e transmitted si~nal to * upper level code.
/
;

else if(lmessage_received) /* send messa~e undelivered siqnal * to upper level code.
*/

~;~82S~6 else{ /* process received message first */
transmitter_done = YES;
transmitter_bus~ = YES;
break:
de~ault:
break;
}

if(messa~e_received){
message_received = NO;
if(receiver_buffer~2] == 0 receiver_buffer[2] == 1)l /* a data message relatin~ to the use of the * telephone facilities, line 1, line 2, or * intercom, has heen received. This could be * a reservation for use, or a release of a * facilit~ previously bein~ used by re~uesting * a hang~up or to PUt the line on hold.
* Each station must keep track of all usa~e status * so that status indicators will be controlled * correctly and so access ~o facillties al-ready * in use will be denied~
*/
;

}
else if~receiver-buffer[2] == 2)t switch(receiver~buffer~3]){
case 2: /* line 1 dial acknowled~e */
/* if this station is usinq line 1, * send a dialin~ comPlete si~nal to * the upper level code.
*/
break;
cas~ 3: /* line 2 dial acknowled~e */
/* if this station is usinq line 2, * send a dialinq complete si~nal to * the upper level code.
*j break;
case 4:
switch(receiver_buffer[4])~
case 2: /* rin~ line 1 */
/* send line 1 rin~ siqnal to * uPper level code.
*/
break;
case 3: /* rinq line 2 */
/* send line 2 rin~ si~nal to * u~per level code.
*/
break;

~ 6 - 33 ~
case 4: /* off hook line 1 acknowledqe */
/* if this station is tryinq to * reserve line 1, send a * confirmation signal to the * upper level code.
*/
break, case 5: /* off hook line 2 acknowledge */
/* if this s~ation is tryinq to 19 * reserve line 2, send a * confirmation si~nal to the * u~Per level code.
*/
break;
}
break;
case ~: /* page received */
/* send intercom pa~e si~nal to uPper * level code.
*/
break:
}
}

/* Other code that is called in the main loop would be * key pad scanninq and switchhook transition detection that * would cause this station to ~o off hook on a line or dial.
* Examples of this would be if this station tried to qo off * hook on a currently idle line 1. The action would be * buffered until the transmitter is idle, at which time a * line 1 in use reservation message would be sent.
*

* massa~e_to_tx(STATUS_CHG, '1', '0', '9') *

* If the successful transmission of the above me~saqe is * confirmed, then the station will wait for an * acknowledgement message from the master station * be~ore proceeding. However, if a data channel messa~e * is received while the status chan~e messaqe was in the * transmitter, then the messa~e will be * returned to the * u~per level code untransmitted. If the newly received * message does nothing to make line 1 unavailable, the off * hook attem~t will be re~eated. If this station was off * hook on line 1 or 2 and dialinq keys were Pressed on the * keypad, ~he ~ollowinq code would be invoked when the * transmit~er was idle.
*

* message_to_tx(UPDATE, '2', line, di~ o_dial):
* The station would then buffer u~ any subsequent dial key * presses ~ntil a dial acknowled~ement was received from the * master station.
}

~X~Z~ 6 messa~e_to_transmit(mess_tyPe, func_code, data_l, data_2) int mess_tyPe /* message tyPe to transmit */
int ~unc_co~e; /* messaqe content */
int data_l, int data_2, /* called whenever there is a messaqe to transmit, * and the transmitter is not busy.
{
/* same as the subroutine of the same name in the master */

transmitter()t /* same as the subroutine of the same name in the master */
}

15 receiver ( ) {
/* same as the subroutine of the same name in the master */
}

Claims (9)

1. Telephone apparatus for installation at a subscriber's location and interconnection with a wire-pair that extends from the subscriber's location to a telephone switching system, the telephone apparatus including a master station and one or more remote stations bridged onto the wire-Pair in a parallel electrical connection, each station including a hybrid circuit for interconnecting electrical signals, at baseband, between the wire-pair and a microphone and loudspeaker, CHARACTERIZED IN THAT
the master station comprises:
a processor for controlling the operation of the master station in accordance with a stored program, said processor exchanging signaling information with the telephone switching system over the wire-pair at baseband, and exchanging status and command information with the remote stations over the wire-pair at frequencies above baseband;
a data multiplexer for frequency shifting status information, generated by the processor into the frequency band above baseband;
means interconnecting the data multiplexer with the wire-pair;
a data demultiplexer for frequency shifting multiplexed command information, generated by the remote station, to baseband;
means interconnecting the data demultiplexer with the processor; and the remote station comprises:
a station housing including visual display devices and one or more manually operated switches for generating command information;
a data multiplexer for frequency shifting command information generated by the manually operated switches into the frequency band above baseband;

means interconnecting the data multiplexer with the wire-pair;
a data demultiplexer for frequency shifting multiplexed status information, generated by the processor at the master station, to baseband; and means interconnecting the baseband status information to the visual display device.

2. The telephone apparatus of claim 1 wherein the master station has an AC input impedance whose magnitude approximately matches the impedance of the wire-pair, and wherein the remote station has an AC
input impedance whose magnitude is substantially higher than that of the master station, whereby the remote station does not significantly load AC signals on the wire-pair.

3. Telephone station apparatus for installation at a subscriber's location and interconnection with a wire-pair that extends from the subscriber's location to a telephone switching location, the station apparatus including a master station and one or more remote stations bridged onto the wire-pair in a parallel electrical connection, each station including a hybrid circuit for interconnecting electrical signals, in the audio frequency band, between the wire-pair and a microphone and loudspeaker, CHARACTERIZED IN THAT
the master station comprises:
means interconnected with the wire-pair for detecting the presence of ringing signals thereon and for generating a digital signal indicating the presence or absence of ringing signals;
a data multiplexer having input and output ports, responsive to said digital signal at its input port for frequency multiplexing same into a frequency band above the audio frequency band at its output port;

means interconnecting the output port of the data multiplexer with the wire-pair;
a data demultiplexer, interconnected to the wire-pair, responsive to a frequency multiplexed offhook command signal for converting same into an answer control signal at an output port thereof;
switching means, responsive to said answer control signal for enabling a dc electrical path between wires of said wire-pair; and the remote station comprises:
a manually operated switch for generating an offhook command signal;
a data multiplexer, responsive to the offhook command signal for frequency multiplexing same into the frequency band above the audio frequency band;
means connecting the frequency multiplexed offhook command signal to the wire-pair;
a data demultiplexer, responsive to the multiplexed digital signal, indicating the presence or absence of ringing, for converting same into an alerting control signal at its output port;
means responsive to said alerting control signal for generating an audible alerting signal.

4. The telephone station apparatus of claim 3 wherein the master station has an AC input impedance whose magnitude approximately matches the impedance of the wire-pair, and wherein the remote station has an AC
input impedance whose magnitude is substantially higher than that of the master station, whereby the remote station does not significantly load AC signals on the wire-pair.

5. Telephone station equipment for installation at a subscriber's location and interconnection with a wire-pair that extends from the subscriber's location to a telephone switching location, the station equipment including a master station and one or more remote stations bridged onto the wire-pair in a parallel electrical connection, each station including a hybrid circuit for interconnecting electrical signals, in the audio frequency band, between the wire-pair and a microphone and loudspeaker, CHARACTERIZED IN THAT
the master station comprises:
a data demultiplexer having input and output ports, responsive to a multiplexed digit selection command at its input port for converting same into a dialing signal at its output port;
means interconnecting the input port of the data demultiplexer with the wire-pair;
dialing means, responsive to the dialing signal, for generating electrical signals on the wire-pair indicating a telephone number to be dialed; and the remote station comprises:
a keypad for manually selecting one or more digits to be dialed in establishing a telephone connection;
means responsive to said keypad for generating a digit selection command; and a data multiplexer having input and output ports, responsive to the digit selection command at its input port for frequency multiplexing same into the frequency band above the audio frequency band at its output port.

6. The telephone station equipment of claim 5 wherein the dialing means comprises a dual tone multifrequency generator for generating touch-tone signals.

7. The telephone station equipment of claim 5 wherein the master station has an AC input impedance whose magnitude approximately matches the impedance of the wire-pair, and wherein the remote station has an AC
input impedance whose magnitude is significantly higher than that of the master station, whereby the remote station does not substantially load AC signals on the wire-pair.

8. The telephone station equipment of claim 5 wherein the remote station has an off-hook DC input impedance whose magnitude is significantly higher than that of the master station, and wherein the dialing means comprises a switch for generating dial-pulse signals.

9. Improved apparatus for installation at a subscriber's location and interconnection with a wire-pair that extends from the subscriber's location to a telephone switching location, the apparatus including a master station and at least one remote station bridged onto the wire-pair in a parallel electrical connection, each station including a interface circuit for interconnecting electrical signals, in the audio frequency band, between the wire-pair and a microphone and loudspeaker, CHARACTERIZED IN THAT
the master station comprises:
means interconnected with the wire-pair for detecting the presence of ringing signals thereon and for generating a digital signal indicating the presence or absence of ringing signals;
a data multiplexer having input and output ports, responsive to said digital signal at its input port for frequency multiplexing same into a frequency band above the audio frequency band at its output port;
means interconnecting the output port of the data multiplexer with the wire-pair; and the remote station comprises:
a data demultiplexer, responsive to the multiplexed digital signal, indicating the presence or absence of ringing, for converting same into an alerting control signal at its output port; and means responsive to said alerting control signal for generating an audible alerting signal.