WO1995021498A1

WO1995021498A1 - Private exchange pots line interfaces

Info

Publication number: WO1995021498A1
Application number: PCT/US1995/001333
Authority: WO
Inventors: Earl R. Ii Goodrich; Thomas R. Bayerl
Original assignee: Adak Communications Corporation
Priority date: 1994-02-04
Filing date: 1995-02-02
Publication date: 1995-08-10
Also published as: AU1739695A

Abstract

An analog interface (20) providing POTS device operation from a subscriber premises private exchange (28) connected to a digital network includes an input connected to a ringing signal supply having a DC offset, wherein the input is selectively connected to the ring lead of an analog port (58, 64). A switch is coupled to the input and to the ring lead for selectively connecting the ringing signal to the ring lead. A sensor is coupled across the impedance, the sensor detecting no DC signal during ringing generation when a POTS subscriber terminal is on-hook. However, the sensor detects a DC signal across the impedance when the subscriber terminal device is off-hook. Thus, the sensor detects ring trip and is coupled to a switch in order to disconnect the input lead from the ring lead wherein ring trip is detected.

Description

PRIVATE EXCHANGE POTS LINE INTERFACES BACKGROUND OF THE INVENTION The present invention pertains to private exchanges, and more particularly, to analog interfaces effecting analog POTS device operation from a subscriber premises private exchange connected to a digital network.

Recently, United States telephone service suppliers have invested billions of dollars developing and installing central office switching equipment, software, and human resources which enable the widespread utilization of digital networks, such as the integrated services digital network (ISDN). However, user acceptance of digital network telephone services has been minimal. The primary reason for this lack of acceptance of digital networks is the incompatibility of most telephone customers existing plain old telephone service (POTS) subscriber station equipment with these new digital networks. An additional impediment to widespread acceptance of digital networks is the high cost of obtaining commercially available customer premises equipment which is compatible with the new digital systems.

Known private exchanges connected to a digital network and implementing analog telephone service at a subscriber premises are typically costly and complex. They require installation by a trained professional and provide service for a large number of subscriber lines. Those private exchanges which are not costly have very limited functionality, such that they will not service all types of analog devices. Additionally, they are limited in their ability to service those devices with which they are compatible. Accordingly, despite the considerable effort and large investment already expended in providing a digital telephone network, it remains desirable to provide a fully functional, low cost, subscriber premises private exchange interfacing with a digital network and servicing a limited number of analog lines.

The analog interface represents a significant consideration in the implementation of a private exchange which meets the cost and function requirements necessary to remove the perceived cost barrier impeding widespread acceptance of digital networks. Analog interfaces effecting analog POTS subscriber station functions in existing exchange equipment use dedicated hardware to effect signalling operation for the analog lines. For private exchanges servicing a multitude of analog lines, this hardware is shared by all the lines such that its cost does not represent a significant factor when the cost of the private exchange is calculated on a per line basis. However, where a small number of analog lines are serviced by a private exchange, such as three or fewer analog lines, the cost of dedicated hardware makes it virtually impossible to provide a private exchange capable of servicing a very limited number of analog lines in a cost efficient manner. This, accordingly, represents a significant impediment to providing a cost effective private exchange which services all types of existing subscriber premises analog devices from a limited number of lines.

SUMMARY OF THE INVENTION The present invention overcomes many of the difficulties of existing analog interfaces by providing a low cost, fully functional, analog interface which operates in combination with a general purpose microprocessor to service the private exchange thereby effecting service to all known POTS subscriber equipment.

According to one aspect of the present invention, a private exchange analog line interface includes an input connected to a ringing signal supply having a DC offset. The input is selectively connected to the ring lead of an analog port. A switch is coupled to the input and to the ring lead for selectively connecting the ringing signal to the ring lead. A tip lead is also coupled to the port. A small impedance component is coupled to the tip lead. A sensor is coupled across the impedance, the sensor detecting no DC signal during ringing signal generation when a POTS subscriber terminal device is On-Hook. However, the sensor detects a DC signal across the impedance when the subscriber terminal device is Off-Hook. In this manner, the sensor detects ring trip. The sensor is coupled to a switch to disconnect the input from the ring lead when ring trip is detected.

According to another aspect of the invention, an analog interface for a private exchange includes an analog port and a tip lead connected to the analog port. A ring lead is also connected to the analog port. A forced Off-Hook circuit is coupled to the ring and tip leads, the forced Off-Hook circuit introduces a current through a resistor connected between the tip and ring leads forcing tip and ring current sources to inject current across the tip and ring leads.

According to another aspect of the invention, an analog interface for a plurality of analog lines includes a first analog port and a second analog port for connecting with subscriber devices. A first digital interface converts between analog line signals and digital network signals for the first analog port. A second digital interface converts between analog line signals and digital network signals for the second analog port. A first analog circuit is coupled to the first digital interface by a first internal line. A second analog circuit is coupled to the second digital interface through a second internal line. A conferencing circuit is coupled between the first and second internal lines for effecting local calls and/or conferencing between the first and second analog ports.

The present invention provides an analog interface which effectively supplies the signalling requirements for all types of analog subscriber equipment. The system is highly functional and interfaces with the private exchange general purpose microprocessor to effect the interface operation. Furthermore, the analog interface provides ring signal injection, ring trip detection, forced Off-Hook operation and two analog line conferencing. An important element of the present invention is the means by which the cost of providing all the services of a private exchange is minimized, which facilitates the manufacture of a low cost, fully functional digital network private exchange to thereby eliminate the perceived cost and compatibility barriers which are currently an impediment to the widespread adoption of digital network services, such as the ISDN basic rate interface implementation. These and other objects, features and advantages of the present invention will become clear upon reading the ensuing written description with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit schematic in block diagram form of an analog interface for a private exchange according to the invention; and

Fig. 2 is a detailed circuit diagram of the analog interface according to Fig. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A POTS line interface 20 for a private exchange is shown in Fig. 1. The POTS line interface includes digital interface circuits 22, 24 and 26 connected to the private exchange controller 28 through a local PCM bus 30. The digital interface circuits each include a CODEC which convert between PCM signals on the PCM bus and analog signals in the POTS interface. The analog side of the digital interfaces 22, 24 and 26 are connected to conference circuit 32. Digital interface 22 is connected to Telephone Analog Interface Circuit (TAIC) 34 through analog line 36. Digital interface 24 is connected to TAIC 38 through analog line 40. Digital interface 26 is connected to DTMF detector 42. DTMF detector 42 detects DTMF signals on the PCM bus 30 after they are converted to analog signals by the CODEC in digital interface 26. TAIC 34 is connected to Subscriber Loop Interface Circuit (SLIC) 46 and DTMF detector 48. TAIC 38 is connected to SLIC 50 and DTMF detector 52. DTMF detectors 42, 48 and 52 are connected to private exchange controller 28 through a local data bus 31. SLIC 46 is connected to an analog subscriber device 54 through interface 56 and port 58. SLIC 50 is connected to analog subscriber device 60 through interface 62 and port 64. TAIC 34, SLIC 46, interface 56 and DTMF detector 48 provide analog line functions for analog POTS devices 54 connected to port 58. TAIC 38, SLIC 50, interface 62 and DTMF detector 52 provide analog line functions for analog subscriber devices 60 connected to port 64. A POTS line interface circuit 20 according to the invention is illustrated in greater detail in Figure 2. The functions needed to support an ordinary phone line are known in the industry as the BORSCHT functions, and the acronym us defined as follows:

B Battery feed. DC current source established on the outgoing wire pair on the order of 30mA when off-hook. O Overvoltage. Protections for transient events (lighting, etc.)

R Ringing. About 70 VRMS AC at 20Hz to ring, plus off-hook detection during ringing, known as ring-trip. S Supervisory. Off-hook and line-fault detection through current sensing.

C Coding. Analog to digital conversion. H Hybrid. Incoming simplex to outgoing full-duplex wire pair conversion.

T Test.

The fully functional analog interface must provide all the BORSCHT functions to service all types of analog devices. The analog interface circuit 20 contains circuit 70 shared by both lines. Circuit 70 includes integrated circuit (IC) U312A connected to local PCM bus 30. IC U312A is a frequency divider which divides the 2.048MHz clock signal from the private exchange controller 28 of the private exchange. IC U312A also divides the 256 KHz signal to generate a 128 KHz clock signal for filters in integrated circuits U303 and U304 which are TAICs 34 and 38. Circuit 70 also includes IC U310, a 4x4 crosspoint switch which channels the digitized TXD and RXD signals on the local PCM bus 30 to and from the CODEC ICs U301 and U302 in digital interfaces 22 and 24. The possible connections through crosspoint switch U310 are as follows: Rx/Tx Swap Switch

Address Common Selections

0 Line 1 Rx IDL Rx

1 IDL Tx 2

3

4 Line 1 Tx IDL Rx

5 IDL Tx 6 7

8 Line 2 Rx IDL Rx

9 IDL Tx 10

11 12 Line 2 Tx IDL Rx

13 IDL Tx

14 15 The circuit connected to line 1 (i.e., connected to port 58) are identical to those for line 2 (connected to port 64). Accordingly, only the interface circuit for line 1 will be described in greater detail herein. A digital interface for line 1 contains the COder/DECoder, or CODEC U301, which performs the conversion between the analog voice signals processed through analog port 58 and the pulse code modulated (PCM) data stream for the local PCM bus 30. The PCM data stream is a series of 8-bit samples of the analog signal taken at a sampling rate of 8,000 samples per second. The CODEC conversion is according to a mu-law data compression scheme which improves intelligibility for the given bandwith. Using 8,000 samples per second, analog signals having a frequency up to 4KHz can be digitized by the CODEC without foldover distortion. The 8-bit samples are clocked on and off the PCM bus in 2.048Mbit/s bursts by ILICLK. PCM bus 30 has four time slots Bl, B2, B3 and B4, which are identified by sync, signals. The PCM bursts from CODEC U301 are put into the appropriate time slot (Bl, B2, B3 or B4) by the timing of the synchronizing signal POTSIFS on input pin 12 of IC U301 for line 1 (similarly the CODEC U302 time slot is selected by the synchronization signal POTS2FS for line 2). The bursts are clocked out at an 8,000 bursts per second rate. Digital interface 22 includes an IC U311A which disables the receive data path to perform a mute function for line 1. The mute function disables the receive line which has signals output to POTS device 54, although the transmit line continues to be active.

Conference circuit 32 performs conferencing between the two POTS line ports 58 and 64 with or without connection to the digital local exchange. From the conferencing circuit, access can be made to the digital local exchange. Conference circuit 32 includes ICs U314 and U315, which are triple two-channel analog multiplexers. These multiplexers interconnect the analog signals for the transmit and receive directions on the two POTS lines. The third switch in ICs U314 and U315 is used to inject analog signals from local PCM bus 30 through IC U300 into both analog lines 1 and 2 for tone insertion, voice monitoring or message retrieval functions to the conferenced analog lines. Resistors R360, R339 and R340 perform current summing into the inverting input of IC U317A, an operational amplifier. Resistor R359 sets the circuit gain and maintains the inverting input at 0 volts. Resistors R338 and R301 perform current summing into the inverting input of IC U301. Resistor R304 sets the gain and maintains the inverting input at 0 volts.

TAIC section 34 contains IC U303, a Telephone Analog Interface Circuit (TAIC). This IC has a digitally controlled variable gain amplifier, which allows adjustment of the gain of a signal supplied to the POTS line 1 through port 58. Adjustment of the gain is in 3dB increments, from -15dB to +6dB. The gain set is accomplished through the serial port as selected by signal SPP1 (pin 4). IC U303 also contains a digital filter to provide a steep high frequency roll-off of the analog signal from POTS line 1 which is input through port 58. This roll-off minimizes distortion in the analog-to-digital conversion by filtering out frequency components above the Nyquist frequency of 4 KHz.

The analog output from the auxiliary amplifier in TAIC IC U303 is coupled to the input amplifier of DTMF IC U307 through capacitor C324 and resistor R332. The gain of the internal input amplifier in IC U307 is set by the ratio of resistor R337 to resistor R332. IC U307 decodes DTMF signals originated by the POTS device(s) 54 connected to port 58. When a DTMF signal is detected, a tone present signal (DTMFITP) is output by the DTMF detector 48 to request an interrupt from the private exchange controller 28 (Fig. 1). When the data path is established, the TOE1 signal at pin 10 of IC U307 (Fig. 2) is asserted and the DTMF decoder outputs a resulting digital signal in a 4 bit code onto the

IOBITS bus associated with local bus 31.

The TAIC IC U303 and DTMF IC U307 have an idle mode activated by a signal

POTS1PWDN generated by private exchange controller 28. An inverter U313F inverts the POTSIWDN signal to assert a low signal to IC U303 when power down is desired. This powerdown is selectively activated to conserve power when the main power supply for the private exchange is interrupted.

SLIC IC U306 supplies the BOSH functions for basic support of the two-wire analog phone wire pair connected to port 58. Transistors Q301 and Q302 and resistors R330, R322, R321, R331 and D305 combine with current sources within IC U306 to supply "battery voltage" and current injection to power POTS phone ring PHON1RNG and tip PHON1TIP connection. These outputs also drive audio signals to the analog port 58.

Capacitors C323 and C322 and diodes D302 and D303 provide protection against large transient noises. SLIC 46 includes resistors R305 and R306 which are input resistors connected to internal current mirrors of SLIC IC U306. The internal current mirrors are part of an amplifier circuit which combines with resistors R357, R604, R318, R311 and R316 and capacitors C340, C305 and C318 to form a hybrid circuit which converts the two-wire circuit at the PHON1TIP and PHON1RNG leads to a four- wire circuit input to TAIC IC U303 and provides isolation of the transmit and receive signals. Resistor R365 and diode

D312 set a bias point for the RXI pin 14 of IC U306, which is coupled through resistor

R309. Capacitor C315 provides a bias filter.

SLIC 46 also includes capacitor C308 and capacitor C343 which provide a hold time for ST2 pin 17 of IC U306, which is derived internally from the signal at input pin 18 (STl). Pin 18 is POTSIHKI signal which indicates that the POTS phone is Off-Hook.

POTS1HK2 is an Off-Hook signal which is held by capacitor C308 and C343 to provide a reference compared with POTSIHKI, which comparison is used for detecting repertory dial pulses. IC U313A provides hysteresis for, and inversion of, the ST2 output.

SLIC section 46 also includes circuit 80 which applies a load to the ring and tip lines (PHON1TIP and PHONE 1RNG) to force IC U306 into operation even though a

POTS phone connected to port 58 is not On-Hook.

Circuit 80 is responsive to the forced Off-Hook signal FRCOH1, which is asserted low and applies drive to the base of transistor Q310 through resistor R381. Resistor R375 provides turn-off bias. When transistor Q301 is driven on, the collector drives the base of transistor Q306 through resistor R376. This turns transistor Q306 on, which connects resistor R373 across the ring and tip leads, which are isolated by the ring and tip drive resistors. This provides the load which SLIC U306 detects and responsive to which current is injected.

Circuit 80 allows transmission of caller ID information which can be used by a caller ID readout device connected to line 1 (Fig. 1). The forced Off-Hook circuit 80 also provides for analog loop-back testing and software control of the SLIC power-up functions. Another use for this feature is to smooth the power-up of some electronic POTS phones. Some phones initially draw a current surge when they go Off-Hook, but when their internal power supplies stabilize, their current draw drops below the threshold necessary for the SLIC to detect On-Hook. Applying the auxiliary load briefly after each Off-Hook detection keeps the analog line from chattering when such a phone is connected to port 58. A ringing signal injector 82 selective disconnects the POTS line 1 port 58 from the

SLIC 46 outputs and connects port 58 to the ring generator. When POTS 1 RING and POTSIHKI both have a high logic level, IC U311B goes high and drives the base of transistor Q304 through resistor R369. The collector of transistor Q304 pulls low, applying drive current to the coil of relay X300. The relay switch is therefore pulled in and makes the transfer to connect the ringing signal to port 58. Diode D306 absorbs flyback energy when the relay coil is released.

A ring-trip detector 84 is also connected to the tip lead by relay X300. The ringing signal is a 65 VRMS, 20 Hz ring voltage superimposed on a -28V battery voltage. Since the ringer circuit in the POTS phone is a series LC circuit, the DC bias does not appear on the ring and tip lines until the POTS phone goes Off-Hook, thereby placing a DC load across the line. The amplifier U316A in ring-trip detector 84 detects this DC bias and terminates the ringing signal by driving transistor Q307 to pull POTSIHKI low, which in turn causes relay X300 in ringing signal injector 82 to drop, and transfer the phone from the ring generator to the output of SLIC IC U306. Resistor R356 is a low value resistor (1 watt, approximately 100 Ohms) which is connected in series with the ring circuit on the ground return side (PHON1TIP). While ringing voltage is applied to the PHON1RING lead, and the analog device 54 is On-Hook, the ringing current causes a small AC voltage drop across resistor R356. This small AC voltage is filtered out by resistor R349 and capacitor C334 so that the voltage across capacitor C334 is near zero. When the analog device 54 goes Off-Hook, a DC component is generated across resistor R356. This DC voltage results in a DC voltage across capacitor C334 equal to the divider ratio of resistors R349 and R350 times the DC voltage across resistor R356. This voltage is applied to the inverting input of comparator IC U316A, and compared to the negative voltage on the non-inverting input of IC U316A, which is set by the voltage divider of resistors R347 and R351 from ground to the -5V supply. Resistor R348 pulls the reference slightly lower than the divider ratio would provide, and then raises it when the comparator output switches, to provide hysteresis around the switch point. Resistors R371 and R352 provide pull-up for the open-collector output of comparator IC U316A. When the output of comparator IC U316A goes high due to detection of the DC voltage, resistor R371 can now supply current from the +5V supply into the base of transistor Q307. Transistor Q307 therefore turns on, pulling POTSIHKI low and causing relay X300 to transfer the POTS circuit to the SLIC. The SLIC then senses the load of the Off-Hook phone, turns on, and holds POTSIHKI low. The state change causes an interrupt request in exchange controller 28, which subsequently turns off the ringing signal generator and deactivates the PHON1RING signal.

Diode circuit D315 is a SID AC connected to port 58 to provide protection in severe environments. The phone line port 58 is an RJ14C. Port 58 is connected to port 64 by conductors 90-93 such that both analog phone lines are available to a two-line subscriber device connected at either port 58 or port 64. The analog lines are available on each of ports 58 and 64 in opposite order.

Section 94 contains voltage regulators for quiet analog +/- 5V power supplies from +6.5 and -8V switchmode supply voltages. Capacitors C345 and C346 provide filtering of the input voltages. IC U318 provides regulation of the +5 V analog supply. Capacitor C342 provides additional filtering of the output. Diodes D317 and D316 provide power transient protection by preventing the analog and digital +5V supplies from being more than a Schottky diode drop apart. This prevents activation of parasitic SCR structures in the CMOS devices that have power supply and signals in the different analog and digital environments. IC U319 provides regulation for the -5 V analog supply.

DTMF circuit 42 includes a third detector IC U309, which provides an auxiliary decoding function to allow external devices connected through a digital port to have access for inputting DTMF signals. IC U309 is provided by an IC No. 8870C commercially available from Motorola Corporation, or any other commercially available DTMF detector. The access of the IC U309 is used for voice mail access, changing call forwarding, or any other command functions. Interface IC U300, which is a CODEC, is used only in the digital/analog decode mode. Its input can be either PCM data stream IL1TXD or PCM data stream IL1RXD on local PCM bus 30, as controlled by the 4 NANDgates in U320. The selected output of NANDgate U300C is input to IC U309, which performs exactly the same as the DTMF decoders U307 and U308 in sections 22 and 24, for analog lines 1 and 2. The output of IC U300 can also be connected to the POTS line by the analog switches in conference circuit 32 for injecting call waiting tones and other audible signals into the analog lines associated with ports 58 and 64. IC U312 is a frequency divider configured to divide by four. It generates the 3.579545 MHz DTMF detector clock from the 14.31818 MHz used by the telephone answering machine.

Accordingly, it can be seen that an analog interface for a private exchange is disclosed which is compatible with all known types of analog POTS devices. The analog interface provides forced Off-Hook operation, novel ring-trip detection, and two-line conferencing. Additionally, the analog interface operates with the general purpose microprocessor of the private exchange to effect the analog interface functions.

It is to be understood that the foregoing description of the preferred embodiment of the invention is provided for purposes of description and illustration, and not as a measure of the invention, whose scope is to be defined by reference to the ensuing claims. Thus, those skilled in the art may devise embodiments of the particular concepts presented in the foregoing illustrative disclosure which differ from the particular embodiments shown and described in detail herein, or may make various changes in structural details to the illustrated embodiments. Accordingly, all such alternative or modified embodiments which utilize the underlying concepts of the invention, and incorporate the spirit thereof, are to be considered as within the scope of the claims appended hereinbelow, unless such claims, by their language, specifically state otherwise.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A private exchange analog line interface, comprising: an input connected to a ringing signal supply for inputting a ringing signal having a

DC offset; an analog port including a plurality of terminals; a ring lead connected to a first terminal of said analog port; a switch coupled to said input and to said ring lead for selectively connecting said input to said ring lead whereby the ringing signal is injected to said port; a tip lead coupled to a second terminal of said analog port; a small impedance component coupled to said tip lead; and a sensor coupled across said impedance component, said sensor detecting a DC signal across said impedance component, said DC signal being present during ringing signal generation such that the removal of said DC signal indicates ring trip, said sensor coupled to said switch to disconnect said input from said ring lead when said sensor detects ring trip.

2. The analog interface as defined in claim 1, further including an output coupled to a private exchange controller, said sensor coupled to said output whereby a ring trip signal is supplied to the private exchange controller.

3. The analog interface as defined in claim 1, wherein said switch includes a relay.

4. The analog interface as defined in claim 1 , further including a logic device having an output coupled to said relay and a first input coupled to said sensor whereby said logic device is responsive to an output of said sensor to control said relay.

5. The analog interface as defined in claim 4, wherein said sensor includes a comparator having one input coupled to said impedance component and a second input coupled to a reference potential, whereby the sensed signal is compared to said reference signal by said comparator.

6. The analog interface as defined in claim 5, wherein a transistor is connected between an output of said comparator and an input of said logic device.

7. The analog interface as defined in claim 6, wherein said logic device has a second input coupled to a ring enable signal source.

8. The analog interface as defined in claim 7, further including a transistor connected to the output of said logic device and a terminal connected to a relay coil which controls the state of said switch.

9. The analog interface as defined in claim 2, further including a delay circuit coupled to said output connected to the private exchange circuit and to an Off-Hook detector, whereby said delay circuit outputs a delayed Off-Hook signal to the private exchange circuit.

10. An analog interface for a telephone subscriber, comprising: an analog port; a tip lead connected to said analog port; a ring lead connected to said analog port; a current source circuit coupled to said ring and tip leads; a forced On-Hook circuit coupled to said ring and tip leads, said forced Off-Hook circuit introducing an impedance using a resistor connected between the tip and ring leads to force said current source circuit to inject current into said tip and ring leads when the impedance of said analog port indicates that a subscriber device is connected to said analog port is On-Hook.

11. The analog interface as defined in claim 10, wherein said current source circuit includes respective current feed circuits coupled to each of the tip and ring leads and said forced Off-Hook circuit is coupled to said current feed circuits and between said ring and tip leads.

12. The analog interface as defined in claim 11, wherein one of said current feed circuits includes a first transistor and a first resistor connected in series between a supply terminal and said ring lead and another one of said current feed circuits includes a second transistor and a second resistor connected in series between a supply terminal and said tip lead.

13. The analog interface as defined in claim 12, wherein said forced Off-Hook circuit includes a third transistor and a third resistor connected in series between said first and second transistors.

14. An analog interface for a private exchange servicing a plurality of analog lines comprising: a first analog port for connection to an analog device; a second analog port for connection to an analog device; a first digital interface for converting between analog line signals and digital network signals for a first analog line connected to said first port; a second digital interface for converting between analog line signals and digital network signals for a second analog line connected to said second port; a first analog circuit coupled to said first digital interface by a first internal line, said first analog circuit coupled to said first port; a second analog circuit coupled to said second digital interface by a second internal line, said second analog circuit coupled to said second port; and a conference circuit coupled between said first and second internal lines for effecting a conference connection between the first and second analog ports.

15. The analog interface as defined in claim 14, wherein said conference circuit is coupled to a third digital interface which converts between digital network signals and analog signals to inject audible signals into the analog ports through said conference circuit.

16. The analog interface as defined in claim 14, further including a local PCM bus having a transmit conductor and a receive conductor, said first and second digital interfaces each including transmit and receive ports, and further including a selector for selectively connecting said local PCM bus transmit and receive conductors to the transmit and receive ports of said first and second digital interfaces.