Digital Switching System Interconnecting Buses With Imcompatible Protocols

An Appendix containing a source code listing of 240 frames of control programs which may be used for programming the control processor 16 and the digital signal processors 48 of FIG. 1 is attached hereto. The Appendix contains subject matter which is copyrighted. A limited license is granted to anyone who requires a copy of the programs disclosed therein for purposes of understanding or analyzing the present invention, but no license is granted to make a copy for any other purpose, including the loading of a processing device with code in any form or language.

Reference is made to Ser. No. 07/897,109, filed Jun. 11, 1992 entitled "Digital Switching System Connecting Buses With Incompatible Protocols and Telephone Answering System and Private Automatic Branch Exchange With Integrated Voice and Textual Message Recording".

The present invention relates to systems for connecting buses of information systems having incompatible protocols.

Digital switching systems have been developed which provide non-blocking digital switching in which pulse code modulation is used to digitize information such as voice channels to permit controllable switching of information on the inputs to the outputs of the digital switch. For example a total of 32 voice channels may be pulse code modulated and time division multiplexed so that selected channels on the inputs of the digital switch are connected to different outputs of the digital switch. The switching required to produce the time division multiplexed switching of different channels between selected inputs and outputs is produced by control switching signals applied to the digital switches of the switching matrix.

In the field of telephony numerous manufacturers make voice digitizing boards which convert individual channels (conversation) into a time division multiplexed PCM encoded format in which multiple bits encode respective samples of each channel. On a wire pair typically 24 or 32 different channels are encoded by assigning individual channels representative of a conversation to a particular time slot within a frame of the time division multiplexed PCM encoded information. Switching of the PCM information on a bus is controlled by a digital switch which permits the selective connection of individual channels through the public switched telephone network to particular telephone lines.

The voice digitizing products of manufacturers such as Dialogic Corporation, Natural Microsystems Corporation, Rhetorex and the assignee of the present invention, American Tel-A-Systems, Inc. have incompatible bus protocols which prevent direct interconnecting of buses of systems made by these manufacturers. The buses utilized by the aforementioned manufacturers and others have several possible clock rates such as 1.544 MHz, 2.048 MHz, 4.096 MHz and different 8 kHz synchronization of the frame rate as provided respectively by specifications of integrated circuits manufactured by National Semiconductor or Mitel Corporation and further synchronization signals which define superframes utilized for T1 service which are a combination of multiples of 6 basic frames at the 8 kHz frame rate. As a result of the diversity of the clock rates, synchronization and other timing signals utilized by the various manufacturers there is no compatible bus protocol for interconnecting information systems which have been made by the interconnection of boards provided by a particular manufacturer with an information system made by interconnecting boards from another manufacturer. As a result a user of a system having digitized voice to date is required to make difficult choices as to which manufacturers' products to purchase which limit the overall functional capability of a system and the ability to modify the system in the future as a consequence of the inability to interconnect subsystems formed from products manufactured by different manufacturers.

Circuit boards are marketed which may be inserted into the expansion slot of a standard PC such as a PC-AT to control different types of analog telephone ports. These boards provide analog telephone line port receptacles for connection to the public switched telephone network (PSTN). The analog ports may be for a station port which supplies battery feed capable of applying ring voltage to a line along with normal line status and control functions for the purpose of interfacing two telephones; a loop start port of the so-called "plain old telephone service" which includes all normal line status monitoring; a ground start port which provides a ground start line interface; a DID port which provides all known RS-464 and IEEE standard DID protocols including wink, immediate start as well as DTMF and pulse dialing; an E & M port which may be configured for type I or II signalling and a battery feed port which provides battery but cannot provide ring voltage. These circuit boards have programmable jumpers or switches which permit each of the ports of the circuit to be programmed to have the desired type of line characteristic. The disadvantage of this type of circuit board is that all of the circuits necessary for supporting the types of possible line interfaces which may be provided must be present on the board even though the customer may only want to use a fraction of the overall number of line type interfaces that are supported by the board. As a result, the expense of the circuit board is increased and further the number of analog lines which may be connected to the board is lessened as a consequence of the necessary electronics for supporting all of the lines taking up space which lessens the number of ports which may be on the board.

Another design of a circuit board for interfacing a PC with analog lines does not permit the programming of the characteristic of the line to which the circuit board is to be connected. Each circuit board of this type is hard-wired to support one or more types of analog line interfaces. As a result, this product is not programmable and may be only used for specific applications for specific connections to analog lines which prevents it from being reconfigured by the customers to interface with different types of analog lines.

The present invention is a digital switching system utilizing a digital switching matrix and further an information system having a plurality of connected information buses of different information systems which each have individual bus protocols which are incompatible and different from the protocols of the other buses of the remaining information systems that prevents communications directly between buses of the information systems. A host processor is coupled to each of the plurality of information systems for controlling the information systems with commands causing at least one of the information systems to transmit and at least one of the information systems to receive information by connecting the information bus of the transmitting and the information bus of the receiving information systems with a digital switching matrix. A control processor controls the digital switching matrix by generating control signals controlling switching of the switches of the digital switching matrix in response to commands from the host processor. The invention is particularly applicable to the interconnection of telephony systems having incompatible information buses transmitting information in the form of time division multiplexed PCM encoded digitized voice or data which are made from products of different manufacturers having different bus specifications. Individual telephone conversations, which are digitized by using time division multiplexed PCM encoding to produce multiple channels on pairs of conductors of the bus of the system, may be connected to other pairs of conductors on the bus within one of the connected information systems or to another information system through the switching matrix by suitable programming of the information systems and the control processor.

Control circuitry is provided, which is responsive to the control processor, for providing a master clock signal to the information systems which controls a rate of information transmission and reception on the buses by the information systems and further the generation of master synchronization signals for controlling framing and other master timing signals which are transmitted to the information systems to provide compatible systems clock, synchronization and control signals. The commands from the host processor control the generation of the master clock signal with the host processor commanding the information systems to control the transmission and reception on the information buses under the control of the master clock signal. The master clock signal may be synchronized a with another signal provided on one of the information buses of the information systems to the control circuitry such as, but not limited to, a timing specification provided by the PSTN such as where T1 service is provided. Additionally, the control circuitry also provides the synchronization signals to the information systems which synchronize frames of information transmitted and received on the buses by the information systems.

In a preferred application of the present invention the host processor is a PC and the control processor is commanded by the host processor to perform the necessary switching interconnections of the switching matrix in order to ensure that appropriate connections are made between the buses of the information systems which are connected to the switching matrix which result in the transmission and reception of information as commanded by the host processor to the information systems as a consequence of the commands issued from the host processor to the information systems. As a result of the commands controlling the connection of the buses from the incompatible information systems through the switching matrix and the generation of a master clock signal controlling the rate of transmission of information on the buses as well as synchronization signals, the information systems individually which have incompatible bus specifications are made compatible by the supplying of common clock and synchronization information to all systems. This ensures the compatibility of information transmission on and between the buses of the information systems and the appropriate signal paths through the switching matrix being established between the buses as a consequence of the switching of the switching matrix being under the control of the commands issued by the host processor.

The invention provides distinct advantages over the prior art which does not permit incompatible information systems to be interconnected through their buses as a consequence of incompatible bus protocols. As a result, digitized voice and other forms of digitized information, such as ISDN, may be transmitted between incompatible digital information systems by the supplying of appropriate commands from a host processor such as a PC to the control processor which controls the generation of centralized clock and synchronization information which is transmitted to the individual information systems to provide their clock and synchronization signals. As a result of using a conventional PC and a processor such as a microprocessor, which is part of a circuit board that is connected to the PC as a peripheral device which is controlled by the PC to control the digital switching matrix and the control circuitry to generate the common system timing, the overall cost of the system is reduced while permitting the interconnection of otherwise incompatible information systems in conventional usage in applications such as telephony where digitized voice products are provided by diverse manufacturers of voice digitizing circuit boards used in combination with digital switches to provide diverse telephonic services through customers connected to the PSTN.

The invention further provides a line interface between the PSTN, a personal computer and an information bus which permits the control by a personal computer of the interfacing of multiple types of analog telephone lines with the PSTN. Individual modules are inserted into slots which configure an interface with a particular type of analog telephone line. As a consequence of the module containing circuitry unique to the type(s) of analog line interface created by its insertion in the appropriate slot of the circuit board, the circuit board contains less circuitry than the prior art as a result of specialized circuits for producing the interface with a particular type of analog line being contained in the module as contrasted with the circuit board as in the prior art. Furthermore additional ports for driving additional analog lines are provided as a consequence of the circuit board containing less circuitry than the prior art as a consequence of particular circuitry for configuring a particular type of analog line interface being present within the module which is inserted into the slot of the circuit board. The line interface may be configured by the insertion of the appropriate module to provide an interface with any combination of DID, E & M, source, ground start, loop start or battery feed type analog lines. The line interface provides the low level protocols and signalling necessary for the different line types. Moreover, an interface with a bus is provided for access to other interface boards. A host processor, which may be a PC, controls the control processor of the line interface by suitable commands which are transmitted from the host processor to the control processor.

A digital switching system in accordance with the invention includes a digital switching matrix having an input bus and an output bus with information on the input bus being selectively connectable to information on the output bus in response to control signals applied to switches of the switching matrix; a plurality of information systems with each information system having an information bus for transmitting and receiving information using a protocol which is different from a protocol used by the information bus of each of the other information systems with the information buses of the information systems being electrically connected to the input and output buses of the matrix to permit information transmitted from a conductor of any of the information buses to be received by another conductor of any of the information buses through the information bus, the switching matrix and the output bus; a host processor, coupled to each of the plurality of information systems, for controlling the information systems with commands causing at least one of the information systems to transmit and at least one of the information systems to receive information using the information bus of the transmitting and the information bus of the receiving information systems, the switching matrix and the input and output buses; and a control processor, responsive to commands from the host processor, for generating the control signals controlling switching of the switches of the digital switching matrix. Information transmitted and received on the information buses may be encoded in a time division multiplexed pulse code modulated format to provide a plurality of channels of information with each channel being time division multiplexed. The channels of information may contain audio voice information or data. At least one of the systems may be connected to a public switched telephone network and the network may provide a timing reference to the switching system used to generate a master clock signal by control circuitry to control a rate of transmission and reception on the buses by the information systems. The host processor also transmits commands to a plurality of the information systems and to the control processor to cause information to be transmitted from the bus of one of the information systems through the digital switching matrix for receipt by the bus of another one of the information systems. The host processor transmits commands to one of the information systems and to the control processor to cause information to be transmitted and received by the bus of the information system receiving the commands through the switching matrix. The host processor also transmits commands to at least one of the information systems and to the control processor to cause a plurality of channels of information to be transmitted from the bus of at least of the information systems through the switching matrix to the bus of at least another one of the information systems. The host processor also transmits commands to a plurality of the information systems and to the control processor to cause the plurality of channels of information to be transmitted from the bus of at least one of the information systems through the switching matrix to the bus of at least another one of the information systems. Finally, the host processor transmits commands to the plurality of information systems and to the control processor to cause full duplex communications between buses of at least two of the information systems through the switching matrix.

The invention further includes control circuitry, responsive to the control processor, for providing a master clock signal to the information systems which controls a rate of information transmission and reception on the buses by the information systems. The commands control the generation of the master clock signal with the host processor commanding the information systems to control the transmission and reception on the information buses under the control of the master clock signal. The master clock is synchronized with a signal provided on one of the information buses of the information systems. The control circuitry also provides synchronization signals to the information systems which synchronize frames of information transmitted and received on the buses by the information systems. The commands control the generation of the synchronization signals with the host processor commanding the information systems to synchronize the transmission and reception of frames of information on the information buses under the control of the synchronization signals.

A system in accordance with the invention includes a plurality of information systems with each information system having an information bus for transmitting and receiving information using a protocol which is different from a protocol used by the information bus of each of the other information systems with conductors of the information buses of the information systems being electrically connected to permit information transmitted on a conductor of any of the information buses to be received by another conductor of any of the information buses; a host processor, coupled to each of the plurality of information systems, for controlling the information systems with commands causing at least one of the information systems to transmit and at least one of the information systems to receive information using the information bus of the transmitting and the information bus of the receiving information systems; a control processor, responsive to commands from the host processor, for generating the control signals; and control circuitry, responsive to the control processor, for providing a master clock signal to the information systems which controls a rate of information transmission and reception on the buses by the information systems. The commands control the generation of the master clock with the host processor commanding the information systems to control the transmission and reception of information on the information buses under the control of the master clock. The master clock may be synchronized to generate the clock signal with another signal provided on one of the information buses of the information systems. The control circuitry also provides synchronization signals to the information systems which synchronize frames of information transmitted and received on the buses by the information systems. The commands control the generation of the synchronization signals with the host processor commanding the information systems to synchronize the transmission and reception of frames on the information buses under the control of the synchronization signals. Information transmitted and received on the information buses may be encoded in a pulse code modulation format to provide a plurality of channels of information with each channel being time division multiplexed. The channels of information may contain audio voice information. At least one of the information systems may be connected to a public switched telephone network and the network may provide a timing reference to the system used to generate the master clock signal.

FIGS. 1A-1C illustrates a block diagram of a system in accordance with the present invention.

FIG. 2 illustrates a block diagram of the clock generation and control circuitry of FIG. 1.

FIGS. 3A and 3B illustrate a block diagram of a line interface which may be utilized in the third information system of FIG. 1.

FIG. 4 illustrates a block diagram of an E & M module for use in the line interface of FIG. 3.

FIG. 5 illustrates a block diagram of a DID module for use in the line interface of FIG. 3.

FIG. 6 illustrates a block diagram of a source module for use in the line interface of FIG. 3.

FIG. 7 illustrates a ground start module for use in the line interface of FIG. 3.

FIG. 8 illustrates a block diagram of a loop start module for use in the line interface of FIG. 3.

FIG. 1 illustrates a block diagram of a digital switching system 10 in accordance with the present invention. A digital switching matrix, switches and input and output buses 12 is the principal switching element of the digital switching system. The input bus and the output bus is conventional and is not separately illustrated. The digital switching matrix functions such that information on the input bus is selectively connectable to the information on the output bus in response to control signals applied to the switches of the switching matrix by control bus 14 which is connected to a control processor 16 which is described in detail below. A plurality of information systems, which are respectively identified as first, second and third information systems 18, 20 and 22, are connected to the digital switching matrix, switches and input and output buses 12 respectively by a first information system bus 24, second information system bus 26 and third information system bus 28. It should be understood that the present invention is not limited to the interconnection of any particular number of information systems and buses with the first, second and third information systems 18, 20 and 22 and first, second and third information system buses 24, 26 and 28 only being representative of possible information systems and buses which may be lesser or greater in number than the three information systems and buses as illustrated. The first information system 18 in a preferred embodiment of the invention is representative of digital voice processing circuitry manufactured by Dialogic Corporation. The second information system 20 in a preferred embodiment of the invention is representative of digital voice processing circuitry manufactured by Natural Microsystems. The third information system 22 in a preferred embodiment of the invention is representative of a group of interconnected line interface boards as described in conjunction below in FIGS. 3-8 which are manufactured by the assignee of the present invention. Inputs 30 are representative of connections between the PSTN and the configuration of circuits which form a complete information system such as a telephony system comprised of interconnected circuit boards for digital processing and switching of audio such as voice communications or data for transmission by protocols such as, but not limited to, ISDN. The digital switching matrix, switches and input and output buses 12 may be implemented by the connection of a plurality of Seimens multi-point switching and conferencing integrated circuits PEB 2245. Each integrated circuit of the digital switching matrix, switches and input and output buses 12 may switch any of 512 time and space division multiplexed input channels to any of 256 time and space division multiplexed output channels to provide PCM encoding. The inputs are connected in parallel to form a 512 channel by 512 channel switching matrix. Each integrated circuit is capable of performing multiple channel conferencing in which several input channels are cross-connected and coupled to appropriate output channels. The conferencing capability of the aforementioned integrated circuits is limited to 64 inputs combined in a maximum of 21 conferences. It should be understood that the present invention is not limited to any particular configuration or operation of the digital switching matrix, switches and input and output buses 12. The first information system bus 24, the second information system bus 26 and the third information system bus 28 each have distinct, unique, different and incompatible protocols which are incompatible with the protocols of the other buses which prevents direct connection of the buses into one system. As a result, the buses 24, 26 and 28 may not be directly connected to transfer information from one information system 18, 20 and 22 to another information system relying upon the timing including clock signals and synchronization signals generated by any one of the information systems. The digital switching matrix, switches and input and output buses 12 provide the capability of connecting conductors of each of the individual first information system bus 24, second information system bus 26 and third information system bus 28 to other conductors of that same bus or alternatively to conductors of another one of the buses and time division multiplexed information, such as voice channels, from one bus 24, 26 and 28 to another bus. The switching connections necessary to connect the input and output buses of the information on the input bus to the output bus of the digital switching matrix to provide appropriate connections between conductors of a single bus 24, 26 and 28 or between the buses is controlled by the control signals applied on the control bus 14 by the local processor 16.

The telephony industry has been characterized by diverse products each developed to provide digital voice processing utilizing time division multiplex PCM encoding. However, the manufacturers of these products have not developed a universal protocol which permits the interconnection of systems each individually configured with one manufacturer's products by bus interconnections to systems configured with another manufacturer's products. The present invention provides the capability of interconnecting information systems through buses which have incompatible protocols which have not been designed to be interconnected. The digital switching matrix, switches and input and output buses 12 provide connections between the conductors of the buses 24, 26 and 28 under the control of the control processor 16. Clock generation and control circuitry 32, which is controlled by the control processor 16, provides a master clock signal to the information systems 18, 20 and 22 which controls a clock rate of information transmission and reception of time division multiplexed PCM encoded information on the buses 24, 26 and 28 by the information systems and further controls the generation of necessary synchronization signals which synchronize the transmission and reception of frames of the time division multiplexed PCM digitally encoded information produced by the information systems on the information buses. In a preferred embodiment where the first, second and third information systems 18, 20 and 22 are respectively made from circuits supplied by Dialogic, Natural Microsystems and from the line interface circuits as described below in conjunction with FIGS. 3-8 of the assignee, a 1.544 MHz, a 2.048 MHz clock signal and a 4.096 MHz clock signal, an 8 kHz synchronization pulse marking the beginning of frames conforming to National Semiconductor timing specifications which may be used for information systems made from circuits supplied by at least the assignee, an 8 kHz synchronization pulse marking the beginning of frames conforming to Mitel timing specifications which may be used for an information system configured from Natural Microsystems circuits and synchronization signals which define a superframe used for T1 service which marks multiples of 6 basic frames. Additionally, the clock generation and control including sync circuitry 32 may be synchronized to an external timing reference from one of the information systems 18, 20 and 22 or from a source external to the information systems such as PSTN timing for T1 service used to generate the master clock signal to control the clock rate and timing of frames of information transmission and reception on the buses by the information systems. In this circumstance the master clock signal and synchronization signals may be synchronized to any external frame synchronization time reference. The external timing reference is routed to the clock generation and control including sync circuitry 32 by control of the control processor 16. Removal of the external clock reference causes the clock generation and control including sync circuitry 32 to revert to providing internally generated timing signals. Each of the information systems 18, 20 and 22 may provide the external timing reference through their connection to the PSTN which results in the timing reference being fed to the clock generation and control including sync circuitry 32 through the control processor 16 as described above. In any event, the clock generation and control including sync circuitry 32 supplies a common clock, synchronization and any other timing signals to some or all of the information systems 18, 20 and 22 either by means of signals generated as a consequence of the control of the control processor 16 as receiving commands from the host processor 34 as described below which also controls the switching connections produced by the digital switching matrix, switches and input and output buses 12 and the functioning of the first, second and third information systems 18, 20 and 22 as operating to transmit or receive information through the switching matrix and input and output buses or from an external source such as T1 timing.

The host processor 34 is coupled to each of the information systems 18, 20 and 22. The host processor 34 controls the information systems with commands causing at least one of the information systems 18, 20 and 22 to transmit and at least one of the information systems to receive information using the information system buses 24, 26 or 28, the switching matrix, switches and input and output buses 12. The different functions of the system 10 which are controlled by the host processor 34 are specified by a command set inputted from the host processor. The mnemonics of the command set are not discussed herein and may be varied. The control processor 16 is responsive to the commands issued from the host processor 34, which may be a PC and preferably may be PC-AT, for generating the control signals outputted on the bus 14 to the digital switching matrix of the digital switching matrix, switches and input and output buses 12 for controlling the switching of the switches in the switching matrix and the generation of the clock signals and synchronization signals and any other timing signals or control signals produced by the clock generation and control including sync circuitry 32. Suitable control programs for the preferred control processor 16, and digital signal processors 48 described below are respectively contained in frames 1-207 and 208-240 in the attached Appendix when the processor is the Hitachi 64180 and the digital signal processors are Analog Devices ADSP 2101. The clock generation and control including sync circuitry 32 is connected to the digital switching matrix, switches and input and output buses 12 by the switch clock and sync bus 36. Similarly the clock generation and control including sync circuitry 32 applies a common clock signal and synchronization signals respectively to at least some and in many applications all of the first, second and third information systems 18, 20 and 22 through the first information system clock and sync bus 38, the second information system clock and sync bus 40 and the third information system clock and sync bus 42. Finally, the clock generation and control including sync circuitry 32 produces clock and sync signals, which are applied through the DSP clock and sync bus 44, to a plurality of digital signal processors 48 as described below. A DSP serial bus 46 connects the digital switching matrix, switches and input and output buses 12 with the digital signal processors 48. The information transmitted on the first, second and third information system buses 24, 26 and 28 is preferably encoded in time division multiplexed pulse code modulated format to provide a plurality of channels of information which may be for example twenty-four or thirty-two in number without limitation with each channel being time division multiplexed in frames. In a preferred embodiment of the present invention, the PCM encoding of information channels by time division multiplexing is digitally encoded audio provided by the information systems 18, 20 and 22 which are made from circuits manufactured by manufacturers such as Dialogic, Natural Microsystems and the assignee of the present invention as described below. It should be understood that the invention is not limited thereto and may for example receive information in the form of data coming from the PSTN in an ISDN format. The host processor 34 controls both the local processor 16 and the first, second and third information systems 18, 20 and 22 PG,24 through commands. The commands are decoded by the first, second and third information systems 18, 20 and 22 to control which of the information systems is transmitting information under control provided by the clock generation and control including sync circuitry 32 through the digital switching matrix and input and output buses 12 to the receiving information system. It should be understood that the information systems 18, 20 and 22 may be connected through the digital switching matrix, switches and input and output buses 12 under the control of the control processor 16 which in turn is controlled by the host processor 34 for controlling the digital switching matrix and input and output buses and the clock generation and control including sync circuitry 32 to cause conductors or information of one of the information system buses 24, 26 and 28 to be connected to other conductors or information in the same information system bus so that interconnects may be made in the same information system or to cause conductors or information of one of the information system buses to be connected to conductors or information of another of the information system buses. The aforementioned operation permits the interconnection between information systems to provide interconnection of buses which each have protocols which are incompatible with the protocols of the buses of the other information systems when operating within each of their respective information systems with timing supplied internally to the information system 18, 20 and 22 for controlling information transfer on its information system bus 24, 26 and 28.

To perform the diverse foregoing functions, the host processor 34 transmits commands to a plurality of the information systems 18, 20 and 22 and to the control processor 16 to cause information to be transmitted from the information system bus 24, 26 or 28 of one of the information systems 18, 20 or 22 through the digital switching matrix, switches and input and output buses 12 for receipt by the bus of another one of the information systems. Alternatively, the host processor 34 transmits commands to one of the information systems 18, 20 and 22 and to the control processor 34 to cause information to be transmitted and received by the bus of the information system(s) receiving the command through the digital switching matrix, switches and input and output buses 12. Furthermore, the host processor 34 transmits commands to at least one of the information systems 18, 20 and 22 and to the control processor 16 to cause a plurality of channels of information which are produced by the time division multiplexed PCM encoding to encode each channel in a multiple bit format such as 8 bits per channel to be transmitted from at least one information system bus 24, 26 and 28 of the information systems through the digital switching matrix, switches and input and output buses 12 to at least one bus of at least one of the information systems. Moreover, the host processor 34 transmits commands to a plurality of the information systems 18, 20 and 22 and to the control processor 16 to cause the plurality of channels of information which are encoded in the time division multiplexed PCM format to be transmitted from at least one information system bus 24, 26 and 28 of one of the information systems 18, 20 and 22 through the digital switching matrix, switches and input and output buses 12 to at least one information bus of another of the information systems. Additionally, the host processor 34 transmits commands to a plurality of information systems 18, 20 and 22 and to the control processor 16 to cause full duplex communications between information system buses 24, 26 and 28 between buses of at least two of the information systems through the switching matrix. Duplex communications are achieved by the setting of transmitting and receiving signal paths from at least a pair of information systems 18, 20 and 22 through the digital switching matrix, switches and input and output buses 12.

The clock generation and control including sync circuitry 32 is responsive to the control processor 16 to provide a master clock signal to the information systems 18, 20 and 22 which control a rate of information transmission and reception on the information system buses 24, 26 and 28 by the information systems. The commands provided by the host processor 34 control the generation of the master clock with the host processor commanding the information systems 18, 20 and 22 to control the transmission and reception of information on the information system buses 24, 26 and 28 under the control of the master clock. The master clock may be synchronized from a timing reference from one of the information systems 18, 20 and 22 or an external timing reference not under the control of the host processor 34 such as that provided by T1 service through the connection 30 to the PSTN. The clock generation and control including sync circuitry 32 also provides the synchronization signals which synchronize frames of the time division multiplexed PCM encoded information transmitted and received on the buses by the information systems 18, 20 and 22. The commands from the host processor 34 control the generation of the synchronization signals with the host processor commanding information systems 18, 20 and 22 to synchronize the transmission and reception on the information system buses 24, 26 and 28 under the control of the synchronization signals. A PC interface 50 interfaces the control processor 16 to the host processor 34. Preferably the interface 50 is an ISA bus interface which is in accordance with standards for a PC-AT. The bus interface 50 supports shared interrupts and uses both the input/output and memory address spaces on the host address bus and control 63 and the host data bus 65 of the host processor 34. The bus interface 50 provides communications between the host processor 34 and the control processor 16. Write-only input/output locations are used in controlling interrupt generation and memory base addresses. The input/output base address is switch selectable in hexadecimal respectively at input/output addresses between sequential hexadecimal addresses 0200-0370 which jump in increments of 0010 in hexadecimal address locations. Additional circuit boards within the system 10 such as the third information system 22 share interrupts mapped to the above-described input/output addresses. The base input/output address is used to reenable the interrupt control circuitry 61 that generates interrupts 58 to the host processor 34 by the host processor writing to the base input/output address after each interrupt is serviced. The base memory address for the dual ported ram 54 of the system 10 with respect to the host processor 34 is set by the host processor 34 writing to the base input/output address plus 1. Data bits D0-D7 on the host data bus 65 map to the address bits A15-A22 of the host address bus and control 61 to set aside 32K address space for the system 10. For example to map the dual ported RAM into base address location D0000 in hexadecimal of the host processor 34, data byte 1A in hexadecimal would be written to the base input/output address plus 1. There is no hardware to prevent mapping into memory below 640K. On power-up the dual ported RAM 54 cannot be accessed by the host processor 34. A jumper switch (not illustrated) may be used to control the mapping of the dual ported RAM 54 with respect to address bit A23 on the host address bus and control 63. When this jumper switch is installed, address bit A23 must be high to access the board, in which case the board is mapped into memory above 16M. The dual-ported RAM 54 may have 2K of memory with the address for the system 10 being mapped into 32K the base memory by the previously described input/output strobe. The ISA bus interface (PC-AT) 50 may contain a hex switch not illustrated to determine the 2K address space within the 32K of base memory used by a particular circuit board with the hex switch having hexadecimal settings 0-F with each hexadecimal value representing a multiple of 2K that is added to the base address of the system as determined by the previously described input/out strobe.

The shared interrupt allows the control processor 16 to generate an interrupt to the host processor 34 by writing to the dual-ported RAM address 54 of 0000 in hexadecimal. The interrupt is cleared when the host processor 34 writes to location 0000 in hexadecimal and also writes to the input/output base address. The host processor 34 can generate an interrupt to the control processor 16 by the writing of 0001 in hexadecimal into the dual-ported RAM 54. The control processor 16 clears the interrupt by writing to 0001 in hexadecimal. Interrupt requests are generated by the interrupt control circuitry 61 and are issued to the host processor 34 on the interrupt lines 58 and are issued to the control processor 16 on the interrupt line 59.

The system 10 supports the shared interrupts provided in the PC-AT Technical Manual as follows: When address 0 of the dual-ported RAM 54 is written into, an interrupt is generated (edge-triggered). If more than one circuit board such as other boards in a third information system 22 attempts to generate the interrupt, only one interrupt is actually generated. Furthermore, if one board generates an interrupt and another board tries to generate an interrupt before the first one is serviced, the second interrupt will not occur until the interrupt circuitry is rearmed. The interrupt service routine must determine which board is interrupting. After finding the appropriate board, it must perform all software tasks required in servicing the interrupt. When these tasks are completed the interrupt service routine must write to location 0 of the dual ported RAM 54 to enable the board to generate another interrupt. The last step in the process is to write into the input/output base address shared by all boards. This enables the interrupt generating circuitry on all boards. If any interrupts are pending (i.e. from a second board needing service before the first board's interrupt service routine is complete), another interrupt will be generated at this time and the normal interrupt service routine functions should be executed. This methodology allows only one board to be serviced in a given interrupt service routine. However, even if all of the boards in the system 10 request an interrupt simultaneously, a fresh interrupt is generated for each board until all have been serviced. If desired, the interrupt service routine can service all boards within one call to the interrupt service routine. If this method is chosen, the input/output strobe should not be generated until the last board is serviced. This will prevent multiple interrupts from occurring. The combination of the dual-ported RAM 54 and the interrupt control circuitry 61 provides a complete communication mechanism which allows the control processor 16 to send messages to the host processor 34 and the host processor to send messages to the control processor. These messages provide all the control functions to a particular board to interface the host processor 34 with a control processor 16.

The control processor 16 may be any suitable microprocessor but an Hitachi 64180 is utilized in conjunction with the source code in the Appendix at frames 1-207. The control processor 16 provides access to the dual-ported RAM 54, RAM 56 and ROM 60. In the aforementioned Hitachi microprocessor a total of 64K of memory is provided. All device specific functions are implemented via the input/output interface of the control processor 16. These communications include communications with the digital signal processors 48, the clock generation and control including circuitry 32, switching control of the digital switching matrix, switches and input and output buses 12 and serial input/output communications. The ROM 60 stores the application program. The RAM 56 is used for variable storage and temporary information. The dual-ported RAM 54 provides the interface to the host processor 34 as described above. The control processor 16 controls all hardware specific aspects of the system. For example, if a port in a circuit board of the third information system 22 is to be connected to a port of another board of the third information system, the control processor 16 receives a command from the host processor 34 to make this connection. The control processor 16 translates the information into the appropriate format and communicates with the digital switching matrix, switches and input and output buses 12 via the appropriate input/output port. The control processor 16 also controls the functions of the digital signal processors 48 and determines the master clock source for the system 10 such as for transmitting information on the information system buses 24, 26 and 28 in time division multiplexed PCM format.

The bus 28 of the third information system bus and bus interface circuitry 22 is designed to transfer time division multiplexed PCM encoded information to and from the boards within the hardware illustrated in FIGS. 3-8 and to other peripherals. The interface includes an appropriate master clock which may be 2.048 MHz but the invention is not limited thereto, a transmit data signal, a transmit synchronization signal, a receive data signal and a receive synchronization signal. These signals are provided on the information system bus 28 via a 10-pin ribbon header. A plurality of third information systems each of which is represented by an individual block 22 can be connected to the bus 28 to provide expansion. The clock generation and control including sync circuitry 32 must provide the master clock and synchronization signals to the third information system 22. As explained above a time reference pulse such as a synchronization signal representing the basic frame rate utilized for T1 service may be provided through the PSTN interface 30 as used for T1 service. The interface between the first information system bus 24 and bus interface circuitry 18 is intended in a preferred application to interface with Dialogic Corporation products. In this circumstance it provides a mechanism to transfer information between the digital switching matrix, switches and input and output buses 12 and products which are connected to the information system 18. It should be understood that the first information system 18 is not limited to a particular bus specification with regard to a particular manufacturer such as Dialogic Corporation. The second information system 20 and bus interface circuitry is intended in a preferred application to interface with Natural Microsystems products. It should be understood that the invention is not limited thereto. In a preferred embodiment up to 256 inputs and 256 outputs are coupled to the second information system bus 26. Clock and synchronization signals for the second information system bus 26 may be generated by the second information system bus and interface bus circuitry 20 or alternatively under the control of the clock generation and control including sync circuitry 32 when the host processor 34 is controlling the transfer of information either within the second information system 20 or between the second information system 20 and the first and third information systems 18 and 22. Finally, the clock source provided by the second information system 20 may be used to synchronize the clock generation and control including sync circuitry 32 to slave the master clock signal for at least one of the first and third information systems 18 and 22. In this case, the clock generation and control including sync circuitry 32 acts as a master for the first and third information systems 18 and 22 but is slaved to the second information system 20. Any information system may provide the clock source to the clock generation and control including sync circuitry so that the clock generation and control including sync circuitry acts as a master for the remaining information systems.

The digital signal processors 48 are used to provide tone facilities to the system 10 and to boards within the system that do not have built-in tone functions. The digital signal processors 48 may be an Analog Devices ADSP 2101. A suitable control program for a digital signal processor is contained in the above-referenced Appendix for the aforementioned Analog Devices integrated circuits at frames 208-240. The digital signal processors 48 are not required to operate the digital switching matrix, switches and input and output buses 12 and provide additional support for other functions of the overall system not involving switching. For example the digital signal processors 48 perform functions such as DTMF generation and detection, call progress generation and detection and MF generation and detection. The digital signal processors 48 are connected to dual-ported RAM 62 via a DSP address and control bus 64 and a DSP data bus 66. Furthermore the dual-ported RAM 62 is coupled to the address bus and control 14 and the data bus 52 of the control processor 16. The dual-ported RAM 62 is configured into a first-in, first-out memory which stores commands from the control processor 16 and information to be transmitted to the processor such as acknowledgement and indicating tones which have been detected.

A plurality of switches which are not illustrated may be used to program functions in the system 10. A switch may be used to provide a 2K offset in the 32K address space of the system 10. A switch sets the input/output base address. Another switch permits the programming of interrupts for determining the interrupt priority in the overall system. A switch may be used to connect an interrupt pull-up resistor with only one switch being installed on one board for a given interrupt. If more than one board shares an interrupt only one board will have this switch. Finally, a switch may be connected to address location A23 of the address bus and control 14 to permit access for memory above 16 megabytes.

FIG. 2 illustrates a block diagram of the clock generation and control including sync circuitry 32 of FIG. 1. Like reference numerals identify like parts in 25 FIGS. 1 and 2. Clock generation circuitry 100, which may be a Mitel integrated circuit MT8941, provides output signals comprising clock signals of 1.544 MHz, 2.048 MHz, 4.096 MHz and an 8 kHz synchronization signal in response to an input from a first oscillator 102 having a frequency of 16.384 MHz and a second oscillator 104 having a frequency of 12.352 MHz. The foregoing outputs are connected in parallel to clock translation circuitry for the first information system 104, clock translation circuitry for the second information system 106, clock translation circuitry for the third information system 108, switch clock translation circuitry 110 for the digital switching matrix, switches and input and output buses 12 and DSP clock translation circuitry 112 for the digital signal processors 48. It should be noted that the communication paths between the clock generator circuitry 110 and the first, second and third information system clock translation circuitry 104, 106 and 108 are illustrated as being bidirectional. However, in practice the design of the information systems will determine if communications are only in one direction from the clock generator circuitry 100 or in two directions between the clock generator circuitry and the information system clock translation circuitry. When communications are bidirectional, the information system is functioning as a master clock with the clock generator circuitry being slaved to the master information system and remaining information systems being slaved to the clock generator circuitry. The function of the translation circuitry 104, 106, 108, 110 and 112 is to vary pulse length and to provide time shifting and inversion of the signals generated by generator 100 to provide clock and sync signals having the particular format for driving the buses 24, 26 and 28, the switching matrix, switches and input and output bus 12 and the digital signal processors 48. The clock translation circuitry 104, 106, 108, 110 and 112 is respectively connected to the information system buses 24, 26 and 28 and the switching matrix switches and input and output buses 12 and the digital signal processor 48 with buses 113, 115, 117, 119 and 121. The clock generator circuitry 100 provides common clock and synchronization for permitting the interconnection of the information system buses 24, 26 and 28, with appropriate clock rates and frame rates as defined by sync pulses to permit the interconnection of information systems 18-22 having incompatible bus protocols with the protocols of the remaining information systems when they are operated as independent stand-alone systems as defined by their manufacturers. The external frame reference from one or more of the information systems 18, 20 and 22 is coupled from the buses 24, 26 and 28 to a selector 122 which selects a particular external frame synchronization reference from available references which may be from any information system which is connected to the clock generator circuitry 100 to permit slaving to the external source such as with T1 service resulting from a connection to the PSTN or to timing from one of the information systems. As has been stated above, the translation circuitry modifies the clock signal outputs and sync signal from the clock generator 100 to have appropriate pulse length duration and shifting and inversion to be compatible with the particular characteristics of the information system to which the information system buses 24, 26 or 28 is connected while still being overall compatible with all of the other information systems. In other words the translation performed to interface the clock signals and sync signals between the clock generator circuitry 100 and the individual information systems 18, 20 and 22 does not alter the standard timing output by the clock generator circuitry to a point where the operation of the individual buses is incompatible when they are interconnected between information systems 18, 20 and 22 by the digital switching matrix, switches and input and output bus 12.

FIG. 3 illustrates a block diagram of an implementation of a third information system 22. Like reference numerals identify like parts in FIGS. 1-3. It should be understood that a preferred embodiment of the third information system 22 may be configured into seven groups of five interconnected circuit boards each having the circuitry of FIG. 3. However, the third information system 22 is not limited thereto. The individual groups of five circuit boards in accordance with FIG. 3 may be interconnected with a 10-pin ribbon cable to the individual interfaces contained within the block represented by the third information system bus and bus interface circuitry 22 of FIG. 1. As a result, the groups of five circuit boards having the circuitry of FIG. 2 may be interconnected into a complete third information system 22 which may be further interconnected with other information systems 18 and 20 as illustrated in FIG. 1 to provide multiple conferencing capabilities making the system of FIG. 1 extremely flexible for current or future voice and data applications and expansion as a result of permitting the interconnection of otherwise incompatible systems. It should be noted that many of the parts of FIGS. 1 and 3 are common and perform substantial the same function. When a part is not described in detail its function should be understood to be similar to that described above with regard to FIG. 1.

The third information system 22 functions as a general purpose line interface for up to six analog telephone lines which can be any combination of DID, E&M, Source, ground start, loop start or battery feed as described above. These connections are indicated by the RJ21X connector to telco and local audio devices 202. Individual line interface modules 204, which are discussed below in conjunction with FIGS. 4-8, are plugged into the ports in the circuit board represented by FIG. 3 to configure a particular audio line 206 as determined by the characteristic of the line interface modules 204 described below to interface with the type of analog line service provided by the line interface module.

As a consequence of splitting the circuitry between the line interface modules 204, which are removable from the ports of the third information system, the quantity and type of circuitry required to be present within the individual board implementing the third information system 22 is reduced thereby reducing the cost of the board and further providing space for additional line ports. For example, the present invention, as illustrated in FIG. 3, provides a 50% increase in the number of ports which are available over a prior art circuit as described above which had all of the circuitry for interfacing the five different types of analog line interfaces of the present invention resident on the circuit board. The address space of the control processor 34 and host processor 100 as implemented in the dual-ported RAM 54, RAM 56, ROM 60 and dual-ported RAM 62 is similar to that discussed above with regard to FIG. 1 both with regard to input, output and memory space on the PC bus. The memory space of the third information system 22 may be in common with the system of FIG. 1 in which the third information system is configured.

The control processor 100 is preferably an Hitachi 64180 microcontroller like the control processor of FIG. 1. The system provides access to 32K of ROM, 30K of RAM and 2K of dual-ported RAM as respectively implemented in ROM 60, RAM 56 and dual-ported RAM 54. The ROM 60 provides program storage for the intended application of the third information system 22. The RAM 56 is used for variable storage of temporary information. The dual-ported RAM 54 provides the interface between the control processor 100 and the host processor PC 34.

The control processor 100 controls all hardware specific aspects of the system. For example, if a port to which one of the line interface modules 204 is connected is set up as a DID port, the control processor 100 may receive a command from the host processor 34 to establish a connection in the DID port. The control processor 100 translates this information into an appropriate format and communicates with the DID port via one of the CODECs 210. Each CODEC 210 is connected to the internal PCM bus 212 and to the serial CODEC control interface bus 214. The serial CODEC control interface bus 214 is connected to a parallel to serial interface for CODECs 216 which provide communications with the control processor 100. The CODECs 210 are also connected to the line interface modules 204 through the combination of digital module control bus 218 and audio signal path 220 which includes transformer coupling circuitry 222. The transformer coupling circuitry 222 provides high voltage protection to the electronics of the third information system 22 and DC isolation.

The CODECs 210 utilize a serial communications interface through the serial CODEC control interface bus 214. The parallel to serial converter 216 permits the control processor 100 to address the CODECs. The CODECs provide numerous functions utilized in a telephony environment for establishing telephony connections. The CODECs 210 provide digital control outputs and sensor inputs. The outputs are used to control relays and other circuitry (not illustrated) necessary to seize and release telephone lines, inject ringing, etc. The inputs provide information regarding presence of loop current and presence of ring voltage, etc. The particular function of each signal is dependent upon the telephone type (DID, LOOP, START, etc.). The CODECs 210 also control the audio path 220 and perform A/D and D/A conversion of the audio signal. The analog side is transferred via transformer coupling 222 to the line interface module 204 to the telephone company or telephone instruments. The digital side through internal PCM bus 212 is transferred to interface logic and connector 230 which is connected to the third information system bus 28. The third information system bus 28 permits connection to other boards containing other third information systems 28 or to different line interface modules 204 through the internal PCM bus 212. The internal PCM bus 212 uses time division multiplex PCM encoding (either A-LAW or U-LAW). For example, the system may contain 32 devices which each transmit information on a single pair of wires. Each device communicates on one or more channels with the CODECs 210 controlling the time slots of a particular channel on which a conversation or other audio or data information may be transmitted.

The digital signal processor 232 provides tone resources required when connecting to telephone lines. These resources include DTMF generation, DTMF detection and call progress generation. Additionally, limited three-way conferencing is included in the capability of the digital signal processor 232. The call progress tones include all standard precise call progress tones required in the United States. The digital signal processor 232 sends tones directly over the internal PCM bus 212 to the CODECs 230 present in the third information system contained in FIG. 3. The digital signal processor 232 also receives tones from either the CODECs 230 or from the bus 28 through the interface logic and connector 230. Call progress tones are output continuously with no intervention from the control processor 100. Other functions are controlled by the control processor 100 via the FIFOs 62 for the digital signal processor 232 which in a preferred application are present as two 512 byte FIFOs. A standard command set is used to implement all of the functions performed by the digital signal processor 232. The digital signal processor 232 is used in some applications to provide PCM clock to the CODECs 230 present within the third information system 22 and to the bus 28. The clock source is controlled by suitable switches (not illustrated) which are within the third information system of FIG. 3. The state of one of the switches is read by the control processor 100 which then sends an appropriate command to the digital signal processor 232 to either provide the master clock, or slaved to an external clock.

The third information system bus 28 is a time division multiplexed bus which is designed to transfer PCM encoded information to and from the individual third information systems 22 as illustrated in FIG. 3 and to other third information systems and to the overall system of FIG. 1 as described above. The interface logic and connector 230 includes an appropriate master clock (usually 2.048 MHz but this is not a requirement), a transmit data signal, a transmit data synchronizing signal, a data receive signal and a receive synchronization signal. These signals are available for use by other third information systems 22 as well as the system of FIG. 1 via a 10-pin ribbon header. Expansion of the system beyond six modules 204 may be made through connections through the third information system bus 28. When a plurality of the third information systems 28 are connected together into a single system it is necessary that one of the third information systems such as that illustrated in FIG. 3 provides master clock and synchronization signals to the third information system bus 28 interconnecting the other third information systems assuming that operation independent of the system of FIG. 1 is occurring. A switch may be selected to program which of the third information systems 22 is to be the master. As a result all other third information systems 22 will be slaved to the master clock provided from the third information systems which has the program switch.

The RJ 21X connector to the telco and local audio devices 202 interfaces to telephone lines and peripherals. The connector 202 interfaces internally to the five types of line interface modules 204 as described below in conjunction with FIGS. 4-8 to provide interfaces with selectable types of audio lines as described above (DID, loop, start, etc.). The modules 204 transmit audio and signalling information to the CODECs 210.

A plurality of different types of modules are provided for different kinds of telephone lines and peripherals. The modules 204 are designed to affect the audio path as little as possible. The currently designed modules as described below are DID interface, a loop start interface, ground start interface, E & M interface, and source interface. The DID interface provides 48 volt battery to the telephone company as well as battery reversal provisions and current sensing for off hook detection. The loop start interface provides battery sink in the off hook state. Ring detection is available when on hook and current detection is available when off hook. This interface functions like a standard telephone set. The ground start interface provides all functions that the loop start interface provides with ground detection and seizure capabilities required by ground start lines. The E&M interface supports E&M types I and II. The audio path is untouched. The source interface provides battery to a standard telephone. If a ring generator is available, it can also ring the telephone to alert the user of an incoming call. Battery is applied at all times and current detection is always available.

The use of modules gives significant flexibility to the overall usage of the third information system 22. Since each module 204 supports only one or two line types, it is unnecessary to include circuitry for all types of analog line interfaces on a given module. Moreover, if special purpose functions are required, a new module can be designed to provide these functions. As a result it is not necessary that the individual third information systems 22 contain all of the circuitry necessary to support all of the analog line functions provided by the different types of line interface modules 204. Moreover, since telephone lines are occasionally exposed to hazardous voltages the use of the modules 204 makes repairs more straightforward such as if a component on a module is damaged the module can be easily replaced. Additionally, if this component were soldered into the main third information system 22 the repair would be much more difficult and time consuming. The third information system 22 includes a line interface connector 240 which permits a 48 volt power supply and ring generator to be connected to the board. The power supply and ring voltages are bussed to the line interface modules 204 by bus 242.

Multiple line interfaces are provided to the third information system 22 by the plug-in modules 204 for each of six ports. The modules 204 are attached to the main board containing the remaining circuitry of the third information system 22 of FIG. 3 with two 16-pin connectors. The signal names and pinouts for these connectors are as follows:

FIG. 4 illustrates a block diagram of a suitable E&M module 300 for use as a line interface module 204 in FIG. 3. Like reference numerals identify like parts in FIGS. 1-8. The current sense and protocol select circuitry 302 contains jumper switches that are used to partially select the desired E&M protocol such as E&M Types I and II. The current sense and protocol select circuitry 302 in conjunction with jumper switches detects the presence of current in E and SG leads. The seize relay and protocol select circuitry 304 contains jumper switches that are also used to partially select the desired E&M protocol such as E&M Types I and II. The seize relay and protocol select circuitry 302 in conjunction with the jumper switches controls the flow of current in M and SG leads.

FIG. 5 illustrates a block diagram of a suitable DID module 400 for use as a line interface module 204 in FIG. 3. The battery feed circuitry 402 provides DC current to the audio interface 220 but acts as a high impedance to AC signals that represent audio information. The current sense circuitry 404 detects the presence of current flowing from the line interface power connector 242 through the battery feed circuitry 402 and the audio interface 220 to indicate line seizure and release by external devices. The relay drive 406 is used to activate and deactivate relay 408 that controls the polarity of the voltage and current that is supplied to the audio interface 220.

FIG. 6 illustrates a source module 500 for use as a line interface module 204 in FIG. 3. The ring insertion circuitry 502 includes relay controls that activate and deactivate the application of ring voltage to the audio interface 220. The ring insertion circuitry 502 also monitors the audio interface 220 for DC current and removes ring voltage when DC current is present. The ring insertion circuitry 502 provides an indication to the digital module control 218 of the current state of the ring insertion circuitry. The battery feed circuitry 504 and current sense circuitry 506 are similar to circuits 402 and 404 described in conjunction with FIG. 5 above. The relay drive 508 controls relay 510 to control connection of the audio 220 to the audio 206 by closing relay 206 so that audio information can be transmitted through the module 500.

FIG. 7 illustrates a ground start module 600 for use as a line interface module 204 in FIG. 3. The ring detect circuitry 602 provides a signal to digital module control 218 that indicates the presence of ring voltage on the audio interface 220. The ground sense circuitry 604 provides a signal to digital module control 218 that indicates when the TIP lead connected to the audio interface 220 is connected to ground. The current sink and current detection circuitry 606 provides a DC current path to the audio interface 220 when a connection is established. The ground assert circuitry 608 connects the RING lead connected to the audio interface 220 to ground through a current limiting network. A signal is provided to the digital module control 218 to indicate the presence of DC current. Relay drive 610 controls relay 612 to control connection of audio 220 to audio 206 through the module 600 by closing relay 612. The ground sense circuitry 604 is normally connected to the TIP line by the relay 612 which also closes the other contact to permit connection of the ground assert circuitry 608 depending upon the state of relay 616 as described below. Relay drive 614 controls closing of relay 616 to control connection of the ground assert circuitry 608 to the RING line when relay 612 is in its normal state in which audio 220 is not connected to audio 206 through the relay 612.

FIG. 8 illustrates a loop start module 700 for use as a line interface module 204 in FIG. 3. The ring detect circuitry 702 and current sink and current detection circuitry are identical to circuits 602 and 606 in FIG. 7. The relay drive 706 controls relay 708 which is normally open to connect the audio 220 to the audio 206.

While the invention has been described in terms of its preferred embodiments, it should be understood that numerous modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims. For example, the number of information systems each having incompatible bus protocols which may be interconnected via the digital switching matrix, switches and input and output buses 12 may be varied to connect diverse types of information system. While a preferred application of the present invention is interconnecting systems configured from circuits utilizing digitized voice, it should be understood that the present invention is not limited to information encoding voice. For example other forms of pulse code modulated data or otherwise digitally encoded data may be transmitted such as data in accordance with ISDN specifications. It is intended that all such modifications fall within the scope of the appended claims. ##SPC1## ##SPC2##