WO1994006215A1

WO1994006215A1 - Communication device connected to an interface circuit employing opto-isolators

Info

Publication number: WO1994006215A1
Application number: PCT/AU1993/000463
Authority: WO
Inventors: Michael John Roberts
Original assignee: Michael John Roberts
Priority date: 1992-09-09
Filing date: 1993-09-09
Publication date: 1994-03-17
Also published as: GB2285899A; GB9504458D0; AU4809593A; AU671667B2; GB9219112D0; GB2285899B

Abstract

An apparatus for communicating over the telephone line (6) via socket (8) has a microprocessor (10), a memory comprising p-ROM (12), and RAM (14), a modem (20), a parallel port (16) and interface (17), a serial port (18) and interface (19), a telephone interface circuit (24) and a power supply (26) in one assembly. The apparatus operates automatically with a program stored in its memory and provides facilities for message transmission, reception and storage. Telephone interface circuit (24) uses only a pair of opto-couplers and non inductive components, complies with the requirements of many telephone administrations and is able to receive, store and print messages without the need of an attached computer.

Description

COMMUNICATIONDEVICE CONNECTED TO ANINTERFACE CIRCUITEMPLOYING OPTO-ISOLATOR FIELD OF THE INVENTION

This invention relates to a device for communication of facsimile, data, digitised voice and other messages over a communications network and to a communication network interface circuit for use therewith.

BACKGROUND OF THE INVENTION

Modems (modulator-demodulators) are known in the telecommunications field as devices for converting digital signals from a computer system into electrical analogue signals suitable for transmission over telephone lines and other communication networks and vice versa. Modems rely on an attached computer for presentation and encoding the message as well as for instructions concerning dialling, call progress and connection with the called or calling computer.

In addition, modems sending data over a switched telephone network must communicate with the exchange over the twisted wire pair provided for the usual telephonic communication. In full duplex

communication the transmitted and received data are superimposed. In most telephone line interfaces the transmitted signal is inverted and added to the received signal to cancel it on the received side in a hybrid circuit. The hybrid circuit usually employs an isolation transformer which is used to electrically separate the equipment and line sides at the

subscriber. This solution is unsatisfactory.

Hybrid imbalance occurs as the strength of the transmitted signal which is sent across the

transformer to the line and back again (and which is to be cancelled) is uncertain. It varies because of frequency dependent phase shift, component variations in the manufacture of the transformers and with changes in the transformer's core flux density. The latter is a function of the line DC current which in turn varies with the length of the line from the exchange.

In addition, the subscriber equipment side of the modem must separate the transmitted and received data, send on/off hook signals and detect a ring signal from the twisted pair of the telephone line.

SUMMARY OF THE INVENTION

In one aspect of the invention these problem are overcome with a simplified telephone line to

subscriber equipment interface circuit employing a pair of opto-isolators, or opto-couplers.

Throughout the specification the terms opto-isolator and opto-coupler will be used interchangeably. Also the term data can simply mean a signal, for example an analogue tone signal when used in relation to the telephone interface.

In another aspect, the invention combines a small computer system with a modem in one assembly so that the assembly is able to perform the above supervisory functions normally requiring connection to a computer allowing simple computer programs to be used to transmit facsimile, data and messages without needing an intermediate communications program.

In addition this invention is able to deliver received messages directly to a printer and can store and automatically relay received messages without the need of an attached computer.

According to a first aspect of the present invention there is provided a telephone line to subscriber equipment interface circuit comprising a receiving opto-isolator and a transmitting opto-isolator, said receiving opto-isolator coupling received data in the off-hook state and the detection of a ring signal when in the on-hook state of the subscriber equipment;

wherein said detection is sensed by the

photo-transistor of said receiving opto-isolator;

said transmitting opto-isolator providing a line seize mode for initial connection, a line hold mode in the off-hook state and transmitting data in said line hold mode, wherein data being transmitted is also applied across the photo-diode of said receiving

opto-isolator.

According to a second aspect of the present invention there is provided a device for communication of facsimile, data, digitised voice and other messages over a communications network including an address, data and control bus connected to which is a modem, a micro-processor, a memory system, a non-volatile program storage, a parallel interface for connection to an external printer, a serial interface for

connection to an external computer, a communication network interface, and a power supply bus, with a program stored in said non-volatile program storage, which encodes and decodes messages as well as manages the sending and receiving of facsimile, data or other messages over the communication network.

The apparatus is able to transmit messages delivered to it by a computer system without relying on the computer for management of the transmission procedure By simplifying the task of the attached computer, a word processing or simple text printing program normally provided with a small computer or a word processing system or an electronic agenda manager is all that is necessary to send messages. The apparatus is able to receive messages. It

automatically answers an incoming call, negotiates the appropriate speed and protocol and receives the message for storage and/or printing without there being any need for an attached computer. In a typical embodiment of the invention, several programs for controlling printers are provided and a means is included in the apparatus to automatically detect which type of printer is attached and then select the appropriate printer driving procedure and signalling method.

A typical embodiment of the apparatus uses its memory system to store messages. The apparatus can relay a stored message to a modem, a facsimile machine or a duplicate hereof. In one embodiment of the invention, such message store and forward facilities can be controlled by the tones generated by a remote or local tone dialling

telephone. Two operations are provided in a typical embodiment of the invention. It can automatically relay a stored message after it has been received or it can accumulate stored messages until commanded to unload them to a remote apparatus . Thus the apparatus can provide a similar function to the telephone answering machine in the field of facsimile and data communication.

DESCRIPTION OF THE DRAWINGS

Figure 1 shows a block diagram of the device according to the invention;

Figure 2 shows a flow chart of the program steps for operating the device of Figure 1; and

Figure 3 shows a block diagram of the telephone interface circuit according to the invention.

PREFERRED MODE OF PERFORMING THE INVENTION

The device of Figure 1 has a power supply bus 2 for supplying power to the components of the device , an address, data and control bus 4 and a telephone line outlet 8 for connection to telephone line 6.

Connected to bus 4 is the central processing unit (CPU) 10 operated by the program stored in

non-volatile memory 12, which may be a programmable read only memory (PROM) or read/erase/write memory such as FLASH or EEROM or any other memory storage device well known in the art for this purpose. A random access read-write memory (RAM) 14 is provided and can be made non-volatile with a back up battery or a non volatile random access memory such as FLASH memory can be used.

Two externally accessible interfaces are provided for connection to a printer and a computer, viz. parallel port 16 having a standard parallel interface socket 17, a parallel port being commonly used to drive printers (although a serial port is sometimes used for this purpose), and a serial port 18, for example of a RS 232 format, which is commonly used to enable a computer to communicate (provide data input/output), having a standard serial interface socket 19. The modem 20 is the means for conversion of signals between the digital circuitry of CPU 10, memory 12, 14 and ports 16, 18, and the telephone line 6 (or other communication network) accessed via telephone line socket or outlet 8, for example of a JB11 type. The telephone interface circuit 24 (see Figure 3) provides isolation and bidirectional signal coupling.

The power supply 26 comprises a voltage regulator with smoothing filters and can be supplied with an

unregulated or partly regulated voltage through an external socket 28 provided from an external

transformer or battery. Alternatively a battery may be incorporated within the device.

In one embodiment of the invention a Zilog super integrated circuit is used for the central processing unit 10, incorporating the serial port 18 and parallel port 16 providing input and output, as well as counter and timing functions. An Hitachi 512 kilobyte

pseudo-static memory with external circuitry to manage bank switching and a 32 kilobyte programmable read only memory make up the memory system 12, 14. Modem 20 is a Rockwell single chip modem and operates with a telephone interface circuit 24 built with discrete components .

Figure 2 shows the program steps whereby the invention manages outgoing and incoming messages. The

procedures which the invention performs are controlled by instructions stored in the programmable read only memory 14 (see Figure 1).

The program of instructions begins when power is applied, the program performing a self-test before setting the circuitry into a prearranged state

(initialisation 100). The program then determines one of three courses of action steps 102, 104, or 106. These steps are: send a message if there is one waiting in memory (step 102), interact with a computer if there have been any signals requiring such

interaction (step 104), or analyse an incoming ring signal from a telephone or other network (step 106). If none of these are required then the program places the circuitry into a power saving condition (step 108) and waits for one quarter of a second. The process steps 102-106 are then repeated.

In step 102, a stored text or facsimile message is sent and the control of the device is returned to the main loop 100 and thence to its idle state 108 as required.

In step 104 data are sent to the device in the form of standard ASCII text or binary coded characters. The first few characters are interpreted in step 110.

If the first three codes are "NNN" (tilde is ASCII character number 126) the message is treated as executable code. It is loaded into memory and the process run (step 112), returning the device to the main loop 100 once completed.

If the first three text characters are "FAX" (step 114) the device stores the text and extracts the destination telephone number from the message. In this example of the device the number to be used as the destination for the message is the first group of digits in the message and this may include

parentheses, spaces and other punctuation marks commonly used in telephone numbers, The letters P or T can be included to indicate pulse or tone dialling respectively.

After extracting the number the device dials it and establishes communication with a facsimile transceiver at that number. Upon successful negotiation of the CCITT T30 procedure the device encodes and transmits the message (step 102). An automatic retry procedure is started up if the first attempt to deliver the message is unsuccessful.

If a printer is attached to the device at the time the message is transmitted the device instructs it to print a replica of the image which it is transmitting to the remote facsimile transceiver. This replica is printed at the same time as the message is being transmitted and in this embodiment the replica is produced by graphically encoded means rather than just sending ASCII characters to the printer. At the end of each page a report is printed stating the connected number, the dialled number, the duration of the connection and whether the message transmission was successful or failed. By this means the operator knows that his message has been sent and has an original copy with exactly the same typeface and format as the received document.

In a typical installation the device is attached to the serial port of the computer and drives a printer from its parallel port 16. In order for the computer to be able to print messages without changing any connections, the device becomes transparent to

messages if the key letters 'FAX' or the modem command letters 'AT' are omitted (step 116). In this

transparent mode messages passed from the computer's serial port, are received by the device which

retransmits them to an attached printer through its parallel port 16.

The method by which the device receives messages depends on whether the calling device is a modem or a facsimile transceiver. Messages to be received by the device are either stored in the device's memory or printed or sent to an attached computer. This example of the device deals with an incoming facsimile message by negotiating the CCITT T30 protocol then receiving the message into its memory. At the time the image part of the message arrives the device checks to see if there is a printer available and automatically runs a test to select the appropriate printer driver as explained below. If the message can be printed the device concurrently receives the message into its memory, manages the message acknowledgment and line disconnection and keeps printing from its memory. If the message cannot be printed the device retains it in memory. In the embodiment described up to 15 pages of A4 text can be stored but this can be varied depending on the available installed memory. At a later time if a printer is connected to the parallel port the device will detect this and print the stored message(s). The device provides a telephone interface circuit 24 of novel design (see the description below with respect to Figure 3), one feature of which is the manner of detection of a ring signal. As more fully explained below a ring signal is coupled as a weak analogue signal to the receiver on the equipment side. There it is converted into digital form and processed by CPU 10 using a software routine (step 100).

Once a ring signal is detected the device

automatically answers (goes "off-hook") (step 124) and proceeds to process any tones or signals received, placing the device in the appropriate state commanded. This may include commands to receive a facsimile transmission (step 126); to act as a simple modem for data input/output (step 128) or to act as a relay facility (step 130) as specified in commands sent from a remote location employing DTMF signalling. In this embodiment of the invention there are two versions of the message relay facility:

(a) Message redirect (step 131) causes any received facsimile message to be stored then transmitted to a number which has been previously programmed into the device by tone dial signals or a message from a local or remote computer.

(b) Remote unload or facsimile retrieve (step 132) is a means by which one or more received facsimile messages are held in the device's memory system until a command in the form of tone dial (DTMF) signals is received over the telephone line from a remote

telephone, modem, facsimile machine or local computer. Whereupon, provided this command incorporates the correct password, the stored messages are transmitted over the line using the Group 3 facsimile procedure.

External switches or push buttons can be used to alter or interrupt the functioning of the device. In this case routines in the software (step 134) would check for and process the interruption. Typical features which may be provided are : to reset the device (step 136); to change the control of the printer (attached to the parallel interface) from being under the command of the device to being under the command of an attached computer (step 138); or to place the device in a facsimile mode, ready to receive a facsimile transmission (step 140). The reset (step 136) would do a 'warm re-boot' restarting the device at the initialisation stage (step 98) without performing any self-testing. Step 138 is intended to toggle the printer between these modes. Step 140 invokes module 126. In addition to the above, other features may be included in the software routines. For example, when ringing is detected (step 106) and in parallel with the program which detects DTMF (dual tone

multi-frequency) signalling (step 124), a program could determine when the "off-hook" state has been initiated (ringing ceased) whereafter a check for the presence of voice in the subsequently received signal is performed. If so, it returns operation of the device to idle allowing the telephone connection to operate normally.

Another facility automatically determines whether to pulse or tone dial the number. This is useful for the redirecting of facsimile messages in countries where only pulse dialling is available and a remote hand held tone generator is used to command the device over the telephone line. A remote tone telephone can load the redirect number into the device but as there is no obvious command to indicate whether the redirect number employs pulse or tone dialling it is more convenient that the device automatically employs the correct dialling procedures such that this aspect is transparent to the user. The device has means to distinguish between two general classes of printer, laser printer or ink-jet printer. By using a combination of tests on the electrical signals on the printer interface and timing the signals coming from the printer in response to sent commands the apparatus can detect which type is connected and automatically employ the appropriate internal printer driver to print replicas and received message in a graphics mode. Another feature of the apparatus is a facility by which graphic images can be printed on a remote laser printer. Information prepared in a computer for printing on a local laser printer can be conveyed to another remote device of the type described herein so that it can be printed remotely. This of course can be done by conventional modems however the user of the computer system must first save his file, then

establish contact with the remote computer and modem. The remote computer in turn must save the file then arrange the appropriate driver for the printer there. All these matters are transparent to the operator of the computer who has merely to print his graphics or text as usual for it to be conveyed to a remote like apparatus and printer. By this means high quality laser printer images can be conveniently transmitted from a computer to a remote laser printer without there being any need for a remote computer to drive the laser printer. This is an improvement on Group 3 facsimile conditions because the image compression can be more efficient and very high quality reproduction can be achieved. This facility is possible because the device manages all the communications. Thus the operator of the computer just has to print his

document in the same way he would do if the remote laser printer were connected directly to his computer.

There is also a means by which the shape of characters which are generated by the text received from a computer can be changed. Similar facilities are well known in printers as "soft fonts" and the instant device seeks to emulate the way a printer can accept downloaded character fonts. By this means a user can use one of the well developed word processing software packages to compose his messages using a variety of font types and sizes.

The device has means for accepting facsimile commands and pre-encoded facsimile image files which can be conveyed to remote receiving apparatus. These are becoming known as the Telephone Industries Association Service Class 2 and Class 3 command set. The device has the ability to send graphic files to a remote facsimile transceiver. These can be in the format commonly used with laser printers or the tagged image file format. The user has just to send the file to the device through the serial port of his computer and the device manages the transmission. In order to achieve this the device must be given a destination number. This is in a similar way to that used with the automatic transmission of text as described above. A brief message is prepared which contains the key letters 'FAX' and then the destination telephone or facsimile number. When this file is sent through the serial port it is intercepted by the device. If the message is just one line the device will not transmit this as a text message but it will retain the

extracted telephone number. If the very next message is a graphics file this will be sent as a Group 3 facsimile to the remote apparatus. Any other message will abort the graphics send condition and thus the device will once again become transparent to messages sent from the computer to a printer through its serial port.

Thus the device, acting as a graphics printer, accepts a graphics print message, encodes it and transmits the image to a remote facsimile transceiver. All of these operations need no computer programs other than the normal printing program provided with computer design and drawing software systems .

Referring to Figure 3 there is shown the interface circuit by means of which the signals from the

equipment side are conveyed to the telephone line and vice versa. This circuitry is commonly known as a telephone access arrangement and usually a line transformer is needed to provide isolation between equipment and line. In this invention the transformer is eliminated and just two optical couplers are used, one for all functions required for transmitting and another for all functions associated with receiving. This technique offers superior safety and noise isolation as well as reduced distortion of signals and improved matching to a variety of line currents and impedances.

As shown, the two connections between the equipment side 40 and the line side 42 are the transmitting optical coupler 44 and the receiving optical coupler 46. Transmitting optical coupler 44 has photo-diode 48 on the equipment side 40 and photo-transistor 50 on the line side 42, while the receiving optical coupler 46 has photo-diode 52 on the line side 42 and

photo-transistor 54 on the equipment side 40.

On the equipment side 40 the receiver comprises photo-transistor 54 with a common collector output, providing the received data 32 and ring signal 34 while the transmitter applies the transmit data 30, on-off hook signal 36 and line seize signal 38, across photo-diode 48. On the line side 42, the circuit includes a

characteristic impedance 60, which is designed to match the reference impedance used to test 'insertion loss'; a current sense 62 which ensures a stable operating current is maintained through the

photo-diode 52; a variable shunt 64 which conducts most of the line current when the hook switch 66 is switched on; a ring sense 68, which passes a small part of an incoming ring signal to the base 58 of the photo-transistor 50; a transmitter load 70 passes a signal to the line 22 in such a way that equal

amounts of the signal appear on both sides of the photo-diode 52. By this balance bridge the net transmit signal current flowing through the

photo-diode 52 is very small and consequently a negligible proportion of the transmit signal is received on the equipment side 40. The components of the transmit load 70 balance bridge can be either fixed, manually variable or automatically variable to obtain perfect balance according to the complexity of the circuit which performance and economic measures dictate.

For impedance 60, the circuit employs a universal line termination which satisfies the insertion and 'return' loss criteria of most administrations. Many

administrations require that the 'insertion loss' and the 'return loss' lie within certain limits over a range of frequencies. It is difficult to achieve a universal solution with a line transformer because it has a frequency dependant inductive component in its impedance and the telephone administrations generally require the telephone interface to be matched to a characteristic impedance which is capacitive.

A universal line termination is achieved with a circuit which bypasses the DC line current through a low resistance and forces the signal (AC component) to pass through an impedance comprising resistive and capacitive components only and which is typical of the equivalent circuit used by most telephone

administrations to test 'insertion loss' and 'return loss'. This is sometimes called the 'reference impedance'. The sending optical coupler 44 is operated in a linear mode at three distinct steady state levels. These are controlled by the current through the photo-diode 48 located on the equipment side: (a) in the case of no steady state current the

electronic switch on the line side is off. This is commonly called "on hook".

(b) when the level of current is on at a level

sufficient to operate the switch on the line side the apparatus is "off hook" and is able to transmit signals which are superimposed on the steady current through the photo-diode 48. Because the

photo-transistor 50 is then in an active state, it conducts current in proportion to the current in the photo-diode 48. Thus signals are transferred from the equipment side 40 to the line side 42 and are

transmitted to the telephone exchange. ( c) when it is necessary to answer a telephone call or do pulse dialling the steady state current through the photo-diode 48 is increased. This causes an

additional switch 72 on the line side 42 to conduct and reduces the line resistance to a lower level.

This is often called 'line seize' condition.

The sending optical coupler 44 pulse dials by

establishing a current through the photo-diode 48 at a line seize level and switching the current on and off to generate make and break signals.

This causes the hook switch 66 to be either open or closed in a low resistance condition suitable for pulse dialling make and break signals. Referring now to the receiving functions of interface 24, the receiving optical coupler 46 has two

functions:

(a) it operates in linear mode to receive analogue signals; and

(b) it couples the incoming ring signal to the

equipment side 40. The receiving opto-coupler 46 detects ring signals in the following manner. By connecting the telephone line to the base 58 of the photo-transistor 50 of the transmitting optical coupler 44 through a network including a small capacitor large variations of line current such as ring signals cause brief periods of conduction of this photo-transistor. This causes small pulses of current to be passed through the line side photo-diode 52 of the receiving optical isolator 46. On the equipment side 40, the receiving

photo-transistor 54 is connected in a high gain circuit when the equipment is on hook. By means of this high gain, the very small ring pulses derived from the line can be detected and their periodicity and cadence analysed. By these means the ring signal is able to be detected without using an additional opto-isolator.

The receiving opto-coupler 46 receives data signals as follows. The receiving circuit comprises an

electronic hook switch 66, and a variable current shunt 64 for the steady state component of the

telephone line current in parallel with a constant impedance path 60 for the signal component of the line signal. Photo-diode 52 is included in the signal path and a small constant current is passed through this photo-diode 52 to cause it to conduct at a chosen steady state level which is independent of the amount of the total telephone line current. A negligible part of the signal component of the line current also passes through a parallel path 70. By means of a network of filters and negative feedback low resistance path 64 presents a high impedance to short term current variations which are forced to pass through the photo-diode 52 and thus appear as signals at the emitter of the photo-transistor 54 on the equipment side 40. When the electronic hook switch 66 is conducting the steady state current shunt 64 adjusts to whatever line current is left over from the small steady current passing through the photo-diode 52.

The transmit load 70 is a circuit of resistive and capacitive components connected across photo-diode 52. It provides a simplified and consistent means for cancelling the transmitted signal in the receiver photodiode 52. Thereby avoiding the difficulties discussed above for the prior art modems using line transformer isolation techniques.

The two path circuit arrangement presents a low resistance which satisfies the requirements of the various telecommunications administrations and a higher AC impedance to correctly terminate the

telephone line. The constant operating conditions across the opto-coupler enables it to operate in linear mode and faithfully reproduce the received signal across the load on the equipment side.

The above description is not to be considered limiting and other variations are possible within the knowledge of a person skilled in the art.

Claims

1. A telephone line to subscriber equipment interface circuit comprising a receiving opto-isolator and a transmitting opto-isolator, said receiving

opto-isolator coupling received signal in the off-hook state and the detection of a ring signal when in the on-hook state of the subscriber equipment; wherein said detection is sensed by the photo-transistor of said transmitting opto-isolator; said transmitting opto-isolator providing a line seize mode for initial connection, a line hold mode in the off-hook state and transmitting signal in said line hold mode, wherein a signal being transmitted is also applied equally across the photo-diode of said receiving

opto-isolator.

2. A device for communication of facsimile, data, digitised voice and other messages over a

communications network including an address, data and control bus connected to which is a modem, a

micro-processor, a memory system, a non-volatile program storage, a parallel interface for connection to an external printer, a serial interface for

connection to an external computer, and a

communication network interface, and further including a power supply bus, and a program stored in said non-volatile program storage, which encodes and decodes messages as well as manages the sending and receiving of facsimile, data or other messages over the communication network.

3. The device as claimed in claim 2 wherein said device is able to transmit messages delivered to it by an external computer system without relying on said computer for management of the transmission procedure.

4. The device as claimed in claim 3 wherein said device is able to receive messages automatically including answer incoming calls, negotiate the

appropriate speed and protocol and receive the message for storage, and/or printing without external

assistance.

5. The device as claimed in claim 4 wherein said program includes means for controlling a printer attached to said parallel interface and a means to automatically detect which type of printer is attached and to select the associated printer driving procedure and signalling required therefor.

6. The device as claimed in claim 5 further including a received data forwarding facility wherein said memory stores messages for subsequent relay to a remote location being a modem, a facsimile machine or a duplicate hereof.

7. The device as claimed in claim 6 wherein said received data forwarding facility can be controlled by tones generated by a tone dialling telephone apparatus and includes an ability to automatically relay, to a predetermined number, a stored message after it has been received, or to accumulate stored messages until commanded to unload said messages to a specified apparatus by said remote apparatus.

8. A device for communication of facsimile, data, digitised voice and other messages over a telephone network including an address, data and control bus connected to which is a modem, a micro-processor, a memory system, a non-volatile program storage, a parallel interface for connection to an external printer, a serial interface for connection to an external computer, and a telephone network interface, and further including a power supply bus, and a program stored in said non-volatile program storage, which encodes and decodes messages as well as manages the sending and receiving of facsimile, data or other messages over the communication network, said

telephone network interface comprising a receiving opto-isolator and a transmitting opto-isolator, said receiving opto-isolator coupling received data in the off-hook state and the detection of a ring signal when in the on-hook state of the telephone subscriber equipment; wherein said detection is sensed by the photo-transistor of said transmitting opto-isolator; said transmitting opto-isolator providing a line seize mode for initial connection, a line hold mode in the off-hook state and transmitting data in said line hold mode, wherein data being transmitted is also applied across the photo-diode of said receiving

opto-isolator.