NZ233360A

NZ233360A - Autoanswer videotex terminal

Info

Publication number: NZ233360A
Application number: NZ23336090A
Authority: NZ
Inventors: Guy Bindels
Original assignee: Alcatel Australia
Priority date: 1990-04-19
Filing date: 1990-04-19
Publication date: 1992-05-26

Description

<div class="application article clearfix" id="description"> j Priority | 9;;,.A^j-. ..ttf..SjO.. ■ .&ss: (^....bi.0.AtT..0a.i.i./o.s../ .tfOI+,ml/Uh 2 6 MAY j P.O. •r-.o: I'" NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION "VIDEOTEX TERMINAL" I ^APflj<)qn o WE, -ST-A^DARD-TEbE-PHONE'S--A'ND-&A-B'LBS-PT'Y-r-IrIM-ITED, A Company of the State of New South Wales, of 252-280 Botany Road, Alexandria, New South Wales, 2015, Australia, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 1 "7 T T 0 d 0 t> This invention relates to videotex terminals such as those called "minitel", and in particular to such a terminal connected to the public switched telephone network (PSTN) and fitted with an automatic answering facility. Typically, a communication between a minitel terminal and an information provider is established, via the PSTN, from a subscriber's telephone set. The telephone set is connected, via the minitel terminal, to a telephone network connector switch. Initially, the telephone is connected to the subscriber's line, while the minitel terminal Itself Is not connected to the network. A flip-flop switch in the minitel terminal ensures this function. This same controlled flip-flop switch is used to connect the minitel terminal to the telephone line, once the connection is established. The procedure to access the PSTN for connection to the information provider occurs via the telephone set with the handset lifted. The subscriber dials either the number of the database centre to which the provider belongs, or the provider number. The call is routed via the telephone network, and detected by the switching centre which connects the provider. An answer tone is sent by the provider to the subscriber. Acknowledgement of this tone by the subscriber confirms that the connection is established with the provider. Then, the subscriber switches the telephone line to his minitel terminal, using the "connect" key designed for this purpose on this terminal, which controls the abovementioned flip-flop, and finally hangs up his telephone . This call signal transmitted to the telephone line has specific characteristics, in order to be detected. It cor-5 responds to the application of a low-frequency a.c. voltage between the network line wires, generally at a frequency of 50 Hz and a level between 25V and 96V. The answer tone is also defined, having a 2100 Hz frequency . 10 The minitel terminal and the provider being connected via the network, data exchanges between the minitel terminal and the provider can then take place via the established connection. For this data exchange procedure between the minitel 15 terminal and the provider, the minitel terminal is fitted with a modem connected via a duplexer to a common data transmission/reception channel, and a processing unit built around a microprocessor connected to the modem. In addition, it includes an electrical isolation transformer coupl-20 ing the common transmit/receive channel to the two telephone line wires. A current generator, mounted between the terminals of the transformer windings connected to the line wires, maintains a continuous current on the telephone line, as soon as the connection is established and for the dura-25 tion of this connection, for detection by the exchange of the subscriber's telephone line engaged state. 3 awr 0 3b The processing unit is built around the microprocessor also coupled to the minitel terminal data entry keyboard and screen and possibly to a printer. The modem is assigned to the conversion of digital data - which corresponds to the 5 data entered from the keyboard and the data displayed on the screen - into analog data signals transmitted in either direction over the telephone line. The duplexer transmits these signals by splitting the common transmit/receive channel coupled to the line into two individual transmit and re-10 ceive channels connected to the modem. Bi-directional signal transmission via the telephone line is performed by assigning two frequency bands, for each of the signal transmission directions. These two frequency bands are centred on a 420 Hz frequency for the signals 15 transmitted from the minitel terminal to the provider, and on a 1700 Hz frequency for those transmitted from the provider to the minitel terminal. The transmission occurs at low speed, 75 bauds, from the minitel terminal to the provider, and at a higher speed, 1200 bauds, from the provider 20 to the minitel, and in both directions with a low voltage level of IV maximum peak value. The transformer isolates the minitel terminal from the telephone network and is matched to the telephone line specifications for data signal transmission, in order to present 25 with the current generator or the duplexer transmit channel, an impedance matched to the telephone line impedance, generally set at 600 ohms. £3 3 3 To transform such a conventional minitel terminal, into an auto-answer terminal requires that the circuits assigned for data signals, with the electrical isolation transformer between them and the network, be associated with circuits assigned for call signal detections, the latter having their own electrical isolation between them and the network. Compared with a conventional minitel terminal, such a minitel terminal fitted with an auto-answer facility costs more, due to the call signal detection function and the required electrical isolation which is Inherent to this function . An object of the present invention is to produce a cost effective auto-answer videotex terminal integrating a call detection. According to the invention there is provided an autoanswer videotex terminal connected to the PSTN and featuring a first pair of wires assigned to call signal transmission, a second pair of wires assigned to data transmission and a controlled flip-flop switch, initially coupling the first pair of wires to the network, then, coupling the second pair to the network once the connection is established. The said terminal includes call signal detection and data signal transmission devices connected via a modem to a processing unit, electrical isolation devices between the data transmission devices and the network for the data signal, characterised in that the said call detection devices include an operational amplifier, the output of which is con nected to a device for recognition of the detected call signal, and the signal input is coupled to the said first pair of wires via the said electrical isolation devices for the data signal and a capacitor compatible with the call signal frequency and an attenuation resistor for the said call signal level, associated with the said electrical isolation devices for the terminal to initially operate in auto-answer mode, and in that it also includes firstly some means for shunting the said attenuation resistor and secondly some means for impedance matching to reduce the impedance of the said capacitor, controlled by the said flip-flop switch when a call signal is recognised, for the terminal to operate in data transmission mode. Preferably, the operational detection amplifier applies a high and a low threshold to its reference input to switch its output state, matched to shape the detected signal into square waves having a frequency equal to that of the said call signal and a set duration of at least a few milliseconds . Preferably, the second impedance matching means consist of a current generator designed to absorb a current on the telephone line once the connection has been established, and having a limited finite impedance of approximately 1800 ohms. In order that the invention may be readily carried into effect, an embodiment thereof will now be described in relation to the drawings. In this drawing, the single figure shows the circuits of an auto-answer minitel terminal according to the present invention. The auto-answer minitel terminal according to the present invention is connected to two lines (A1 and A2) of the PSTN via a switch (1) from which three wires (LI Tl, T2 and L2 respectively) run to the auto-answer minitel terminal. Two of these wires (Tl and T2) correspond to the pair of wires connected to a subscriber's telephone, not shown here, for sending the dial signals to the line. The two other wires (LI and L2) correspond to the wires connected to the circuits of a conventional minitel terminal for bi-directional transmission of data signals to the telephone line (A1 A2), once the connection previously requested is established. In the minitel terminal, a changeover switch (2) connected to wire (T2) connects this wire (T2) either to wire (L2) or wire (LI Tl). It corresponds to the flip-flop switch of a conventional minitel terminal, which, when in the break position, connects the telephone to the network, via wire (L2) connected to wire (T2), and which is manually actuated by the "connection" key of the conventional terminal once the connection is established. This changeover switch (2) is shown in the break position, making it possible for the subscriber's telephone to send a dial signal, and for the auto-answer minitel terminal according to the invention to detect a call. In the other position whereby it connects wire (T2) to wire (LI Tl), it is said to be in the make position, for data transmission. The auto-answer minitel terminal consists, on the one hand, of a call signal detector (3), and on the other hand, of a duplexer (4) and a modem (5) for data signals. Duplexer (4) transmits data signals in either direction, between a common transmit/receive channel (6)and two individual transmit (7) and receive (8) channels. These two individual channels (7 and 8) are connected to model (5). A processing unit built around a microprocessor (10), (only the latter is shown here) is connected at one end to the modem for processing of the data signals to be transmitted or received, and at the other end, via a controlled switch (11) to a call signal detector (3)> for recognition of these signals. This microprocessor (10) is also coupled to a keyboard, a screen and a printer not illustrated here, for the entry of data to be transmitted and the display of data to be transmitted or received by the minitel terminal and for their printout as required. In this auto-answer minitel terminal, an electrical isolation transformer (13), adapted for bi-directional transmission of data signals to the telephone line via switch (2) in the make position, couples the common transmit/receive channel (6) for duplexer (4) to wires (LI L2). This transformer has a first earthed winding (14), and defines at its other terminal the common transmit/receive channel (6) for duplexer (4) to wires (11 L2). This trans former has a first earthed winding (1H), and defines at its other terminal the common transmit/receive channel (6). Its second winding (15) is coupled to wires (LI L2). A current generator (16) mounted in parallel between wires (LI L2), on the winding (15) terminals of this transformer (13), enables the switching centre to detect the engaged state of the line. Transformer (13) is also used to couple call signal detector (3) and telephone line (A1 A2), or wires (Tl T2) and switch (2) then in the break position. In order to take into account the characteristics of the call signal generally operating at a 50 Hz frequency with a minimum voltage level of 25 V, winding (15) of this transformer is connected between wires (Tl T2), to switch (2) in the break position, via a capacitor (17) and a resistor (18). This capacitor (17) has a value of luF approximately, selected according to the frequency of the call signal received from the telephone line and to be detected by detector (3) of the auto-answer minitel terminal. Resistor (18) has also a selected value of aproximately 33 kohm, for suitable attenuation of the level of the call signal transmitted by the transformer. In this auto-answer minitel terminal, featuring an isolating transformer (13) used both for data signals and call signals, a controlled switch (19) is mounted between the resistor (18) terminals. Closure of this switch (19) is controlled when switch (2) is placed in the make position to B shunt resistor (18) and thus shorting the attenuation it generates when the terminal is transmitting data. The three switches (2, 11 and 19) of this terminal are automatically controlled by microprocessor (10), as illus-5 trated by the arrow and the microprocessor item number in brackets. To the break position of switch (2) corresponds a C**) break position of switch (11) for which threshold detector (3) is connected to microprocessor (10) ensuring the terminal call signal recognition, and a break position of switch 10 (19) whereby this switch (19) is open and does not shunt resistor (18). In the terminal, switch (11) can be removed, thus microprocessor (10) is connected to modem (5) and detector (3) via two independent connections. In a way similar to the assembly associated with resis-15 tor (18), the auto-answer minitel terminal is fitted with means to reduce the capacitor (17) impedance when the terminal transmits data. These means may consist of an additional capacitor mounted with an associated controlled switch, not illustrated, in parallel on capacitor (17). 20 Nevertheless, these means consist preferably - as illustrated - of a current generator (16), featuring a selected impedance, not infinite, to match the Impedance given by the transformer to the telephone line impedance set at between *100 and 900 ohms. 25 The auto-answer minitel terminal circuits are described below. 10 T "7 T 0 0 i In this terminal, modem (5) and microprocessor (10) are known circuits. Microprocessor (10) is also provided for the recognition of the frequency of the detected call signal it receives from detector (3) via switch (11) closed. As such, duplexer (4) is also a known circuit. It includes two operational amplifiers, one for transmission (20) and one for reception (21) of data signals between channels (7 and 8) and channel (6), i.e. between modem (5) and transformer (13). Both amplifiers have a reverse feedback resistor (22) or (23) depending on the case, between their negative input and their output. The output of transmit amplifier (20) is connected to common transmit/receive channel (6) via a resistor (24), the negative input of the receive amplifier (21) is connected to this same channel (6) via a resistor (25). In addition, the output of the transmit amplifier is coupled to the positive input of the receive amplifier via a resistor (26), this positive input being earthed via resistor (27), to shape the receive data signal using the transmit/receive signal present on channel (6) and the transmit signal issued from amplifier (20). Current generator (16) includes a Darlington pair (transistors 30 and 31)» controlled via a diode bridge rectifier (32) globally referenced and a resistor bridge with resistors (33 and 34). The output transistor (31) of this Darlington pair has its collector connected to that of transistor (30) at one of the terminals of the diode bridge, 11 V to which resistor (33) is connected, and has its emitter connected via a resistor (36) to the other opposite terminal of the diode bridge, to which the other resistor (34) is connected, and which controls transistor (35). The two 5 other terminals of the diode bridge rectifier are connected to wires (L2 and LI), via switch (2) in the make position ^ for this latter wire (LI). Transistor (35) collector is connected, with transistor (30) base, via resistor (33) > to one of the terminals of the 10 diode bridge, its emitter is connected via resistor (37) to the other terminal of the diode bridge. In this current generator, resistors (33 and 34) are equal and are more powerful than resistor (37) and resistor (36). By suitably selecting the value of these resistors, 15 for instance R33 = 10 kohms, R37 = 143 ohms and R36 = 25-5 ohms, current generator (16) is given a finite impedance Z of approximately 1800 ohms. ^ This current generator (16) which maintains, in a known way, a current on the telephone line, for recognition by the 20 exchange of the engaged state of the connection, also allows reduction of the impedance of capacitor (17) during data ^ transmission. Call detector (3) includes an operational amplifier (40). Its negative input is connected to transmit/receive 25 channel (6) defined on the terminal of winding (34) of transformer (13), not earthed. Its positive input is connected on the one hand to a resistor bridge made up of re- 12 »<y «y sistor (41) connected to a 5V dc current source, resistor (42) connected to earth, and on the other hand on its output i""*^ via resistor (43) and diode (44). The output of. this ampli fier (40) is also connected to microprocessor (10) via re-5 sistor (45) and switch (11) in the break position. A Zener diode (46), mounted between the output of resistor (45) and '^1 earth, limits the output signal level of detector (3) to a TTL compatible value. In this call signal detector (3), the feedback arm with 10 diode (44) and resistor (43) is inhibited when operational amplifier (40) output is in high state, and is conducting when its output is in low state. As a result, a high threshold value is present on this positive input which flips the amplifier output signal from a high to a low 15 state, hence a low threshold value will flop the amplifier (40) output to the opposite state. This high threshold value is defined by resistors (41 and 42) and is selected so as to correspond to the near-linear and increasing part of the positive alternance of the call signal present on chan-20 nel (6). As for the low threshold value, it is defined by resistors (41, 42 and 43) and is selected close to 0 volt. These high and low threshold values allow to obtain on the operational amplifier (40) output, square signals having the call signal frequency and lasting more than 4ms, for 25 processing of call frequency recognition by microprocessor (10). 13 »$' T T C' 3 o o o Resistors (41, 42 and 43) have been given the following suitable values: R41 = 100 kohms, R42 =5-1 kohms and R43 = 150 kohms. For a +5Vdc voltage, the high threshold level is then Vs+ = 240 mV. The auto-answer minitel terminal being initially set for call signal reception by switches (2, 19 and 11) in the break position, current generator (16) is shorted, wires (12 and T2) being connected via switch (2). The call signals present on wires (Tl and T2) are significantly attenuated by resistor (18) and transmitted via capacitor (17) and transformer (13) to channel (6). As a result of the earth returned to resistor (24) and transmit amplifier (20) in the break state of duplexer (4), this resistor (24) is in parallel on winding (14) of transformer (13) and constitutes the load resistor on winding (14) for these call signals. The value of resistor (24) is selected to be almost equal to 1 kohm. This value intervenes with the global impedance of transformer (13), capacitor (17) and resistor (18) in the resulting impedance of the circuit. The signal present on channel (6) is thus detected by operational amplifier (40). The output signal of operational amplifier (40) brought back to TTL compatible level by Zener diode (46) is transmitted to microprocessor (10). The pulse frequency of the detected signal defined by the frequency of the call signal, can then be directly recognised by microprocessor (10). Recognition of a call signal by the microprocessor places switches (2, 19 and 11) in the make position. Call detector (3) is preferably isolated from microprocessor (10) by switch (11). Then, via model (4), microprocessor (10) 5 actuates the transmission at 2100 Hz of the answer tone through the line to the calling terminal which, when de-tected by the latter, enables the subscriber to connect his own minitel terminal to the telephone network. The called auto-answer minitel terminal and the calling 10 minitel terminal are then connected, and data exchange between them occurs with the auto-answer minitel terminal acting as a provider, ie. transmitting on the high frequency band centred on 1700 Hz, and receiving data signals from the calling terminal in the low frequency band centred on 420 15 Hz. The data signal transmitted by the minitel terminal can Initially consist of a welcome page for the calling termi- , ! ^ nal. Such a welcome page is initially stored in memories associated with microprocessor (10) in the processing unit. 20 This welcome page invites the calling terminal to follow the data exchange procedure with the called auto-answer termi-nal. 25 15 </div>

Claims

<div class="application article clearfix printTableText" id="claims"> 233360 What wc claim is:

1. An auto-answcr videotex terminal eonncctcd to the public switched telephone network and having a first pair of wires assigned to call signal transmission, a second pair of wires assigned to data transmission, and a controlled flip-flop switch coupling initially the first pair of wires to the network, then coupling the second pair to the network once the connection is established, the said terminal including call signal detection means, data signal transmission means connected via a modem to a processing unit, electrical isolation means between the data transmission means and the network for the data signal, wherein said call detection means includes an operational amplifier the output of which is connected to means of recognition of the detected call signal and a signal input of which is coupled to the said first pair of wires via the said electrical isolation means for the data signal and a capacitor compatible with the call signal frequency and a resistor for the attenuation of the level of the said call signal, associated with the said electrical isolation means for the terminal initially operating in an auto-answer mode, and wherein said terminal further includes means for shunting the said attenuation resistor and impedance matching means to lower the impedance of the said capacitor, controlled by the said flip-flop switch when a call signal is recognised, for operation of the terminal data transmission mode.

2. An auto-answer videotex terminal as claimed in claim 1, wherein said means of recognition of the detected call signal comprise a microprocessor connected to the output of the said detection operational amplifier, a detected call signal level limiter being connected to said output of said detection operational amplifier.

3. An auto-answer videotex terminal as claimed in claim 2, wherein said detection operational amplifier applies a high and a low threshold to its reference input to change over its output state, matchcd to the shaping of the detected signal into a 233360 square wave of frequency equal to that of the said call signal and of a duration set at a few milliseconds minimum.

4. An auto-answer videotex terminal as claimed in claim 3, wherein said operational amplifier applies to its rcfcrencc input a resistor bridge connected between a 5 voltage source and earth and an arm with resistor and diode connecting it to its output, defining the said high and low thresholds.

5. An auto-answer videotex terminal as claimed in any one of claims 1 to 4, and including a current generator absorbing a current on the telephone line once the connection has been established and for the duration of the operation of the terminal 10 in said data transmission mode, wherein said second impedance matching means are made up of the said current generator designed to present a limited finite impedance. 15 20 25

6. An auto-answer videotex terminal as claimed in claim 5, wherein the impedance of the said current generator is selected to be substantially equal to 1800 ohms.

7. An auto-answer videotex terminal, substantially as herein described with reference to the figure of the drawing. STANDARD TELEPHONES AND CABLES PTY. LIMITED B. O'Connor Authorized Agent P5/1/1703 18 </div>