An auto-answer videotex terminal includes data signal transmission and processing circuits and an isolating transformer between them and a telephone network. It also includes circuits for detecting and recognizing the ringing signal from the network which are connected to the network by the isolating transformer for the data signal circuits. A capacitor and an attenuator resistor appropriate to the frequency and the level of the ringing signal transmitted by the network are associated with the data signal circuits. Further, selectively operated circuits bypass the attenuator resistor and reduce the impedance of the capacitor when the ringing signal is recognized.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The present invention concerns videotex terminals such as the "Minitel" 
terminal used in France. It is more particularly concerned with a terminal 
of this kind connected to a public switched telephone network and equipped 
with an auto-answer device. 
2. Description of the prior art 
The connection between a videotex terminal and a data server via a public 
switched telephone network is usually set up using a subscriber telephone. 
The telephone is connected via the terminal to a telephone network jack. 
Initially the telephone is connected to the telephone subscriber line 
while the terminal itself is not connected to the network. A switch in the 
terminal performs this function. The same switch is operated to connect 
the terminal to the telephone line once the connection has been set up. 
The public switched telephone network is accessed to set up the connection 
to the server by means of the telephone with the handset off-hook. The 
subscriber dials the number of the data center for the server or the 
server's own number. The call is switched through the telephone network 
and is sensed at a connection center which makes the connection to the 
server. A return tone is sent by the server to the subscriber by way of a 
response. The subscriber hears this tone and recognizes that it confirms 
that the connection to the server has been set up. The subscriber then 
switches the telephone line to his terminal by pressing the key provided 
for this purpose to operate the aforementioned switch and then hangs up 
the handset of his telephone. 
To enable it to be detected at the receiving end the ringing signal sent 
over the telephone line has defined characteristics. It represents the 
application of a low-frequency alternating current voltage between the 
line wires, usually at a frequency of 50 Hz and at a level between 25 V 
and 96 V. 
The response tone is also predefined. Its frequency is 2100 Hz. 
When the terminal and the server are connected via the network data can be 
exchanged between the terminal and the server using the link so set up. 
To enable the exchange of data between the terminal and the server the 
terminal incorporates a modem connected via a duplexer to a common data 
transmit/receive channel and a microprocessor-based processor unit 
connected to the modem. It further comprises an isolating transformer 
coupling the common transmit/receive channel to the two wires of the 
telephone line. A current generator connected between the terminals of the 
transformer winding connected to the line wires maintains a direct current 
on the telephone line immediately the telephone connection is set up and 
throughout the duration of the connection, to enable the central office to 
sense that the telephone line is busy. 
The processor unit is connected to the data entry keyboard and to the 
display screen of the terminal and optionally to a printer. The modem 
converts digital data representing data entered at the keyboard and data 
displayed on the screen into analog data signals transmitted in one 
direction or the other on the telephone line. The duplexer transmits these 
signals and splits the common transmit/receive channel connected to the 
line into two separate transmit and receive channels connected to the 
modem. Bidirectional transmission of the signals on the telephone line is 
achieved by assigning two frequency bands for the two signal transmission 
directions. The frequency bands are centered on 420 Hz for the signals 
transmitted from the terminal to the server and 1700 Hz for those 
transmitted from the server to the terminal. Transmission from the 
terminal to the server is at a relatively lower rate of 75 bauds and 
transmission from the server to the terminal is at a relatively higher 
rate of 1200 bauds, at a low peak level in either direction of 1 V 
maximum. 
The transformer isolating the terminal from the telephone network is 
matched to the characteristics of the telephone line transmitting the data 
signals so as to have in conjunction with the current generator (or with 
the transmit channel of the duplexer) an impedance matched to the defined 
impedance of the telephone line, which is usually 600 ohms. 
Adapting a "conventional" videotex terminal of the kind described to 
provide an auto-answer videotex terminal entails associating with the 
circuits assigned to the data signals and isolated from the network by the 
isolating transformer circuits assigned to the ringing signals to be 
detected, separately isolated from the network. 
Compared with a conventional videotex terminal, the cost of an auto-answer 
videotex terminal of this kind is increased by the cost of implementing 
the ringing signal detection function including the isolation specific to 
this function. 
An object of the present invention is to provide an auto-answer videotex 
terminal with ringing signal detection incorporated into the terminal at 
reduced cost. 
SUMMARY OF THE INVENTION 
The present invention consists in an auto-answer videotex terminal adapted 
to be connected to a public switched telephone network by a first pair of 
wires adapted to carry ringing signals and by a second pair of wires 
adapted to carry data signals, said terminal comprising a switching unit 
adapted to connect said first pair of wires to said telephone network 
initially and to connect said second pair of wires to said telephone 
network after a telephone connection is set up, data transmission means, a 
modem, a processor unit connected to said data transmission means by said 
modem, isolating means for data signals between said data transmission 
means and said telephone network, ringing signal detection means, an 
operational amplifier in said ringing signal detection means, detected 
ringing signal recognition means connected to an output of said 
operational amplifier, a capacitor compatible with the ringing signal 
frequency and a resistor for attenuating said ringing signal associated 
with said isolating means for a call answering operating mode of said 
terminal, an input of said operational amplifier connected to said first 
pair of wires by said isolating means, said capacitor and said attenuator 
resistor, and means for bypassing said attenuator resistor and means for 
reducing the impedance of said capacitor, adapted to be commanded with 
said switching unit when said ringing signal is recognized for the data 
transfer operating mode of said terminal. 
In accordance with another characteristic of the invention the detector 
operational amplifier reference input has a high threshold and a low 
threshold for changing its output state to produce the detected signal in 
the form of pulses at a frequency equal to that of said ringing signal and 
having a specified duration of at least a few milliseconds. 
In accordance with a further characteristic of the invention said impedance 
reducing means comprise a current generator adapted to absorb current from 
the telephone line when the connection has been set up and which has a 
limited finite impedance of substantially 1800 ohms. 
The characteristics and advantages of the present invention will emerge 
more clearly from the following description of one embodiment shown in the 
appended drawing.

DETAILED DESCRIPTION OF THE INVENTION 
The auto-answer videotex terminal in accordance with the present invention 
is connected to two wires A1, A2 of the public switched telephone network 
by a connector 1 connected to the auto-answer videotex terminal by three 
wires L1T1, T2 and L2. Two of these wires, T1 and T2, correspond to the 
pair of wires connected to a subscriber telephone (not shown) to send 
dialling signals over the line. The other two wires L1 and L2 correspond 
to the wires connected to the circuitry of a conventional videotex 
terminal for bidirectional transfer of data signals on the telephone line 
A1, A2 when the requested connection has been set up. 
In the videotex terminal a switch 2 connected to the wire T2 connects the 
wire T2 to the wire L2 or the wire T2 to the wire L1T1. It corresponds to 
the switch of a conventional videotex terminal which when idle connects 
the telephone to the network via the wire L2 connected to the wire T2 and 
which is operated manually when the relevant key on the conventional 
terminal is pressed, once the connection has been set up. The switch 2 is 
shown in its idle position in which it enables a dialling signal to be 
sent by the subscriber telephone and in which it enables the auto-answer 
videotex terminal in accordance with the present invention to detect a 
ringing signal. In its other, active position it connects the wire T2 to 
the wire L1T1 for transferring data. 
The auto-answer videotex terminal includes a ringing signal detector 3. It 
also includes a duplexer 4 and a modem 5 for data signals. The duplexer 4 
handles the transfer of data signals in both directions between a common 
transmit/receive channel 6 and two separate transmit and receive channels 
7 and 8. The individual channels 7 and 8 are connected to the modem 5. A 
processor unit pu (not shown in full) based on a microprocessor 10 is 
connected to the modem to process data signals transmitted or received and 
is also connected through a switch 11 to the ringing signal detector 3, to 
enable it to recognize ringing signals. The microprocessor 10 is also 
connected to a keyboard, a screen and a printer (not shown) for entry at 
the keyboard of data to be transmitted and for display on the screen of 
data received and to be transmitted by the videotex terminal, together 
with their printout, if required. 
In the auto-answer videotex terminal an isolating transformer 13 for 
bi-directional transmission of data signals on the telephone line through 
the switch 2 in the active position connects the common transmit/receive 
channel 6 for the duplexer 4 to the wires L1, L2. The transformer has a 
first winding 14 which is grounded and defines at its opposite terminal 
the common transmit/receive channel 6. Its second winding 15 is connected 
to the wires L1, L2. 
A current generator 16 between the terminals of the winding 15 of the 
transformer 13 and shunting the wires L1, L2 enables the central office to 
sense the busy state of the telephone line. 
The transformer 13 also couples the ringing signal detector 3 and the 
telephone line A1, A2 or the wires T1, T2 when the switch 2 is in its idle 
position. To allow for the characteristics of the ringing signal, which is 
usually at a frequency of 50 Hz and has a level of at least 25 V, the 
winding 15 of this transformer is connected between the wires T1, T2 when 
the switch 2 is in the idle position via a capacitor 17 and a resistor 18. 
The value of the capacitor 17 is substantially 1 .mu.F, being chosen to 
suit the frequency of the ringing signal received from the telephone line 
and to be detected by the detector 3 of the auto-answer videotex terminal. 
The value of the resistor 18 is chosen to attenuate as necessary the 
ringing signal transmitted by the transformer. Its value is around 33 
kohms. 
In this auto-answer videotex terminal the isolating transformer 13 is used 
for the data signals and also for the ringing signals. A switch 19 is 
connected across the resistor 18. The switch 19 is closed when the switch 
2 is in the active position to shunt the resistor 18 and so bypass the 
attenuation that it otherwise provides when the terminal is transferring 
data. 
The switches 2, 11 and 19 of the terminal are controlled automatically by 
the microprocessor 10 as symbolically represented by the arrows marked 
(10). The idle position of the switch 2 corresponds to an idle position of 
the switch 11 in which the threshold detector 3 is connected to the 
microprocessor 10 which can recognize the ringing signal to the terminal 
and to an idle position of the switch 19 in which this switch is open and 
does not shunt the resistor 18. The switch 11 could be dispensed with, the 
microprocessor 10 then being connected to the modem 5 and to the detector 
3 by two independent connections. 
In a similar way to the circuit associated with the resistor 18, the 
auto-answer videotex terminal is equipped with means for reducing the 
impedance of the capacitor 17 when the terminal is transferring data. 
These means may comprise an additional capacitor with an associated switch 
(not shown) shunting the capacitor 17. It is preferably in the form shown 
by the current generator 16, however, which has a selected, finite 
impedance to match the impedance of the transformer to that specified for 
the telephone line, which has a value between 400 and 900 ohms. 
The circuits of the auto-answer videotex terminal are described 
hereinafter. 
In this terminal the modem 5 and the microprocessor 10 are known circuits, 
the microprocessor 10 being further adapted to recognize the frequency of 
the detected ringing signal which it receives from the detector 3 through 
the closed switch 11. 
The duplexer 4 is also a circuit known as such. It comprises a transmit 
operational amplifier 20 and a receive operational amplifier 21 for data 
signals between channels 7 and 8 and the channel 6, in other words between 
the modem 5 and the transformer 13. 
Each amplifier has a respective feedback resistor 22 or 23 between its 
negative input and its output. The output of the transmit amplifier 20 is 
connected to the common transmit/receive channel 6 through a resistor 24. 
The negative input of the receive amplifier 21 is connected to the same 
channel 6 through a resistor 25. The output of the transmit amplifier is 
connected to the positive input of the receive amplifier through a 
resistor 26, and this positive input is also grounded through a resistor 
27 to enable generation of the receive data signal from the 
transmit/receive signal present on the channel 6 and the transmit signal 
from the amplifier 20. 
The current generator 16 includes a Darlington circuit with two transistors 
30 and 31 controlled via a diode rectifier bridge 32 and a resistor bridge 
comprising resistors 33 and 34. The output transistor 31 of the Darlington 
circuit has is collector connected to that of the transistor 30 and to one 
terminal of the diode bridge, to which the resistor 33 is connected. Its 
emitter is connected through a resistor 36 to the opposite terminal of the 
diode bridge, to which the other resistor 34 is connected and which 
controls the resistor 35. The other two terminals of the diode rectifier 
bridge are connected to the wires L2 and L1 (through the switch 2 when it 
is in the active position in the case of the wire L1). 
The collector of the transistor 35 and the base of the transistor 30 are 
connected through the resistor 33 to one terminal of the diode bridge. The 
emitter of the transistor 35 is connected through a resistor 37 to the 
other bridge terminal. 
In this current generator the resistors R33 and R34 are of the same value 
which is very much higher than that of the resistor 37 or the resistor 36. 
The current generator 16 is given a finite impedance Z in the order of 
1800 ohms by appropriately choosing the values of these resistors, for 
example: R33=10 kohms, R37=143 ohms and R36=25.5 ohms. 
When the connection has been set up, the current generator 16 absorbs 
current on the telephone line in the conventional way so that the central 
office can recognize the busy state of the connection. It also reduces the 
impedance due to the capacitor 17 when transferring data. 
The ringing signal detector 3 includes an operational amplifier 40. Its 
negative input is connected to the transmit/receive channel 6 at the 
non-grounded terminal of the winding 14 of the transformer 13. Its 
positive input is connected to a resistor bridge comprising a resistor 41 
connected to the 5 V supply rail +Vcc and a resistor 42 connected to 
ground. This input is also connected to its output through a resistor 43 
and a diode 44. The output of the amplifier 40 is connected through a 
resistor 45 and the switch 11 when the latter is in the idle position to 
the microprocessor 10. A Zener diode 46 connected between the output side 
of the resistor 45 and ground limits the level of the output signal from 
the detector 3 to a TTL-compatible value. 
In the ringing signal detector 3 the feedback circuit comprising the diode 
44 and the resistor 43 is turned off by a high level at the output of the 
operational amplifier 40 and turned on by a low level at this output. This 
produces at its positive input a high threshold value which causes the 
output signal of this amplifier to switch from the high level to the low 
level and a low threshold value causing the opposite switching of the 
output level of the amplifier 40. The high threshold value is defined by 
the resistors 41 and 42 and is selected to correspond to the substantially 
linear and rising part of the positive half-wave of the ringing signal on 
the channel 6. The low threshold value is defined by the resistors 41, 42 
and 43 and is near 0 V. 
These high and low threshold values make it possible to obtain at the 
output of the operational amplifier 40 a signal whose pulses at the 
frequency of the ringing signal have a duration greater than 4 ms so that 
they can be processed by the microprocessor 10 to recognize the ringing 
frequency. 
The resistors 41, 42 and 43 advantageously have the following values: 
R41=100 kohms, R42=5.1 kohms and R43=150 kohms. 
For a voltage +Vcc=+5 V the high threshold level is then Vs+=240 mV. 
With the auto-answer videotex terminal initially set to receive the ringing 
signal with the switches 2, 19 and 11 in the idle position, the current 
generator 16 is bypassed and the wires L2 and T2 are connected by the 
switch 2. Ringing signals on the wires T1 and T2 are strongly attenuated 
by the resistor 18 and transmitted through the capacitor 17 and the 
transformer 13 to the channel 6. With the transmit amplifier 20 of the 
duplexer 4 in the idle condition the resistor 24 is grounded and is 
therefore in parallel with the winding 14 of the transformer 13 and 
constitutes the load resistor on the winding 14 for these ringing signals. 
The value of the resistor 24 is substantially 1 kohm. The impedance of the 
circuit as a whole is determined by its value and the overall impedance of 
the transformer 13, the capacitor 17 and the resistor 18. 
The signal on the channel 6 is detected by the operational amplifier 40. 
The output signal from the operational amplifier 40 is reduced to a 
TTL-compatible level by the Zener diode 46 and transmitted to the 
microprocessor 10. The frequency of the pulses of the detected signal, 
which depends on the frequency of the ringing signal on the line, can then 
be recognized directly by the microprocessor 10. 
Recognition of the ringing signal by the microprocessor of the terminal 
changes the switches 2, 19 and 11 to their active state. The ringing 
signal detector 3 is preferably isolated from the microprocessor 10 by the 
switch 11. The microprocessor 10 then causes the 2100 Hz response tone to 
be sent via the modem over the line to the calling terminal where it is 
detected to enable the subscriber to connect his videotex terminal to the 
telephone network. 
The called auto-answer videotex terminal and the calling videotex terminal 
are then connected and data is exchanged between them with the auto-answer 
videotex terminal functioning as a server, in other words transmitting in 
the high frequency band centered on 1700 Hz and receiving data signals 
from the calling terminal in the low frequency band centered on 420 Hz. 
The data signal transmitted by the videotex terminal may begin with a 
"greeting" page for the calling terminal. A greeting page of this kind is 
initially memorized in memories associated with the microprocessor 10 in 
the processor unit. It invites the calling terminal to continue the 
procedure for exchanging data with the called auto-answer terminal.