GB2149615A - Telephone exchange - Google Patents

Abstract

A mini-telephone exchange to serve, typically one exchange line and five extensions uses a semiconductor matrix array (2) as the switching means, with microcomputer (6) control. The system does not need an operator. The matrix array is single-wire. The system has a power failure detector (9) which in response to a local power failure operates a relay which (a) connects one extension (Ex. 1) to the exchange line circuit (1) and (b) disables the rest of the exchange. <IMAGE>

Description

SPECIFICATION Telephone exchange The present invention relates to an automatic telecommunications exchange for use as a PABX when a relatively small number of lines has to be provided.

A need exists for a small telephone exchange for use in small boarding houses, public houses, large private houses and small business, or as an attachment to a larger PABX. Such a system should not need an operator, and should be relatively cheap.

It is an object of the invention to provide such a telephone exchange.

According to the invention there is provided an automatic telephone exchange which includes a single stage co-ordinate switching array formed by semiconductor cross-points via which communication connections are set up, a number of subscribers' line circuits connected to the array, at least one line to a remote exchange also connected to the array, a microcomputer so programmed as to control the setting up of connections through the array, a power supply circuit driven from a local supply such as the public electrical supply which supplies direct current for the operation of the exchange circuitry and alternating current to serve as ringing current therefor, and a power failure circuit responsive to failure of the power supply or part failure thereof to connect one of the subscriber's line circuits to the line to the remote exchange and also to disable the rest of the exchange.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which Figure 1 is a functional block diagram of a telephone exchange embodying the invention.

Figure 2 is a simplified circuit of one of the extension line circuits.

Figure 3 shows schematically how a power failure is dealt with.

Figure 4 is a relay driver circuit used in the exchange of Fig. 1.

Figure 5 is a tone generator as used in the exchange of Fig. 1.

Figure 6 is a ring trip detector which serves the line circuits.

Figure 7 shows the cross-point array used in the exchange of Fig. 1.

Figure 8 shows schematically the network circuit, i.e. exchange line circuit, of Fig. 1.

The exchange shown in the functional block diagram, Fig. 1, has one line to the public telephone network, which is connected via a network circuit 1 to a cross-point array 2, formed by two (8 x 2) matrices, as will be described later. There are five internal extensions Ex.1 to Ex.5, which are also connected to the array 2. Each of the extensions' line circuits includes a constant current generator and loop detector, as will be seen later. Also connected to the array 2 are a- line load unit 3, a DTMF (Dual Tone Multi-Frequency) receiver 4 and the tones supply unit 5. The system is controlled by a microcomputer 6.

The other units shown in Fig. 1 are the power supply 7, which supplies power at + 5 Volts and + 24 Volts DC, and 70 Volts AC to the system. The AC supply is used to produce the ringing current, which is available to the entensions' line circuit via a ring trip detector 8.

The connection from Ex. 1 to the network circuit 1 is used in conjunction with a power failure detection circuit 9. If the mains fails, a relay in the circuit 9 releases, and completes the power fail connection so that Ex. 1 is connected directly to the circuit 1, the remainder of the exchange then being disconnected. Thus in such circumstances, Ex. 1 can function as a normal telephone.

In the exchange shown a connection is shown via which the microcomputer 6 supplies ringing relay switching signals to the line circuits and via which it responds to dial pulses and lock switch information from the extension. As already mentioned, the ringing current reaches the extension via the detector 8, while DTMF signals from extensions reach the microcomputer 6 via the array 2 and the DTMF receiver 4.

Note that an exchange embodying the principles of the invention may have more than one line to a remote exchange, and may have more than five extensions.

Figure 2 shows the subscriber's line circuit, of which there is one per extension. The main purpose of this is to signal to the microcomputer the off-hook state of an extension (logic high), so these circuits are basically constant current generators and loop detectors. Each line circuit includes relays (not shown) for switching the ringing current. The main extension, Ex. 1, also has two extra relay contacts so that in the event of a power failure it is automatically connected to the public network as an ordinary telephone.

When an extension's handset is off-hooked, a loop between its a and b wires is completed, which controls transistor Tr.1. When the line is on-hook, Tr.1 produces a constant current at its collector as its base is biased to + 23 volts by ZI and RI in series. On off-hooking, the collector of Tr.1 goes low, to control a Schmitt trigger Tr.2. The normal state of Tr.2 is with its emitter and base at 24 volts, so its collector and hence output to the microcomputer is low.

When off-hooking occurs, it drives the emitter of Tr.1 low, Tr.2 is switched on so its output is now at + 24 volts, which is applied to the microcomputer, with this connection from the junction of a potential divider R2-R3 "caught" at 5 volts via diode Dl.

With the relay contact in the condition shown and the extension off-hook, speech passes to the cross-points via Cl and C2, of which Cl shorts HF to ground, and C2 passes audio frequency so that Cl and C2 are a low pass filter.

Each extension is recognisable by the microcomputer since each one has its own port, so there is no ambiguity as to which telephone is off-hook.

When an extension is dialled by a loop-disconnect telephone, the dial pulses produced appear as a train of pulses on the output shown to the microcomputer. However, when a DTMF telephone is used the signals are sent through the speech channel where they are detected by the DTMF receiver.

When an extension is to be rung, the relay is switched to its reverse position under microcomputer control, so that the ringing is applied to the telephone bell.

We now describe power failure operations, see Fig. 3. If either of the DC supplies fails, a relay RL8 in the power failure detector 9, Fig. 1 operates to reverse its contact RI 8-1 to RL 8-4 to directly connect that extension to the network circuit, so that the extension now functions as an ordinary telephone line. Since the network is now directly connected to Ex.1, all of the rest of the exchange is disabled. At the same time a power failure alarm is sounded.

The relays used in the exchange have to be controlled from electronic equipment, so driver circuits are needed, and one of these is shown in Fig. 4.

Here we have a Darlington amplifier Tr3-Tr4 to control the relay RL.

The essential elements of the tone generators, 5 in Fig. 1, are shown in Fig. 5. This uses two Schmitt oscillators, based on NAND gates NI and N2. The two tones which have to be provided are dial tone and busy tone. The third tone used, ringing tone, is taken directly from the 70 volts supply - see Fig. 1.

The basis of each of the Schmitt circuits in that its input is high, so its output is low. Hence for NI the capacitor C4 discharges through the diode D3, so the input goes low. Thus the output goes high and C4 charges up through the resistor R5, so that the input goes high, which brings the output low.

Thus the device NI gives a continuous train of 5 volt pulses. This circuit NI gives a continuous note of 111 Hz which is used for the dial tone. The other circuit N2, which does not use a diode, gives a 1.42 Hz pulse which, when fed to another Schmitt NAND device N3 with the dial tone, produces an interrupted signal for use as busy tone.

Next we consider the ring trip detector, as included in the line circuits, see Fig. 6. This consists of an amplifier whose input is derived from the 70 volt 50 Hz ringing current, and whose bias comes from the 5 volt DC supply. If the handset is lifted during ringing, the DC loop is completed. The increased load reduces the signal input to the detector, Fig. 6, which cuts the amplifier off and provides a steady low DC to the microcomputer to "tell" it that the called line has replied. The microprocessor now cuts off the ringing supply and connects the called line, e.g. by control of the appropriate relay.

Some of the telephones served may be pushbutton sets using DTMF, in which case the composite signal formed by two tones for each digit is sent over the line to the exchange where it is decoded. The basic method of discriminating between frequencies using digital logic is by counting the number of cycles of a reference clock in one period of the frequency.

The DTMF signal is passed via a band splitting filter to separate the high and low band frequencies, and each frequency thus separated is squared. The squared waves are then compared with a fixed clock to determine their frequencies.

They are then converted to a four-bit binary num ber which represents the value of the original digit.

This circuit is not shown as it is based on a Mitel MT8870 integrated circuit, which includes the filter and the decoding circuits.

The cross-point array is shown in Fig. 7, and is a solid state matrix in which SGS integrated circuits are used, the example shown in Fig. 7 using two M099 devices, each of which is a 2 x 8 matrix of which each switch can be individually set, and has a latch to maintain it set. The switch address and control bits are loaded into an internal shift register when the enable inputs shown in Fig. 7 are low.

In each case the address bits consist of three input selection bits and a single output selection bit. A fifth control bit (D) defines whether the chosen switch is to be opened or closed. In addition, a sixth bit (R) is used as "an all switch reset", reset occurring on the low-high transition of the enable inputs when both D and R are zero. During normal selection the R bit is high.

As can be seen in Fig. 7, the two matrices are joined to form an 8 x 4 matrix, with the five extension telephones connected to the first five horizontal lines. Note that the matrix array, and indeed the entire switched path from the network circuit (1, Fig. 1) to the extension line circuits (Ex.1 Ex.5) is single wire.Two more of these horizontal lines are used for dial and busy tones and the last one is ieft to put an extension on hold. In an enhanced version of the exchange, a musical tone is put on to the hold line for the benefit of the caller.

There are four other lines, one of which, Trunk, is used for the external line to the public network.

A second line is used to connect DTMF signalling from an extension to the DTMF receiver, 4 in Fig.

1. A final pair of lines are used for making a combination of interconnections between extensions.

Two connect two telephones via the matrix array, two control commands are used. Thus to connect extension 2 to extension 4, firstly a connection is made from extension 2 to line A of the lines used for making interconnections, and then a connection is made from extension 4 to line A. Using this method, external line, hold, DTMF line and tones may be connected to the extensions, all under the control of the microprocessor.

We now describe the external line interface, included in the network circuitry 1, Fig. 1 This provides isolation between the extension handsets and the public network, and gives isolation between the ringing current and the microcomputer.

For a connection to a public exchange line, it gives voltage-controlled completion of the current loop from the exchange.

Connection of wave frequencies from the public exchange into the exchange uses an isolating transformer.

For an incoming call, ringing current from the public network is detected via an optical isolator, which gives the microcomputer a pulse train input for the duration of the ringing current. On receiving this pulse train, the microcomputer switches the ringing relays to the appropriate extension.

When the extension set being rung replies, that extension is connected to the public network via the cross-points and the isolating transformer IF. Thus in this case relies RL8 and RL7 reverse their contacts from the states shown in Fig. 8.

For an external call, relays RL6 and RL7 are energised of which RL7 connects the external line interface to the public network while RL6 completes the loop. Dialling is effected by pulsing RL6, under microcomputer control.

The diode bridge is provided to ensure that the system from the public network with the correct polarity as "seen" by the PABX.

For a conference connection involving three lines, which can include the exchange line, the microcomputer causes three connections to be set up each as indicated above, in which case the line load 3, Fig. 1, may be used to adjust loading.

Claims (13)

1. An automatic telephone exchange which includes a single stage co-ordinate switching array formed by semiconductor cross-points via which communication connections are set up, a number of subscribers' line circuits connected to the array, at least one line to a remote exchange also connected to the array, a microcomputer so programmed as to control the setting up of connections through the array, a power supply circuit driven from a local supply such as the public electrical supply which supplies direct current for the operation of the exchange circuitry and alternating current to serve as ringing current therefor, and a power failure circuit responsive to failure of the power supply or part failure thereof to connect one of the subscriber's line circuits to the line to the remote exchange and also to disable the rest of the exchange.

2. An exchange as claimed in claim 1, and which can serve both lines equipped with telephone sets with pulse type dials and with telephone sets with digit sending equipment which emits digit representing signals in dual-tone multifrequency (DTMF) form, in which pulse-type signals are applied to the controlling microprocessor directly from the line circuits, and in which DTMF signals are applied to the microcomputer via the cross-point array and a DTMF receiver which converts the DTMF signals into a digital form.

3. An exchange as claimed in claim 1 or 2, and in which the tone generator which supplies at least the dial tone and the busy tone is an electronic circuit connected to the cross-point array.

4. An exchange as claimed in claim 3, and in which the tone generator includes a first Schmitt oscillator which supplies a steady tone for use as a dial tone, a second Schmitt oscillator which supplies an interrupted signal, and a NAND device to which the outputs of both Schmitt oscillators are applied and which thus produce an interrupted signal for use as a busy tone.

5. An exchange as claimed in claim 1, 2, 3 or 4, and in which speech and other transmissions via the cross-point array is single wire.

6. An exchange as claimed in claim 1, 2, 3, 4 or 5, and in which a subscriber's line circuit includes a first transistor which responds to its line offhooking to switch a Schmitt trigger which in turn signals to the microcomputer that the line has off hooked.

7. An exchange as claimed in any one of claims 1 to 6, and in which each subscriber's line and the or each line to a remote exchange has its own port connected to the microcomputer.

8. An exchange as claimed in any one of claims 1 to 7, and in which electro-mechanical relays used in the exchange are each driven via a Darlington amplifier.

9. An exchange as claimed in any one of the preceding claims, in which ring trip detection means includes an amplifier connected to the speech wires of an extension which is normally operative, and in which when the subscriber offhooks the amplifier is cut-off as a result of which a signal is sent to the microcomputer to indicate that the called line has replied, whereupon the microcomputer emits a signal to cut the ringing supply to that line.

10. An exchanged as claimed in any one of claims 2 to 9, and in which the DTMF receiver includes a band-splitting filter to separate each digit's signal into a high and a low frequency portion, and for each said portion a separate frequency detector.

11. An exchanged as claimed in claim 10, and in which each said frequency detector determines the frequency of a signal applied to it by squaring that signal and counting the number of cycles of a reference frequency during one cycle of that signal.

12. A private automatic telephone exchange substantially as described with reference to the accompanying drawings.

New claims or amendments to claims filed on 18 Jan 1984.

New or amended claims:

13. An automatic telephone exchange, which includes a co-ordinate switching array formed by semiconductor cross-points via which communication connections are set up, a number of subscribers' line circuits connected to the array, at least one line to a remote exchange also connected to the array, processing means so programmed as to control the setting up of connections through the array, a power supply circuit for the exchange driven from a local supply and which supplies direct current for the operation of the exchange circuitry and alternating current to serve as ringing current for the exchange, and a power failure circuit responsive to failure of the power supply to the exchange or part failure thereof to connect a selected one of the subscriber's line circuits to a said line to the remote exchange and also to disable the rest of the exchange, in which the exchange can serve both lines equipped with telephone sets with pulse-type number sending means and with telephone sets with digit sending equipment which emits digit-representing signals in dual-tone multifrequency (DTMF) form, in which pulse-type sig nals are applied to the controlling processing means directly from the line circuit, and in which DTMF signals are applied to the processing means via the cross-point array and a DTMF receiver which converts the DTMF signals into digital form.