GB2249919A - Modem system - Google Patents

Abstract

A modem system couples a telephone line BT with a computer connection COMP and is fully powered from the telephone line with low power consumption in a quiescent state. The modem system is fully powered from the telephone line on ring signal detection or on detection of signals from the computer, using supply 14 via regulator 15. A phone ring is detected at 16, and phone signals pass through a signal amplifier 10 at 12 to a modem 10 23 coupled to the computer via an isolation interface 25; computer signals pass through units 25 and 23 to amplifier 11 in unit 10 to drive a modulator 13 across the BT line. Units 24 and 26, which power units 10 and 25, are fed by supply 14 and are energized only when activity is detected, either by ring detector 16 or from the computer. A microprocessor 17 is put into the stand-by state when there is no activity. <IMAGE>

Description

Modem System Background The present invention relates to modems; that is, to circuits which couple a computer to a telephone line, modulating digital signals from the computer into a form which is compatible with the telephone system and demodulating such modulated signals received from the telephone system back into a digital form.

Such modems obviously require power to operate, A modem is coupled to a computer, and computers have in the past normally been powered from a mains supply. It has therefore been conventional for modems to be powered direct from a mains supply, from the computer, or from a battery.

With the advent of portable computers, a mains supply may well not be available, and the modem then has to be powered from the computer. Although this is tolerable, it results in an additional power drain on the computer battery, which is undesirable. In particular, it is sometimes desirable to operate a computer and modem in such a manner that the combination is permanently sensitive to incoming signals from the telephone system (so that it can receive incoming calls at whatever time they may arrive). Although the computer itself can be largely powered down and arranged to switch on when an incoming call arrives, the modem has to be permanently powered up to detect such incoming calls, and this imposes a severe load on the battery of either the computer or the modem itself.

The public telephone system operates at a substantial voltage, and it has been proposed to power certain kinds of telephonic apparatus from that system.

An obvious example is the normal telephone handset; and it has also been proposed to operate call barring apparatus in that manner. However, the power available from the public telephone system is limited, and the extent to which it can be used is subject to strict regulation by the public telephone authority.

While, therefore, it would be convenient to be able to power a modem from the public telephone system, it is recognized that in practice this is not feasible.

The present invention According to the present invention there is provided a modem system for coupling a telephone cable and a computer, the system comprising: signal amplifier and modem IC means for passing signals between the computer and the telephone cable and converting their form appropriately, a ring detection unit for detecting ring signals on the telephone cable, a power supply unit connected across the telephone cable via regulating means which present a high AC impedance, and power switching means for powering the signal amplifier and modem IC means from the power supply unit on ring signal detection.

(The use of the terms "modem" or "modem system" on the one hand and "modem chip" or "modem IC" on the other is well established, the former denoting a complete system and the latter an integrated circuit or chip (IC) in the system which performs the specific task of converting between computer signals in logical form and telephone signals in tone form.) Preferably the power switching means are also responsive to signals from the computer to powering the signal amplifier and modem IC means from the power supply unit.

Preferably the regulating means is switched between different DC impedances on ring signal detection or other initiation of communication. The system may include a microprocessor and an associated ROM for controlling its operation, and an isolation barrier through which signals are coupled with the computer and auxiliary power supply means energized across the barrier through the power switching means. Preferably also the microprocessor is switchable between a stand-by state in which its power consumption is low and an active state in which its power consumption is relatively high, with the system including startup means which switch the microprocessor into the active state on ring detection or on other initiation. The present system also avoids the use of a transformer for maintaining a DC return path for the telephone cable.

The crux of the present invention thus concerns energy management.

The power available from a public system telephone cable is dependent on whether the cable is active or not - that is, whether or not a call is being made.

When a call is being made, a relatively large amount of power can be drawn from the telephone cable, but when the cable is inactive, the telephone authority's regulations permit only a very small amount of power to be drawn from it.

This low power is inadequate to power a modem system using ordinary technology.

The present system nevertheless uses ordinary technology, and overcomes the power problem by being divided generally into two subsystems, the first of which includes means for detecting incoming signals from either the telephone cable or the computer and the second of which contains the bulk of the remainder of the system and is powered down when the system is inactive, the first subsystem detecting when the system is to become active and thereupon causing the second subsystem to be powered up.

This system comprises a modem system for coupling a telephone cable and a computer, wherein the telephone cable is AC-coupled to signal amplifier means for injecting signals into and receiving signal from the telephone cable, and also to main power supply means via regulating means which present a high AC impedance and switch between different DC impedances in dependence on the state of the system, the system preferably including a microprocessor and associated ROM for controlling its operation.

Isolation In conventional mains-powered modem systems, adequate isolation between the BT cable and the main body of the modem system is required by BT. In the present system, there is no source of mains power. However, the present system will normally be connected to a computer, and although the present system is designed to work with battery powered computers, there is nothing to stop it being used with mains powered computers as well. There is therefore still a need for adequate isolation. Most computers do not incorporate such isolation, so isolation must be included in the present system.

The present system thus requires isolation. This in itself could be achieved by using suitable passive coupling devices such as opto-couplers.

However. the present system also has to generate sufficient power on the computer side of the isolation barrier to produce the call-indicate signal and receive data from the computer. For this, an active power transfer across the isolation barrier is required, by means of a transformer. This isolation power supply forms part of the second subsystem, which draws no power until the system becomes active.

Specific embodiment Further preferred features of the invention will become apparent from the following detailed description of a modem system in accordance with the invention, given by way of example and with reference to the drawings, in which: Fig. 1 is a general block diagram of the system; and Figs. 2A to 2F together are a simplified circuit diagram of the system, partially in block form.

General system arrangement (Fig. 1) Referring to Fig. 1, the system couples a telephone cable (typically a British Telecom one) BT with a computer cable COMP. The signals on the BT cable are standard telephone (AC) signals and dialling pulses; the signals on the computer cable are of the V28/V24 type, having essentially two lines, one for transmitting and the other for receiving. (As will be seen below, this cable actually has two further lines - a call-indicate line and a return line.) The BT cable is connected to a signal transmit and receiver amplifier block 10, comprising a transmit amplifier chain 11, a receive amplifier chain 12, and a modulator transistor 13. The transmit and receive amplifier chains are crosscoupled so that the signal amplifier block 10 presents the right AC impedance (and high DC impedance) to the BT cable, and to prevent excessive feedback of the transmitted signal into the receive path.

The BT cable is also connected to a main power supply 14 via a regulator circuit 15. The main power supply 14 extracts power from the BT cable for operating the rest of the modem system; the regulator circuit 15 minimizes the power drain from the BT cable when the system is inactive and at other times when the power drawn from the BT cable must be minimized, and presents the right DC impedance (and high AC impedance) to the BT cable.

The BT cable also has a ring detector circuit 16 connected to it.

The modem system is controlled by a microprocessor 17, which is initialized by an initialization block 19 including a power supply monitor 20 and a latch 21. The microprocessor is coupled to a ROM 18 which stores the appropriate control sequences. The monitor 20 disables the microprocessor until a stable power supply is available, so preventing unexpected conditions from arising in.

the event of a power failure.

The microprocessor has a small speaker unit 22 coupled to it, giving an audible indication of what the system is doing.

The microprocessor and the signal amplifier block 10 are coupled to a modem IC 23 which converts data signals between the computer format from the computer and the telephone format on the telephone cable (subject to amplification, etc). The modem IC is a conventional component performing a standard function.

A subsidiary power supply block 24 is energized from the main power supply 14, and provides an auxiliary source of power which is electrically isolated from the telephone cable. This subsidiary power supply energizes an interface block 25, which couples the computer cable to the modem chip 23.

This is required to ensure isolation confirming to the appropriate regulatory requirements between the computer and the telephone cable, and also to provide power for driving a call-indicate signal to the computer without imposing any power drain on the computer.

The subsidiary power supply 24 is powered from the main power supply 14.

It is controlled by the microprocessor, which turns it on only when the system is active. This occurs when the latch 21 detects signals from the ring detector 16 (indicating an incoming call on the telephone line) or the isolation unit 25 (indicating that the computer wants to make an outgoing call).

The signal amplifier circuitry 10 and the modem chip 23 are powered from an amplifier power supply 26, which is, like power supply 24, powered from the main pair supply 14 and controlled by the microprocessor, which turns it on only when the system is active. The powering down of the amplifier circuitry 10 and the isolation circuitry 25 and the disabling of the microprocessor 17 reduce the power consumption of the system in the inactive state to a level which can be supplied by the telephone cable in the inactive state.

Main power supply < 14, 15 - Fig. 2A) The telephone cable is a BT (British Telecom) cable terminating at three terminals BT-A, BT-B, and BT-SHUNT. Of these, the main two are BT-A and BT-B, between which the telephone signals appear. A substantial DC voltage appears between these terminals (-50 V when idle, 10 V when active), and this voltage is utilized by the present circuitry as its sole power source.

The terminals BT-A and BT-B are connected to a diode bridge D6 via protective resistors R41 (which is voltage sensitive) and R43. This diode bridge is used to ensure consistent polarity for the system regardless of the polarity of the BT supply. The output of the bridge D6 appears between a main or system earth or chassis line EARTH and a signal line LINE. A main power supply 14, producing a power supply voltage on a line &commat; (relative to EARTH) for the rest of the system, is connected in series with a regulator circuit 15, for controlling the main power supply circuit, between lines e and EARTH.

The main power supply 14 comprises a large capacitor pair C26-28, the voltage across which is regulated by a voltage divider R34-R35 which controls a voltage reference device IC6 which in turn controls a regulating transistor TR4.

The regulator circuit 15 provides two routes for the energization of this main power supply.

A high value resistor R31 is connected directly to it from the signal line LINE, and the current flow through this is sufficient to maintain the power supply in the quiescent or standby state.

An FET transistor TR1 is also connected to the power supply 14 from line LINE. Its gate is pulled up by a high value resistor R26 and is connected to the power supply 22 via two FETs TR3 and TR6 in parallel, a capacitor Ci8 being connected in series with TR6. TR3 is controlled by a signal SLOOP and TR6 by a signal GYR, both generated by the microprocessor.

In the quiescent state, the system is required to present a high DC impr dance to the BT cable. TR3 is held on in this state by the signal /LOOP from the microprocessor. This holds TR1 off, so that only the very small standby current through R31 and R26 is drawn from the BT cable. TR3 is on in this state, but it has no effect on the DC conditions because it is isolated from TR1 by C18.

When the system becomes active, the microprocessor takes signal /LOOP inactive, turning TR3 off. The gate of TR1 is therefore pulled high by R26, and TRI turns on. DC can therefore flow through TR1, and the power supply therefore receives enough current to maintain the supply voltage on line z to energize the rest of the system in the active state. The microprocessor also takes signal GYR active, turning TR6 on. Capacitor C18 therefore holds the gate voltage of TRl constant, and the current through TRI is therefore substantially independent of any voltage variations on line LINE. TR1 thus presents a high impedance to any AC signals on line LINE.

In the active state, therefore, the main power supply 14 and its regulator circuit 15 provide a DC path across the BT cable, of suitable resistance to allow a current within the BT requirements to flow. It presents a high impedance to AC signals, however: the treatment of such signals is discussed later.

The system may become active either because signals are to be transmitted out onto the BT cable or because signals are detected coming in on that cable.

In the former case, signal transmission is normally preceded by dialling. For this, the microprocessor turns TR1 on and off by means of signal SLOOP to generate the required dialling pulses (simulating the opening and closing of a pair of dialling contacts across the BT cable). For this, the microprocessor also takes signal GYR inactive to turn TR6 off, so that the switching of TRl is not slowed down by the effect of C18.

BT bell tinkle shunting (Fig. 2A) If two or more handsets are connected to a simple 2-wire cable, dialling by means of one of them may produce ringing (known as bell tinkle) of the others.

The BT cable therefore usually includes a "shunt" terminal BT-SHUNT, which is used to suppress this. To achieve this suppression, the "shunt" terminal has to be coupled to the BT-A terminal. In the present system, this is achieved by a resistor R45 and the relay contact of an opto-coupler IC13 connected as shown. The relay contact is controlled by an optical coupling from a diode of IC13, which is connected in series with a transistor TR8 as shown across the main power supply (from the main power supply circuit 14, between lines &commat; and EARTH). Transistor TR8 is controlled by a signal /SHUNT from the microprocessor; this signal turns transistor TR8 on, and hence closes the relay contacts of IC13, when dialling is occurring.

Initialization and start-up (16, 19-21 - Figs. 2A and 2B) Incoming calls are detected by a series ring detection circuit connected across the BT cable, in parallel with the diode bridge D6. This circuit consists of a resistor R44, a pair of back-to-back zener diodes D16 and D17, a capacitor C39, and a parallel diode combination of diode D18 and the diode of an optocoupler IC17. The transistor of opto-coupler IC17 is connected to the main power supply, and generates a ring signal RING when a ringing signal is received from the BT cable; that is, when the signal on the BT cable is of appropriate frequency to pass through the capacitor C39 and is large enough to overcome the voltage drop across the zener diodes. The use of an opto-coupler means that the system can deal with either polarity on the BT cable.

The RING signal is fed through an inverter IC15 to produce a signal PRING which is fed to the microprocessor, and is also fed to a NAND gate IC18 which feeds a one-shot circuit IC11. The output of one-shot IC11 is a signal /RESET, which is fed to the microprocessor. On a ring, RING goes active, and triggers the one-shot IC11 to send PRESET active. This signal is fed to a latch circuit LATCH, setting it and sending its output /STBY active. Signal /STBY is fed to the microprocessor and holds it in the stand-by state (in which it draws very little power) when inactive. So /STBY going active takes the microprocessor out of the stand-by state. This is followed by the pulse from the one-shot IC11 ending, so signal /RESET goes inactive, and this resets the microprocessor (which has just left the standby state and become active).

The detection of an incoming call thus takes the microprocessor out of the stand-by state and resets it, whereupon it starts running the program in the ROM to process the incoming signals.

An incoming signal from the computer produces a signal VTXD which is fed to the latch circuit LATCH 21 through NAND gate IC18 like the signal PRING, and thus initiates the same start-up sequence.

The main power supply is sensed by a sensor circuit IC9 connected to the middle resistor of a divider chain R46-R47-R48 connected across the power supply. The output of IC9 is combined with the signal /LOOP and fed to the latch circuit LATCH. IC9 puts the latch circuit, and hence the microprocessor, into the stand-by state, so that the system starts from a known state.

BT signal transmit and receive circuitry (10-13, 26 - Fig. 20 During a call, the BT cable provides a DC voltage from which a DC current has to be drawn, and also either supplies or has imposed on it an AC signal.

As discussed above, the main power supply draws the DC current via its regulator circuit, but presents a high impedance to AC, so having no additional effect on either incoming or outgoing signals. An incoming AC signal passes through capacitor C8; an outgoing AC signal is generated by an FET transistor TR2.

Considering first a signal being received from the BT cable, this passes through C8 and then through C1 and R1 to an amplifier IC4D. IC4D has a feedback resistor R7 and capacitor C5, and produces a signal RXA via an RC circuit R8-Cll. The signal RXA is fed to the modem chip, which converts it from its analogue form to digital form.

Considering now a signal being transmitted to the BT cable, this is generated as an analogue signal TXA by the modem chip 23 from digital form. TXA is fed via an RC filter circuit R24-C18 to the input circuitry R27-R28-C13-C15 of an amplifier IC4A with feedback as shown. This feeds an amplifier IC4B via input and feedback circuitry C3-C4-R4 as shown. This in turn feeds an amplifier IC4C via input circuitry R17-R8-C9-C12-R22 as shown. This in turn drives FET TR2 via R30, TR2 being loaded by R21, which gives feedback to IC4C. The analogue signal TXA is therefore suitably amplified to drive the BT cable via TR2.

The DC level of the output of IC4C is set so that TR2 passes a small DC current, to avoid distortion of the transmitted signal. A resistor R3 provides a feedback path for incoming signals on line 21 to TR2 via IC4C. The components in this loop are chosen so that an appropriate AC impedance is presented to the incoming signal, i.e. to the BT cable. Capacitor C12 serves to prevent external high frequency noise on the BT cable from interfering with the correct termination of the line.

The signal produced by transistor TR2 for transmission will, of course, also be fed into the receive amplifier chain IC4. A capacitor C2 and resistor R2 couple the signal being transmitted from the transmit amplifier chain IC4A IC4B-IC4C-TR2 to the receive amplifier IC4D, and are chosen to achieve substantial cancellation of the transmit signal in the receive path at the output of IC4D.

This circuitry has a substantial power consumption, and it is therefore powered down when the system is inactive. This is accomplished by means of the amplifier power supply circuit 26, which comprises a transistor TR5 connected to the 0' line and a capacitor pair C14-C19. The microprocessor produces a signal /MODPOW when the signal amplifying circuitry 10 is to be energized, and this signal is fed to the base of TR5 via a resistor R39 to turn it on, and hence energize the power supply to the amplifier circuitry 10 (i.e. mainly to the four amplifiers IC4A to IC4D), and also to the modem chip IC1.

Computer interfacing (24, 25 - Figs 2D and 2E) The system couples a computer interface with the BT cable. The computer interface is of the V28/V24 type, which is very simple, consisting of just 4 lines: a transmit line TXD, a receive line RXD, a call-indicate line CI, and a return line RETURN. The computer sends data on the transmit line and receives data on the receive line. The call-indicate line is used by the interface to signal to the computer that a call has been received; the computer then takes appropriate action to receive the data which will follow. For this to be useful, the computer must of course be left permanently on.

With mains powered computers, the computer can simply be left permanently on (powered up), and it will always be ready to respond to the call-indicate signal. With portable computers, however, the internal battery power supply is limited. Such a computer can often be put into a quiescent state in which it uses almost no power but will respond to input signals such as a call-indicate signal by powering up fully. Such a computer can be used in conjunction with a telephone modem system.

However, the modem system also needs to be powered up sufficiently to respond to incoming calls from the BT cable and generate the call-indicate signal. The modem system can of course be provided with a mains power input.

But if mains power is available, then it is also available to power the computer, which will therefore not have to use its battery power. So the situation to be dealt with is one where there is no readily available source of mains power.

The modem system could then be powered from the computer. However, this would increase the power load on the computer, which is undesirable. The modem system could also, of course, be powered from a battery of its own, but this is equally undesirable, as the battery charge would not last long if the modem were left permanently switched on. The present system overcomes this problem by powering the modem system from the BT cable.

As discussed above, the present modem system incorporates isolation between the computer interface and the BT line. The coupling power supply for the computer interface side of this isolation is formed by a transformer T1 with a controlled primary circuit and a rectifying secondary circuit.

In the controlled primary circuit, the main power supply line &commat; is connected, via a decoupling resistor R62 and capacitors C37 and C38, to the primary of transformer T1 in series with a switching transistor T1. A signal V24POW from the microprocessor is fed to an oscillator circuit consisting of an AND gate IC14 and an inverter IC15 coupled in a loop by coupling resistors and capacitors as shown.

Signal V24POW goes active when the system is powered up and turns on the oscillator, which switches TR9 on and off. Thus when the system is powered up, a (square wave) AC signal is applied to T1. The secondary of T1 feeds a pair of capacitors C25 and C27, with zener diodes D10 and D13 across them, via a rectifying diode D15, producing a balanced power supply.

The transmit line TXD carries signals from the computer to the present system, and the receive line RXD and the call-indicate line CI carry signals from the present system to the computer, all relative to the common line RETURN which is connected to the centre of the coupling power supply.

The transmit signal TXD is connected through a resistor R36 to the diode of an opto-coupler IC12 and back to the return line RETURN. The transistor of opto-coupler IC12 is connected in series with a resistor R50 across the main power supply, to generate the transmit signal VTXD which is fed to the modem IC. Diodes D3 and D14 and resistor R14 are protective components; they also hold the RXD line to the computer low (through IC5A), matching the line TXD, when the modem system is inactive.

If the computer wants to make an outgoing call, it puts a signal on line TXD. The current path for this signal is not dependent on the isolation power supply 24, so the signal is coupled to the transistor of the opto-coupler, producing signal VTXD, even if the isolation power supply is not energized.

This signal VTXD is coupled not only to the modem IC 23, but also to the latch circuit 21 in the start-up circuit 19, through gate IC18 (Fig. 2B). Thus the system becomes active on either a ring signal from the BT line or a signal from the computer.

The receive signal VRXD from the modem IC is fed via a resistor R49 to the diode of an opto-coupler IC8 connected to the main power supply. The transistor of opto-coupler IC8 is connected in series with a resistor R40 across the coupling power supply, and feeds an inverter IC5A which produces, through a resistor R12 and protective diodes D5 and Di2, the signal RXD. The ring signal /VRING is similarly coupled through opto-coupler IC10, inverter IC5B, and associated circuitry to produce the signal CI.

Microprocessor, modem chip, and associated components (17, 18, 22, 23 - Fig. 2F) The microprocessor 17 IC7 has two major control inputs /RESET and /STBY, several control outputs, a DATA bus input/output, and an address bus output ADD. (For simplicity, the control inputs and outputs are shown only in the places where they are used.) The ADD bus is coupled to a ROM IC3, the output of which is coupled to the DATA bus. The DATA bus is coupled, via a buffer driver block IC2, to the modem chip 23 IC1, to which the ADDRESS bus is also coupled via a further driver and logic block ICi6. The buffer and driver blocks IC2 and IC16 allow the modem IC to be powered down without affecting the operation of the microprocessor.

The microprocessor has an output SNDEN which drives speaker unit 22 containing a small speaker X3 and a drive transistor TR7. The system can thus generate an audible indication of the function it is performing.

The microprocessor spends part of its time in the stand-by state, in which it does not produce definite outputs. Those of its control signals which have to perform their desired functions during this stand-by state are therefore also provided with pull-up (or pull-down) resistors to the power supply 14, to ensure that they are at the correct level during this state.

The modem chip IC1 converts data signals between the computer format from the computer and the BT format on the BT cable (subject to amplification, etc).

That is, it is coupled on one side to the signals RXA and TXA which are associated with the BT cable, and on the other side to the signals VRXD and VTXD which are associated with the computer interface It performs the necessary conversions under control of the signals received from the DATA bus via the drivers IC2 and the control signals received from the address driver and decoder block IC16. Signal VTXD is received via a diode D8 and a pull-up resistor R23.

Signal VRXD is generated (Fig. 2E) via an inverter IC15, and a NAND gate IC18 and an AND gate IC14, which are controlled by control signals MRXDEN and PTXD generated by the microprocessor.

The functions which the microprocessor and modem have to perform are substantially the same as those which are performed in known modem systems (powered from mains supplies), and are therefore not described in detail.

It will be realized that the system can readily be modified to implement tone dialling. For this, it is necessary to use a modem chip which provides tone signals, and to include suitable additional control sequences in the ROM 18.

Claims (9)

C1i ms

1 A modem system for coupling a telephone cable (BT) and a computer (COMP), the system comprising: signal amplifier and modem IC means (10, 23) for passing signals between the computer and the telephone cable and converting their form appropriately, a ring detection unit (16) for detecting ring signals on the telephone cable, a power supply unit (14) connected across the telephone cable via regulating means (15) which present a high AC impedance, and power switching means (26) for powering the signal amplifier and modem IC means from the power supply unit on ring signal detection.

2 A modem system according to claim 1 wherein the power switching means also powers the signal amplifier and modem IC means from the power supply unit in response to signals from the computer.

3 A modem system according to either previous claim wherein the regulating means is switched between different DC impedances on ring signal detection.

4 A modem system according to any previous claim including a microprocessor and an associated ROM for controlling its operation.

5 A modem system according to any previous claim including also an isolation barrier through which signals are coupled with the computer and auxiliary power supply means energized across the barrier through the power switching means.

6 A modem system according to any previous claim wherein the regulating means comprises a field effect transistor.

7 A modem system according to any previous claim wherein the output of the signal amplifier to the telephone cable is coupled thereto by means of a field effect transistor.

8 A modem system substantially as herein described and illustrated.

9 Any novel and inventive feature or combination of features specifically disclosed herein within the meaning of Article 4H of the International Convention (Paris Convention).