Analog/Digital Telephone Test Set

1. Field of the Invention

The present invention relates generally to the testing of telephone lines and systems, and particularly to telephone test sets for accessing and testing subscriber lines. More particularly still, it relates to a telephone test set capable of testing subscriber lines emanating from different switching centres which use the following modes of signalling: dial pulse; DTMF or tone signalling; and digital command signalling on subscriber lines associated, for example, with the now widely used Northern Telecom DMS-100 switching system.

2. Prior Art of the Invention

Prior art telephone line testing devices may be used on telephone lines by simply connecting them to the tip and ring leads. U.S. Pat. No. 4,194,098 granted Mar. 18, 1980 to Carlson and U.S. Pat. No. 4,682,346 granted Jul. 21, 1987 to Faith et al provides such test sets which are capable of operating on telephone lines which use either of dial pulse or DTMF signalling both of these United States Patents are relevant prior art as far as the dial pulse and DTMF signalling functions are concerned. Both United States Patents are incorporated herein by reference.

On telephone lines switched using a digital telephone switch such as Northern Telecom's DMS-100 such prior art devices cannot be used. A factor in such incompatibility is that although the prior art devices have included universal signalling test systems (U.S. Pat. No. 4,165,450) and occasionally had selective D.C. impedances of 600 ohms and 900 ohms (U.S. Pat. Nos. 4,843,620 and 4,912,755) they have generally had impedances in the range of 70 to 300 ohms. A digital telephone line requires the testing device D.C. impedance to be in the range of 1000 to 2000 ohms. Overcoming such incompatibility would simplify the number and type of devices that would be needed to test telephone lines in various telephone line systems.

Prior art devices for testing the circuitry of telephone lines have included memory dialers. They have also included batteries to provide power to the memory circuitry when the device is not connected to the tip line and ring line as a source of power. Batteries have various replacement and maintenance problems associated with them. Such problems could be overcome by using a different power source for the memory circuitry.

Moreover, prior art devices have included a shunt circuit to limit the current applied to the device. Usually, such shunt circuits have comprised sensing circuits and a silicon controlled rectifier to shunt current from an overcurrent condition between the tip line and ring line.

Additionally, prior art devices have included light emitting diodes in series with voice receiving and transmitting units. Alternatively, the diodes were installed across the tip line and ring line with a switch to momentarily test the polarity of the voltage on the tip and ring lines.

Finally, prior art devices included circuitry producing a short between the tip and ring lines to keep large currents out of the device. Such circuitry could result, if the large current was from an unfused source or a source with a high current fuse (such as 15 ampheres or higher), in damage to the devices and injury to a person using them. Such damage and injury may be avoided by opening the tip and ring lines, not allowing current to flow.

In order to provide a more versatile telephone test set capable of accessing telephone lines and systems employing digital signalling in addition to dial pulse and DTMF tone signalling, the present invention utilizes a programmable microprocessor to accommodate the requirement for one or more digital signalling functions and formats.

Accordingly, the present invention provides an analog/digital telephone test set, wherein the DTMF tone signals generated are applied to a DTMF decoder then to a programmable microprocessor, which converts that decoded DTMF signals to a series of digital command signals to be applied to the telephone line. The format of the signalling commands generated by the microprocessor is predetermined by the therein stored firm- or software. An example of such software for converting DTMF signalling to signalling commands comprising 16 bit data words is given herein as below. In case of Northern Telecom's DMS-100 switching system, the 16 bit data words are transmitted by turning on and off and eight kilo-hertz tone at the rate of 1 kilohertz, all of which is generated by the microprocessor as programmed. Of course, other signalling formats and carrier frequencies may be programmed.

According to a broad aspect of the present invention, there is provided an analog/digital telephone test set for connection to a telephone line having DTMF signalling means for signalling on analog telephone lines, comprising: (a) DTMF decoding means for converting DTMF tones to microprocessor compatible signals; and (b) microprocessor means for responding to said DTMF decoding means in a preprogrammed manner by outputting digital signals for signalling on digital telephone lines.

The preferred embodiment of the present invention will now be described with reference to the annexed drawings, in which:

FIG. 1 is a system functional block diagram of the analog/digital telephone test set according to the present invention;

FIG. 2 is a more detailed block/schematic diagram illustrating different modes of operation of the system shown in FIG. 1;

FIG. 3 is a schematic diagram of the wave shaper; and

FIG. 4 is a schematic diagram of the power supplies.

Referring to FIG. 1 of the drawings, the preferred analog/digital telephone test set of the present invention comprises a polarity indicator 10 adapted for connection across the tip and ring leads of a telephone line to be tested, followed by a full-wave bridge rectifier 11 and an overvoltage protection circuit 12 connected to the positive output of the rectifier 11. Also connected across the tip and ring leads is a monitor circuit 13 which is essentially a voice transformer connectable to a loudspeaker or earphone 14. When not in only monitor mode the loudspeaker 14 is driven by a voice circuit 15 to which a microphone 16 is also connected. A standard push-button dial pad 17 serves as the user's signalling input to an analog signalling circuit 18, which outputs either DTMF tone or dial pulse signals directly onto the tip and ring leads via the rectifier bridge 11; or, in digital signalling mode, the analog signalling circuit 18 applies the generated DTMF tones to a DTMF tone decoder 19, the output of which is read and interpreted by a controlling microprocessor 20. The latter then generates a keyed 8 kHz tone signal encoding the 16 bit data words necessary for signalling the DMS-100 switching machine, which signal is applied to a digital wave shaper 21 before being output onto the tip and ring leads. A dial tone detector 22 detects dial tone and a ringing detector 23 detect dial tone and ringing on the tip and ring leads, respectively, and apply their outputs to the microprocessor 20, which processes such information.

FIG. 2 of the drawings shows in more detail the interconnection of the functional blocks under analog and digital modes of operation. The positive output of the rectifier bridge 11 is connected to the emitter of switching transistor 24, which is controlled via transistors 25 and 26 by the dial pulse output of the analog signalling circuit 18 and which interrupts the line current when switched off, thus causing dial pulse signalling. Unless the latter is occurring the transistor 24 is switched on and is transparent, conducting line power to power supplies 27A and 27D from the bridge 11, as well as conducting signalling from and to the tip and ring leads. When in DTMF tone signalling mode, the DTMF output of the analog signalling circuit is connected via switch 28a to the base of transistor 29 and though its collector to the positive output of the bridge 11, i.e. onto the tip and ring leads to the telephone system. As shown in FIG. 2, switches 28a, 28b and 28c are in the position of digital signalling, thus power is applied to the power supply 27D, and the DTMF output of the circuit 18 is shown connected to the DTMF decoder 19, while resistor R.sub.L disconnected raising the impedance across tip and ring, and the digital signalling output of the microprocessor 20 is applied via the digital wave shaper 21 and the switch 28b as well as the transistor 24 to the bridge 11 and hence onto the tip and ring leads. As voltage at the collector of the transistor 24 exceeds the zero voltage of zener diode 30, the latter conducts causing transistor 31 to conduct and through its collector causing the transistor 25 to switch off, which in turn switches the transistor 24 off, thus removing the high voltage from its collector. As the transistor 31 is turned on, so will transistor 32 be turned off, while the former continues to conduct because of resistors 33 and 34 connecting its base to the positive output of the bridge 11, which is the immediate source of the high voltage. Of course, once the latter no longer exists, the transistor 31 will switch off without untoward consequences.

Referring to FIG. 3, it shows the wave shaper circuit 21 in detail. This circuit is necessary because the microprocessor 20 generates square waves referenced to zero volts. The signalling requirement on a digital line is that a "1" data bit is represented by eight cycles of an 8 kHz sine wave of 1.2 volts peak-to-peak be placed on the tip and ring leads of the telephone line; while a "0" data bit is represented by the absence of the 8 kHz frequency for one millisecond. The microprocessor 20 is a Motorola MC68HCO5J1 and generates the 8 kHz signal at its pin 15 output, which is applied to the base of transistor 35 via two resistors. The base of the transistor 35 is biased via +4 Volts applied from the microprocessor 20 pin 14 (CRTL). The 8 kHz square wave applied from pin 15 is integrated by means of resistor 36 and capacitor 37 into a triangular wave before it is applied to the transistor 35, at the emitter of which diodes 38 and 39 and resistor 40 at its collector smooth the triangular wave into an approximate sine wave. The latter is capacitively coupled to transistor 41 and further to an output transistor 42, the emitter of which is capacitively coupled to the switch 28b for application to the tip and ring leads. In order to conserve power when not signalling, the microcomputer 20 by means of pin 13 (CRTL) is used to power the output transistor 42.

FIG. 4 is a schematic of each of the power supplies 27A and 27D, which yield +5 Volts at the emitter of transistor 43. A memory backup capacitor CM of 0.47 Farads of the type normally used in computers is used to provide memory backup.

As for the circuits not given in detail in the drawing figures, they all are off-the-shelf circuits and are used in the preferred embodiment as suggested by their suppliers in the respective data sheets. These circuits are:

Rectifier Bridge 11: ITT DFO6M

Voice Circuit 15: MOSTEK L3280

Analog Sign. Cct. 18: MOSTEK MK5375

DTMF Decoder 19: MITEL MT8870

Dial Tone Detector 22 and Ringing Detector 23: EXAR XR-L567

The Motorola microprocessor 20 controls the digital mode of operation in the following manner. Once powered by switching the set to digital mode, the processor 20 monitors two of its input pins. It monitors pin 12 to determine if ringing tones are present on the subscribers' line and if so flashes a light emitting diode (connected to pin 16) at a rate of 10 PPS to indicate to the user that ringing is present. The microcomputer 20 also monitors pin 18 to determine if any digits have been dialed. If the microcomputer 20 sees pin 18 go low, (from 5 to 0 volts) it reads the digit that has been keyed on pins 5,6,7, and 8. Each digit is represented by a binary code, the binary bit "1" is represented by 5 volts present and the binary bit "0" is represented by 0 volts present. As digits are keyed they are stored unitl an "*" digit has been keyed. When the "*" digit is keyed it is an indication to the microcomputer 20 that it should send out a 16 bit code over the tip and ring of the subscribers' line to tell the central office which telephone number it wants associated with the subscribers' line. Upon receiving the 16 bit code the central switching office then assigns the requested telephone number to the subscribers' telephone line and returns dial tone. The microcomputer notes that dial tone has been received (pin 17 goes low) and turns on light a emitting diode by placing 0 volts on pin 16. At this point any digit that is keyed on the keypad 17 is automatically converted by the microcomputer 20 to a 16 bit code and sent over the tip and ring lines to the central switching office. If an "*" digit is the first digit dialed the microcomputer sends a default 16 bit code to the central office to select the first or primary telephone number associated with the subscribers' line being used. When the user has completed using the telephone test set they must send an on-hook condition to the central office to inform it to disassociate the used telephone number with the line being used. This is accomplished by keying an "#" digit to the microcomputer which then generates the 16 bit on-hook signal. The microcomputer 20 then turns the light emitting diode connected to pin 16 off.

The entire program for the MC68HCO5J1 microprocessor 20, with commentary, is as follows: ##SPC1##