ARRANGEMENT IN A SUBSCRIBER LINE INTERFACE CIRCUIT

Abstract: A control processor (2) is connected to a subscriber line interface circuit (1) to, on the one hand, control the operating mode of the line interface circuit (1) and, on the other hand, monitor, via a detector output terminal (D) of the line interface circuit (1), the status of a line connected to the line interface circuit (1). The control processor (2) is adapted to control the line interface circuit (1) to, on the one hand, ensure, at a first point of time, that its detector output terminal (D) is in a first signal state and, on the other hand, at that first point of time, initiate measurement of the line voltage. The line interface circuit (1) is adapted, at a second point of time, to bring the detector output terminal (D) to a second signal state after a time interval whose length in a predetermined manner is proportional to the measured line voltage, and the control processor (2) is adapted to convert the time interval during which the detector output terminal (D) of the line interface circuit (1) is in the first signal state, to a voltage value corresponding to the line voltage.

Description: ARRANGEMENT IN A SUBSCRIBER LINE INTERFACE CIRCUIT TECHNICAL FIELD The invention relates generaUy to an arrangement for measuring the voltage across a telephone line connected to a line interface circuit. BACKGROUND OF THE INVENTION Line interface circuits are provided on line interface boards and are controlled by a control processor on the line interface board, the control processor being common to a plurality of line interface circuits. In order to be able to compensate for line resistance dependent attenuation on the line, the control processor needs information about the resistance of the line connected to the respective line interface circuit. A method of obtaining information about the line resistance when the current characteristic and supply voltage of the line interface circuit are known, comprises measuring the line voltage and calculating the line resistance from the measured line voltage. To transfer the line voltage information to the control processor, either an external analog-to-digital converter or an internal analog-to-digital converter in the line interface circuit, is used today. In the case of an external analog-to-digital converter, a separate output terminal is necessary on the line interface circuit to output an output voltage proportional to the line voltage. With an internal analog-to-digital converter in the line interface circuit at least one separate output terminal is needed on the line interface circuit for communication with the control processor. BRIEF DESCRIPTION OF THE INVENTION The object of the invention is to bring about an arrangement for line voltage measurement and, thereby, line resistance calculation which does not require any extra components or any extra output terminal. This is attained according to the invention in that the control processor controls the detector output terminal of the line interface circuit to a first signal state at a first point of time at the same time as the line interface circuit is controlled to initiate the measurement of the line voltage. After a time interval whose lenght is proportional to the measured line voltage, the detector output terminal of the line interface circuit is controlled to a second signal state. The control processor is adapted to convert the time interval during which the detector output terminal of the line interface circuit is in the first signal state to a voltage value corresponding to the measured line voltage. Since the current characteristic and supply voltage of the line interface circuit are known, the control processor can calculate the line current on the basis of this voltage value and, thereby, also the line resistance. Thus, the measurement can be carried out without any extra components or any extra output terminal, i.e. no extra wiring is needed on the line interface board. BRIEF DESCRIPTION OF THE DRAW TNG The invention will be described more in detail below with reference to the appended drawing, on which the single figure shows an embodiment of an arrangement according to the invention for measuring the voltage across a line connected to a line interface circuit. PREFERRED EMBODIMENT The single figure schematically shows a line interface circuit 1 and a control processor 2 for controlling the operating mode of the line circuit 1 via a digital interface 3. The interface 3 has two control output terminals S 1 and S2 for controlling the liite" interface circuit 1 for measuring the voltage across the line (not shown) connected to the line interface circuit 1. The control output terminal S 1 is connected to a switch 4 for switching the switch between a lower and an upper position. Normally, the switch 4 is in its lower position in order to transfer signals to the control processor 2 via the detector output terminal D of an mverting detector output amplifier 5 from the normal detectors of the line interface circuit 1, illustrated in the form of a block 6. These detectors which do not have anything to do with the present invention, are known per se and, therefore, are not described in this connection. When the line voltage is to be measured in accordance with the invention, the switch 4 is switched to its upper position shown in the figure in that the control output terminal SI of the interface 3 is activated (brought to a high signal level) by the control processor 2 as will be described more in detail below. In the embodiment shown, the control output terminal S 2 of the interface 3 is connected to the base of a transistor QI whose collector is connected, on the one hand, to the base of a transistor Q6 and, on the other hand, to the anode of a diode Dl whose cathode is connected to a supply voltage VBB. The emitter of the transistor QI is interconnected with the emitter of a transmitter Q2 whose collector is connected to an interconnection node A. A capacitor Cl is connected between the node A and ground. The node A is also connected to the emitter of a transistor Q3 whose base is adapted to sense the potential on that one of the two wires of the telephone line not shown, whose potential is intended to be closest to ground, usualh' denoted a-wire or tip wire, via a tip wire connection teπninal 7. The collector of the transistor Q3 is connected, on the one hand, to the anode of a diode D2 whose cathode is connected to the supply voltage VBB and, on the other hand, to the base of a transistor Q4 whose emitter is connected to the supply voltage VBB and whose collector is connected to the interconnection point between the base and one collector of a multi-collector transistor Q5. A second collector of the transistor Q5 is connected, on the one hand, via a resistor Rl to ground and, on the other hand, tδ the upper pole of the switch 4. The emitter of the transistor Q5 is connected to a supply voltage VCC. Moreover, a current generator II is connected between the supply voltage VCC and the interconnection point between the emitters of the transistors Ql and Q2. The node A is, furthermore, connected to the cathode of a diode D3 whose anode is adapted to sense the voltage on the second wire, usually denoted b-wire or ring wire, of the line (not shown) connected to the line interface circuit 1 via a ring wire connection terminal 8. The b-wire (not shown), is the wire whose potential is intended to be closest to the supply voltage VBB. Finally, the node A is connected to the collector of the transistor Q6 whose emitter is connected to the supply voltage VBB and whose base, as above, is connected to the interconnection point between the collector of the transistor Ql and the anode of the diode Dl. The base of the transistor Q2 is connected to a DC source 9. Normally, the base of the transistor Q 1 is at a signal level which is lower than the signal level at the base of the transistor Q2, i.e. the transistor Ql is normally conducting, while the transistor Q 1 is cut off. Thus, the current from the current generator II normally flows through the transistor Ql and the diode Dl. This current is mirrored to the collector of the transistor Q6 and will be drawn through the diode D3 in such a manner that the voltaee in the node A will be a diode voltace drop of the diode D3 below the voltage of the ring wire (not shown), i.e. the voltage of the ring wire connection terminal 8. Thus, the capacitor Cl will normally be charged to this voltage. To measure the line voltage across the line (not shown) connected to the connection terminals 7 and 8, the control processor 2 is adapted to apply to the base of the transistor Ql via the control output terminal S2 of the interface 3, a signal level which is high in comparison with the signal level that the DC source 9 applies to the base of the transistor Q2. This point of time is registered by the control processor 2 as a starting point of time for the measurement. At the same time, the control output terminal S I of the interface 3 is activated, bringing the switch 4 to its position illustrated in the figure. Hereby, the input terminal of the amplifier 5 is grounded via the resistor Rl. Therefore, the signal level on the output terminal of the amplifier 5 will be low. Since the amplifier 5 is mverting, the signal level will be high on the output teirninal D which ensures that the signal level on the terrninal D will be high irrespective of its previous signal level. When the signal level on the base of the transistor Ql is higher than the signal level on the base of the transistor Q2, the transistor Ql is cut off while the transistor Q2 becomes conducting. The current from the current generator II will instead flow through the transistor Q2 and start to recharge the capacitor Cl. When the voltage across the capacitor Cl, i.e. the voltage in the node A, reaches such a value relative to the voltage on the base of the transistor Q3, i.e. the voltage on the tip wire connection terrninal 7, that the transistor Q3 starts to conduct, the current from the current generator II will flow through the transistor Q3 and the diode D2. This current is mirrored to the collector of the transistor Q4. The collector current of the transistor Q4 is drawn from the collector interconnected with the base of the transistor Q5 and is mirrored to the second collector of the multi-collector transistor Q5. Thus, the current flows through the resistor Rl and causes a voltage across this resistor. The voltage across the resistor Rl is coupled via the switch 4 in its illustrated position to the input terminal of the amplifier 5 which causes the signal level to become low on the output of the amplifier 5. That the signal level on the output terrninal D goes low is registered by the control processor 2 as the ending point of time for the measurement. Thus, the detector output terrninal D of the line interface circuit 1 goes low after a time interval conesponding to the time it took to recharge the capacitor Cl. During this time interval, the capacitor Cl has been recharged to a voltage which is proportional to the voltage across the line (not shown) connected to the tip and ring wire connection terminals 7 and 8, respectively. The control processor 2 is adapted to measure this time interval whose length is proportional to the line voltage. Based on the calculated voltage value, the control processor 2 is adapted to calculate the line current since the current characteristic and supply voltage of the line interface circuit are known. From the line current value, the control processor is adapted to calculate the line resistance.

Claims: CLAIMS 1. Arrangement in a line interface circuit (1) to which a control processor (2) is connected, the control processor (2) being adapted, on the one hand, to control the operating mode of the line interface circuit (1) and, on the other hand, monitor the status of a line connected to the line interface circuit (1), via a detector output terminal (D) of the line interface circuit (1), characterized in - that the control processor (2) is adapted to control the line interface circuit (1) to, on the one hand, ensure at a first point of time that its detector output terminal (D) is in a first signal state and, on the other hand, at that first point of time initiate measurement of the line voltage, - that the Line interface circuit (1) is adapted, at a second point of time, to bring the detector output terminal (D) to a second signal state after a time interval whose length in a predetermined manner is proportional to the measured line voltage, and - that the control processor (2) is adapted to convert the time interval during which the detector output terminal (D) of the line interface circuit (1) is in the first signal state, to a voltage value conesponding to the line voltage. 2. The arrangement according to claim 1, characterized in that the control processor (2) is adapted to calculate the line resistance from the voltage value conesponding to the line voltage.

Inventors: ERIKSSON HANS, ERIKSSON, HANS
Applicants: ERICSSON TELEFON AB L M, ERIKSSON HANS, ERIKSSON, HANS, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1997
Main classification: H04M1
IPC codes: H04M1, H04M3