Patent GB-1383587-A (GB)
Title: Ringing arrangement for telephone exchange
IPC: H04M3/00
Filed: 1972-04-28
Abstract: 1383587 Automatic exchange systems INTERNATIONAL BUSINESS MACHINES CORP 28 April 1972 [19 May 1971] 19818/72 Heading H4K A subscriber line circuit comprises three limbs; a first and a second limb of which are connected via a telephone line to a ring signal detector situated in a subscribers sub-set and are arranged in operation respectively to conduct on different polarities of an A.C. ringing signal, and a third limb which is connected to the first limb in operation through the voice circuits of the subset to provide a conductive path for a D.C. offhook signal. With the third limb may be associated logic circuitry for detecting the off-hook signal which is arranged to prevent longitudinal noise signals, occurring on an unlooped line, being detected as an off-hook or an on-hook signal. Fig. 1 shows one such line circuit with the loop detection logic circuitry omitted. A ring- ing signal generator GS of 50 Hz is coupled via transformer TS to buses BA and BB which are connected to a first and second terminal respectively of a plurality of line circuits. In each line circuit the two buses are connected via respective limbs of the line circuit to the conductors of a telephone line extending to a subscriber sub-set PT. In each limb is connected a respective thyristor RCA and RCB which have their gates energized by positive peaks of a 20 kHz signal. This signal is supplied by the exchange to a line circuit when the particular subscriber is to be rung. On alternate half cycles of the A.C. signal generated in windings S1 and S2 of transformer TS thyristors RCA and RCB will conduct alternately and supply ringing current via the telephone line to the sub-set, i.e. on positive half cycles thyristor RCA conducts, and current flows from earth via winding S1, thyristor RCA, resistor R1, ring detector DS, diode D2 and resistor R4 to negative battery. The ring detector is simultaneously energized by a circuit including diode D4 and resistor R5. On negative half cycles thyristor RCB conducts to complete a similar circuit and ring detector D5 is simultaneously energized by a circuit including diode D5 and resistor R4. When the subscriber sub-set is looped the D.C. loop current flows from earth via resistor R3, diode D1, the sub-set loop, diode D2 and resistor R4 to negative battery. Hence only the D.C. loop current flows in resistor R3 and the potential across R3 can be used as an on/off hook indication. Fig. 4 shows an arrangement of logic circuitry for providing an on/off hook indication at a terminal D. When the line is not looped transistors T1 and T2 are blocked as well as transistors T3-T6. Thus it is necessary for transistors T1 and T2 to be saturated at the same time to cause transistor T3 to conduct. As soon as this condition is met transistors T4, T5 and T6 become saturated. Transistor T3 which up to now acted as an AND gate now acts as an OR circuit for the detectors constituted by transistors T1 and T2. Because the noise signals on each wire have the same polarity with respect to earth the detection of noise signals is prevented under open loop conditions since transistors T1 and T2 cannot become saturated on the same polarity cycle of the noise signal. Similarly with the line looped noise signals cannot cause transistors T1 and T2 to be simultaneously blocked. The line circuit of Fig. 1 is modified in Fig. 4 as follows. It is stated that the resistors R1-R5 should be of equal value and the resistor R3 should be of a lower value for correct operation of the Fig. 1 circuit. However, if the subscriber's loop circuit is to be perfectly balanced when D.C. passes through it, it is necessary for the resistors located in the two branches of the loop to be equal. This is achieved in Fig. 4, in which resistor R<SP>1</SP>3 has a value equal to the value of R3 and resistor r is equal to the value of resistor R5. When transistor T1 detects a loop current, this transistor controls the conduction of transistor T7 which then short circuits resistor r. This circuit also provides an automatic protection in case of a short circuit between the line wire connected to the power supply and the ground. In Fig. 5 (not shown), a circuit corresponding to Fig. 1 is shown in which positive battery (+V) is used in place of negative battery.