Patent GB-1355129-A (GB)
Title: Individual telephone line equipment
IPC: H04M3/22
Filed: 1971-12-16
Abstract: 1355129 Automatic exchange systems INTERNATIONAL STANDARD ELECTRIC CORP 16 Dec 1971 [23 Dec 1970] 58429/71 Heading H4K A subscriber's line circuit is arranged to give an output dependent on the line condition, e.g. whether the subscriber's line is free, calling, busy or in false call condition, when an interrogation pulse is applied to the circuit. The line L1, L2, has two resistors R1, R2 connected in series with it and the potentials occurring on either side of these resistors are used by means of circuits CT1 and CT2 to change the amplitude of an interrogation pulse received by transistor T1. The changed signal is then applied to threshold circuits DL1-DL3 from whose outputs the condition of the line can be determined. For the purposes of the following description it will be assumed that potential source - U = - 48V and potential source - u = - 6 volts, that the value of resistor R 1  = value resistor R2 = value of the resistance of the looped line L1, L2; that when an interrogation pulse is received by transistor T1 the potential applied to line E1 is charged from - 48v to - 19v and the potential applied to line E2 is charged from -48 to - 30v; finally that the potential on the emitter of transistor T4, v = + 15v. Line free.-This is characterized in that contacts A1, and A2 are closed and the line is not looped through the subscribers set Pt. The points L and M in loop detector circuit CT1 are at - 48v and earth respectively. The point N in feed test circuit CT2 is at earth. Application of the interrogation pulse therefore causes a 29v pulse level to appear at M and an 18v pulse level to appear at N, these pulse levels being transferred via respective diodes D1, D6 to transistors T3 and T4, T5 respectively. These pulses are sufficient to cause all three transistors to turn on and a 111 output is obtained (see Fig. 2). Line calling.-The line is now looped with contacts A1, A2 closed. Because of the values of R1, R2 and the loop resistance point L is now at - 32v and point M at - 16v. When the interrogation pulse is now applied only a -3v pulse level appears at M and transistor T3 does not turn on. As the point N is circuit CT2 is at earth as before transistors T4, T5 are turned on by the interrogation pulse. A 011 output is obtained indicating that the line is looped and fed by source -U via contact A2. Line connected.-When the line is connected through the exchange switching circuit RC it derives its potential source therethrough and contacts A1, A2 are open. If the supply via the network RC is such that L1 is at - 48v and L2 is at earth, point L is at earth and points M and N at - 48v. Reception of the interrogation pulse fails to cause points M and N to go positive and a 000 output is obtained. Likewise, if the line polarity is such that L1 is at earth and L2 is at - 48v, diode D5 and resistor R5 connected to a - U( - 48v) source ensure that L, M and N are all at - 48v and a 000 output is again obtained. at False call condition,-If the subscriber has off-hooked and is not participating in a call, after a time interval means, not disclosed, causes switches A1, A2 to open and switches P1, P2 to close. The potentials at the points L, M and N are then - 28v, - 14v, and - 6v respectively and the interrogation pulse causes point M to reach a pulse level of - 5v and point N to reach a pulse level of + 12v. As transistor T4 requires at least a 15v pulse to turn it on a 001 output condition is obtained. When the subscriber on-hooks, the line is no longer looped but is still being fed by P1, P2. Points L, M, N are therefore at -48v, - 6v and - 6v respectively, and the presence of the interrogation pulse causes a 23v pulse level at M and a 12v pulse level at N. A 101 output condition is therefore obtained which indicates to the exchange equipment that the false call condition has ceased, and as a result P1, P2 will be opened and A1, A2 closed.