Patent GB-839567-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: FORSHAW GEOFFREY
IPC: H04Q3/52
Filed: 1956-10-24
Abstract: 839,567. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd. Oct. 11, 1957 [Oct. 24, 1956], No. 32358/56. Class 40 (4). In a small private exchange in which switching is performed by means of transistors, each subscriber's line SL is matched by way of a line transformer TR1 to a single-wire transmission path which forms part of the line circuit SLC and includes two transistors LT24, LT25, one for the transmission of speech and the other for the transmission of supervisory tones, all the transmission paths being connected to a common lead 12. As shown, all the transistors are of PNP junction type, but with suitable battery connections those of NPN type could be used. The exchange common equipment comprises a pulse, transfer and reset circuit PTR, impulse counter IC, line marker LM, tone generator TGC, time-delay throw-out TDT and call priority circuit PD. Establishment of call.-The looping of line SL produces a negative-going pulse which switches on transistor LT4 in place of LT3. LT5, LT8 are both switched off with the result that wire PDB becomes earthed. LT11 is switched on so that LT16 ceases to conduct and LT25 is switched on to apply dial tone suitably attenuated by R16 from common lead 12. Dial tone is generated by a multivibrator CT13, CT14 which operates until the first dialled impulse is received and is fed through an amplifier CT12. LT14 becomes conducting and switches off LT15, thus earthing wire 10. All wires 10 are commoned so that LT15 is also switched off in all other line circuits and prevents the switching on of LT11 by the initiation of another call. As both LT5, LT6 are switched off LT7 now conducts and connects earth to common lead 11. This switches off CTA in the circuit PTR and allows CT5 to conduct, preparing the circuit TDT for operation. During dialling breaks and makes are passed on by CTA as negative and positive-going pulses to CTB which is switched off at each break. These pulses also control CT1, CT3 to bring the collector of CT2 to negative potential, while CT4, CT11 are switched on as long as impulsing continues. The impulse counter IC comprises three bi-stable trigger pairs arranged to count in binary form, CT25, CT27 and CT29 being normally conductive. Each time CTB is switched off, negative potential applied through R18 changes over CT25, CT26. When CT26 is switched off at the second impulse negative potential applied through R17 changes over CT27, CT28, and so on. At the first impulse, earth through R20 switches on CT8 in place of CT9, with the result that dial tone is cut off and the circuit TDT is disabled. The line marker LM provides an individual marking wire M for each subscriber associated with three transistors. At the end of impulsing CT11 is switched off and negative potential is connected to all the transistors in the line marker, so that, by virtue of connections from the impulse counter, the transistors of a particular marking wire M are disabled and that wire is marked, with the result that the speech transistor LT24 is switched in the called subscriber's line circuit. Negative potential also switches on CT24 which starts a pair of multivibrators CT13, CT14 and CT18, CT19 to generate on common lead 12 interrupted ringing current which passes to the called subscriber and also as ringing tone through LT25 and R16 to the calling subscriber. The reply of the called subscriber switches on LT19 and increases the collector current of LT20. Common lead 13 is earthed, so that CT15 ceases and disables the ringing generator, while LT18 ceases in all the line circuits and causes the speech transistor LT24 to conduct in the calling line circuit. Release.-Assuming that the called subscriber hangs up first, LT19 ceases, negative potential is replaced on common lead 13 and LT24 is switched off in the calling line circuit. When the calling subscriber clears, LT7 ceases and replaces negative potential on common lead 11 to switch on CTA. CT5 is switched off, and a positive-going pulse switches off CT3 for a certain time during which CT4 conducts. When CT4 ceases, CT6 is switched on to reset the calling line circuit by switching on LT13, while CT10 is switched on to disable the marker and thus switch off LT24 in the called line circuit. Failure to dial.-CT5 conducts on the initiation of a call and starts a multivibrator CT37, CT38 in the circuit TDT which delivers a pulse every 4 seconds to a counter similar to the impulse counter IC. After 7 pulses CT40, CT41 and CT42 are all switched off, and, if the subscriber has not dialled, CT43 is also off and CT44 conducts, connecting earth to common lead 14 to switch off LT27 in the calling line circuit. Dial tone is cut off and the calling condition is removed, so that the subscriber must hang up before he can start the call again. Call priority circuit, PD.-PDT1 ... PDT6 are normally conducting and LT10 in each line circuit is off. If, for example, subscriber No. 2 calls, PDB2 is earthed and PDT2 ... PDT6 are switched off, the potential on PDA3 ... PDA7 becomes negative and switches on LT10 in the corresponding line circuits, so that later numbered subscribers cannot call.