Patent GB-949552-A (GB)
Title: Electronic switching telephone system
IPC: H04Q3/52
Filed: 1962-01-19
Abstract: 949,552. Automatic exchange systems; circuits employing bi-stable magnetic circuits. STANDARD TELEPHONES & CABLES Ltd. Jan. 19, 1962 [Jan. 24, 1961], No. 2035/62. Headings H3B and H4K. A cross-point comprises a magnetic core which is fed with scanning pulses, which are normally insufficient to bias the core to one of its states of saturation, but when a control signal is also supplied to the cross-point, the pulses drive the core to that state of saturation, thereby causing the cross-point to be switched on and causing the core to emit pulses indicating that the cross-point is busy. Connection of calling subscriber to link.-A link is pre-allotted by allotter 9 by marking the ALLOT lead. When the calling subscriber goes off-hook, potential developed over resistance 505 marks lead 10c negatively and enables the AND gate 512 so that when this gate is scanned over lead 16 by a pulse allotted to the subscriber, it pulses the off-hook highway, the resulting signal passing through a gate 443, enabled by the allot lead, to enable gate Q 4  of that link so that lead 12a is grounded at this time. Core 532 is normally saturated by current flowing downwards through winding 539 via the base-emitter circuit of thyristor 550, which operates in this instance as a Zener diode. Pulses applied to lead 16 are normally insufficient to drive core 532 to its opposite state of saturation, so that it presents high impedance and the pulse thus received over lead 12a is insufficient to cause Zener diode Z 1  to conduct. However, when lead 12a is earthed, the pulse in winding 536 is increased sufficiently for the pulse induced in winding 538 to trigger the thyristor whose collector is enabled by the marking on 10c. This causes a large current to pass through winding 539 to saturate the core. Winding 536 now presents low impedance and applies a full pulse to lead 12a, and this, applied via gate 441 to the input busy highway, inhibits gate 316 to prevent the call from seizing another link. Although this disables transistor Q 4 , the pulses on lead 12a are now sufficient to break down Zener diode Z 1  to enable gate 441 to maintain the busy marking for the links, so that no link may now seize the busy calling subscriber's line. The allotter is stepped to mark another link by the output of gate 441. Conduction of thyristor.550 enables transistors 527, 528 to connect the incoming and outgoing speech paths 10a, 10b to the vertical multiples 12a, 12b of the link. The output from gate 441 also enables a dial tone gate 457 to pass dial tone to lead 12b via transformer 351. Dialling and busy test.-Dial pulses received over transformer TR2 and leads 10b, 12b are applied to the shift register RA . . . RN, which registers the digit, and also to start circuit 446 whose output inhibits the dial tone at gate 456. Assume digit 2 is dialled. The registered digit enables gate 463 which passes pulses in pulse position B corresponding to the called subscriber and these pass via gate 331. If the subscriber B is busy a pulse in this position is present on the input busy highway and prevents at gate 334 the transmission of the pulse from 331 on to the line seize highway. This pulse also passes via gate 342 to set trigger 346 which enables gate 354 to revert busy tone via transformer 351 and lead 12b. Completion of connection.-If the called line is idle the output of gate 334 is applied to inhibit, at gate 320, the subsequent application of input busy pulses when the connector matrix switches through. The pulse is also applied via gate 442<SP>1</SP> to the vertical control conductor 18a. The pulse is also applied via the line seize highway to gate 400 and all subscriber's gates such as 517. The latter for subscriber B is scanned by the B pulse and applies, via a holdover amplifier, marking potential to the c conductor for that subscriber, and this in conjunction with the signal on 18a fires the corresponding connector cross-point in manner similar to that for the line finder cross-point. The fired cross-point operates via conductor 18a and Zener diode Z1<SP>1</SP> in gate 442<SP>1</SP> to mark the input busy highway in the same way as for a calling subscriber. The output from gate 331 also sets the trigger 350 to enable gate 352 to pass ringing signal to the called line via lead 18b, and to enable gate 352 to pass ringing tone via transformer 351 and lead 12b to the calling line. Answer supervision and release.-When the called subscriber B answers, his loop is detected over resistance 573 to enable gate 406 to pass B-phase pulses to the off-hook highway to gate 340, wherein coincidence with pulses from gate 331 cause it to apply a reset pulse via a timer 341 to trigger 350. This shuts off the ringing signal and ringing tone. The output of the timer also resets the register RA . . . RN. The connector cross-point is now held by potential applied to the control conductor, as a result of detection of the off-hook signal over resistance 573. When a subscriber hangs up, this potential is removed to extinguish the thyristor, and current over resistor 540 and winding 539 restores the core to its original state of saturation so that, as previously described, the pulses on lead 12a are insufficient to break down the Zener diode Z 1 , the pulses thus being removed from the input busy highway, to allow the subscriber to test free. Specification 938,685 is referred to.