Patent GB-722173-A (GB)
Title: Improvements in or relating to automatic telephone exchange apparatus
Inventor: SCOWEN FRANK
IPC: H04M19/02
Filed: 1949-10-05
Abstract: 722,173. Automatic exchange systems. POSTMASTER GENERAL. Sept. 14, 1951 [Oct. 5, 1949], No. 25652/49. Class 40 (4). In an automatic exchange ringing current is applied to a called subscriber's line through an electrical bridge network which is unbalanced to ringing current if the line terminated in the substation apparatus is faultlessly connected to the exchange apparatus, and is substantially balanced if the line terminated in the substation apparatus includes a disconnection, the unbalance current which flows from the bridge circuit causing the application of ringing tone to the calling subscriber's line. Thus ringing tone cannot be transmitted if the called line is disconnected and if the application of ringing current to a called line fails to give rise to the application of ringing tone to the calling line the fault condition is indicated to the maintenance staff at the exchange. Embodiment for use with Strowger equipment. As shown in Fig. 1 (Prov.), indicating circuits (one per called line) are wired to the third bank W3 of the final selector. Contacts h are those of the normal H relay (not shown) and F is the usual ring-trip relay. The bridge circuit comprises hybrid transformer T to terminals 2 and 5 of which are connected the ringing generator RG and battery B1 normally provided. Relay X responds to the unbalance ringing current. Balance Z is equal in impedance to the apparatus connected to terminals 1 and 4 when F is back, H up, and no line is connected to wipers W1 and W2. When the called line is seized (H up) and is intact the unbalance ringing current brings up X which connects ringing tone from RR to the calling line. Relay Y is slow-to-operate and does not pull up during the period when H is up and X is not. On the reply of the called party F operates in the usual way, switches through and disconnects ringing current and ringing tone. If the called line is disconnected X remains down and ringing tone is not connected to the calling line. Relay Y pulls up and locks, lighting lamp IL to indicate the fault condition. The indication is removed by operation of key K1. Embodiment for use in an electronic exchange of the type disclosed in Specification 722,172. The calling subscriber's line OS1, OS2, Fig. 1, is connected via hybrid transformer T1 to the 4-wire path through the exchange over which it is extended to a register R. A marker marks the called line and the register then connects a forward hold signal to lead 1 by raising its potential from - 50 V. to ground potential. The register is then connected to the called subscriber's exchange apparatus S through a ringing and metering unit RM. The forward hold signal is fed back over lead BHR the potential of which rises from - 50 V. to - 40 V. so that tube V1 fires and its anode potential drops below + 40 V. Rectifier W1 thereupon conducts and ringing control generator RC1 transmits a signal over circuit 2 and band-pass filter F1 to tube V2. As described in Specification 722,174, V2 fires and current from ringing generator RG is applied to the called line CS1, CS2 via hybrid transformer T5. If the called line is intact unbalance current flows through R4 and battery B1 thus changing the potential of the private wire P at the frequency of the ringing generator. This wire controls a converter 4 which produces a signal at, say, 10 kc/s. which is normally inhibited (P at ground potential), but is transmitted when lead P has a potential below - IV. Part of the output is applied via transformer T6 and a rectifying circuit to a gating circuit C6, C7, W3, W4, R5, R6 which allows signals from the ringing tone generator RT to be transmitted to circuit 3 during the part of each cycle of the ringing generator during which the rectifier output is positive. Part of these signals passes via filter F2 to rectifier W5 in the register. The firing of tube V1 raises its cathode potential and capacitor C2 is charged sufficiently to cause V3 to fire. However, if ringing tone is being transmitted, the output of W5 prevents V3 firing. Thus if V3 does fire there is a disconnection in the called line or in circuit 3 and the resultant rise in the cathode potential of V3 may operate an alarm or fault recording apparatus (not shown). Part of the output from converter 4 is rectified by W11 to provide an anode supply for tubes V4 to V7, and the rectified voltage from the circuit connected to T6 is applied to the grid of V4. V4 fires, followed by V5, which opens a gate W6, W7, thus connecting ringing control generator RC2 to circuit 2, and charges C9 to fire V6. The drop in its anode potential extinguishes V5 and gate W6, W7 is cloesd. The firing of V6 opens a second gate W8, W9, thus connecting RC3 to circuit 2. This generator which produces an output interrupted at the usual ringing rhythm controls tube V2 so that the ringing current is interrupted in like manner. On the reply of the called party the potential of private wire P falls to - 40 V. and the output from the circuit connected to T6 becomes steady. This allows C10 to become fully charged so that V7 strikes extinguishing V4, V5 and V6 (if fired) so that both ringing control gates are closed and V2 no longer allows ringing current to be transmitted.