Patent GB-336802-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04M19/00
Filed: 1929-10-21
Abstract: 336,802. Coventry Automatic Telephones, Ltd., and Wilman, C. W. Oct. 21, 1929. Automatic exchange systems.-The current supply for an automatic telephone system is derived directly from alternating current mains. The speaking current is rectified and smoothed before use but the current for energizing relays and for stepping switches need be neither rectified nor smoothed. Switches can be made to step at each half wave or if the periodicity is high the current may be first passed through a half wave rectifier and the switches stepped at each wave. This unidirectional pulsating current may be used for ringing, or may step a switch which extends only alternate impulses for ringing, or raw alternating current may be used. Switches responding to dialled impulses are prevented from moving more than one division (two steps) at each impulse. A relay chain is provided for producing tone currents and for breaking up the ringing signal. The invention is described as applied to a small exchange with line finders and single motion connectors having access to 20 lines. Current supply. Current supply may be of four types indicated in the circuit diagrams as 1, 2, 3, and 4. The raw alternating current from AC1, AC2 is transformed by T1, T2 and the output from T1 is rectified by S1-S4, which may be metal rectifiers, and supplied to terminal 1. This rectified current is also smoothed bv inductance I and condensers K1, K2 and supplied to terminal 2. The output from T2 is supplied raw to terminal 4 and through a half wave rectifier S5 to terminal 3. If necessary supplies may be duplicated as shown at 3'. The other sides of the output are shown connected to earth. Seizure of calling line. When a call is originated ST energizes over the calling loop in series with LR which pulls up its armature lr1 only. Relay P energizes from supply 3 and completes a circuit from the half wave supply 3 for MF 'to step wipers WF in search of the line marked by absence of earth on WF4. The use of interrupter springs IF is optional. When the calling line is found Q energizes from supply 3, earthing WF3 to enable LR to energize completely and switch through to the connector. Relay A energizes over the loop from unsmoothed supply 1 and B energizes holding Q. P releases extending start wire 31 over a chain circuit to the next line finder. If all line finders are busy when ST energizes, earth at contact st1 is extended to buzzer relav BZ and coil RC. Buzzer relay BZ by shorting itself at bz1 produces a busy signal which is connected over 36 to the caller. Setting of connection. Relay B having operated C pulls up on supply 1 from earth on WS4, slow relay H energizes from supply 3 and slow relay R pulls up. When the first digit (which is either one or two) is dialled relay A falls away and MS energizes from half wave supply 3 to contact a1 stepping the wipers WS on de-energization. Springs IS need not be provided. When A reoperates at the end of the impulse MS reenergizes from a1 over x1 stepping the wipers again and if the digit dialled is two a further two steps are made for the second impulse. To ensure that A remains energized long enough between the impulses for the switch to step, the relay is held via a3 to earth at WS4. Only two steps can thus be made for each impulse irrespective of the number of half-cycles supplied. If the digit dialled is two, relay C, which is held during impulsing over a2, remains energized from WS4 and the second digit moves the wipers two steps for each impulse, as already described, to one of the sets of terminals 21-20. If the first digit had been one, relay C would have released at the end of the train completing a circuit over c4 for X to energize from supply 1. Relay X locks at x5 and at x2 and x1 changes over the connections from al to magnet MS resulting in MS moving the wipers another step to the 3rd set of terminals. The next digit moves the wipers two steps for each impulse as before described leaving the wipers on one of the sets of terminals 11-10. Called line busy. At the end of the second train relay C releases ana if the line is busy earth on WS3 energizes relay G. Relay C at c5 connects earth to relay chain L, M, N and these relays energize in turn so long as the earth connection is maintained. Relay G connects earth to relay BZ and this relay generates a tone signal as already described which is connected to the calling line via 12 each time relay L energizes to indicate a busy condition to the calling party. Called line free; ringing. If the called line is free G does not operate, H falls away and F energizes from supply 3 every time M of the relay chain L, M, N pulls up. Relay F connects raw alternating current from supply 4 to ring the called subscriber, condenser 40 providing the usual tone leak. When the called subscriber replies D energizes from smoothed supply 2 breaking the circuit of F and connecting supply 2 to relay A. Relays A and D form the usual transmission bridge. Breaking in. Certain subscribers are allowed to break in on engaged circuits. The connection from wiper WF5 on these lines is broken and when a busy line is called, relay H releases when C falls away connecting to the called line but as relay F is disconnected at g5 ringing current is not connected. Release. When the calling subscriber releases relays A, B and Q fall away and MS steps the wipers WS to their home position.