Patent GB-733577-A (GB)
Title: Improvements in or relating to busy-tone circuits for use in automatic telephone exchange systems
IPC: B23D61/02
Filed: 1953-05-01
Abstract: 733.577. Automatic exchange systems. PHILIPS ELECTRICAL INDUSTRIES, Ltd. May 1, 1953 [May 5, 1952], No. 12132/53. Class 40 (4). In a system wherein an intelligence connection between two subscribers is established over a discharge tube BA of a calling subscriber AA and the circuit is completed by a marking circuit TS which applies a firing potential to the ignition electrode f2 of a tube BB proper to the wanted subscriber AB, completion of the connection is prevented if the wanted subscriber is busy, and busy tone is sent to the calling subscriber through elements G3, G4 which are controlled jointly by the voltage on the line S between the anodes of the two tubes, and the anode voltage of an auxiliary tube B3 which is fired on the arrival of the first dialled pulse. The lifting of the handset at AA closes a loop LCA including a battery BT to apply a negative pulse at multiple point QA, and fire one of the tubes, say BA, multipled thereto. As a result the potentials of points P2 and QA are lowered and raised respectively to mark busy both the connecting circuit and the calling subscriber. Subsequent dialling pulses, which are made to be of smaller magnitude by shunting the dial with an impedance, are amplified in a valve B0 and applied to a self-extinguishing tube B4 which in firing applies a positive pulse to each of the marking valves B6, B7, B8 ... over condensers C60, C70, C80 ... Tube B6 which has a standing trigger potential owing to its connection to the junction of resistors R33 and R34, fires and in turn fires tube B3 which remains fired throughout the call. B3 in firing reduces the potential on the striking electrode of tube B6 and when tube B7 fires over condenser C70 in response to the next dialling pulse, tube B6 is extinguished by a positive pulse across condenser C71. Successive tubes are prepared for' firing over resistors R71, R81 ... and each tube in firing extinguishes the preceding tube. Marking potentials at M1 and M2 are ineffective during stepping because the tubes are extinguished before the potential across.the appropriate delay network R5, R6, C3, C4 reaches that necessary for striking, but after the final pulse, in this case the third, trigger f2 of tube BB is raised to its striking potential and the tube fires to complete the connection if the subscriber at AB is free. Busy tone is sent to the calling subscriber over condenser C2, rectifier G3 and tube BA when the potential at P1 remains at 110 volts due to BB not firing. Rectifier G3 is blocked when the potential at point B on resistors R3, R4 exceeds the potential at P1 (93 volts) which exists when the two tubes BA and BB are fired. Pulses arising from tube BB firing and the lifting of the wanted subscriber's handset fire both tubes B4 and B5 and restore all the connecting and marking circuit tubes to. normal except tube B3. Replacement of the handsets sends a positive pulse which clears the tubes BA and BB either directly or by strong negative pulses fed back from the anode of tube B2. Tube B3 prevents the marking circuit from responding to digits dialled by a subscriber whilst he is connected to a second subscriber and is extinguished by a negative pulse from the anode of tube B1 at the end of a call.