Patent GB-1044114-A (GB)
Title: Improvements in and relating to devices for use in an automatic signalling system, particularly a telephone system
IPC: H04M1/515
Filed: 1965-01-22
Abstract: 1,044,114. Automatic exchange equipment. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. Jan. 22, 1965 [Jan. 25, 1964], No. 2918/65. Heading H4K. Digital information signalled by the insertion of resistances and diodes into a two-wire loop line is received in a circuit in which the measurement of the level of the line current is delayed until the line current transients have subsided. In the Figure the circuit shown at A shows a subseriber subset in which one of resistances 16, 17 and one of resistances 18, 19 or the short circuit 20 are inserted in the loop at each depression of a dialling push-button. The receiving circuit shown at B is connected to A via the lines L 1 , L 2  and sequentially applies to the loop, from the central battery, voltages of opposite polarity. The potentials developed across resistances 27 and 28 for one direction of current flow and across resistance 36 for the other charge capacitors 65 and 66 respectively to a value proportional to the line current. The capacitors are then simultaneously discharged through the potential dividers comprising seriesconnected resistors 67, 68, 69 and 70, 71, 72, 73, at the junctions of which are monostable circuits 76, 77, 78 and 74, 75. The number of such circuits triggered depends on the capacitor voltage and hence the resistance inserted in the loop, the outputs of the circuits are connected to driving stages 84-89 connected to the register (not shown). The initial depression of a push-button by removing the microphone and inserting the resistances decreases the line current and by so doing biases transistor 37 to turn it off, so allowing transistors 39 and 48 to conduct. Transistor 48 unblocks transistors 52 and 58 but as yet only transistor 52 comes on to operate relay U. By the now opened contact U, the charge already accumulated on capacitor 65 due to the potential across resistors 27, 28 is isolated. Any line current transients induce an output in winding 105 of the transformer 31 to turn off transistor 107 of the monostable pair 107, 109, the conducting transistor 109 then inhibits the operation of relays U and V. The charge on capacitor 65 is therefore not isolated until the transients have subsided. When relay U does operate contacts U 2  and U 1  effect the reversal of the line voltage and capacitor 66 now charges. If relay V is not inhibited it too comes on due to the now closed contact U 4 , transistor 37 still being held off by contact U 3 . The operation of relay V removes capacitors 66 from the charging circuit and discharges both capacitors through the potential dividers. The charging time of the capacitors, which is made small by the small RC time constant, is the rise times of relays U and V. Relay V is held on whilst relay U de-energizes and is released when the push-button is released after a short delay provided by transistor 109 conducting due to subsequent line transients and holding on relay V by make contact V 4 . The holding period avoids charge reaccumulated on capacitor 65 on the close of contact V 1  from being transferred to resistors 67 to 69 due to contact bounce. If the circuit receives digital information from an impulse dialler the line current at each line interruption has a small value which charges the capacitors 65, 66 to a voltage insufficient to operate any of the monostable circuits 75-78. None of the transistors 89 in the driver stages 84-88 conducts and the capacitor 101 is therefore not discharged to earth but through the relay I which accordingly operates for each dialling impulse. Relay W releases the register after a predetermined time.