GB842057A - Improvements in or relating to two party-line telephone circuits - Google Patents

Abstract

842,057. Sub-station circuits. NOMURA, T. March 18, 1958 [March 20, 1957], No. 8706/58. Class 40 (4). Subscribers on a party line are locked out when one subscriber is using the line due to the discharge of capacitors in the other subscribers' circuits, the charges on which are necessary to operate latching relays before a connection can be obtained to the line. As shown in Fig. 1, a capacitor 20 is charged over high resistance 21 and when cradle contacts 13 and 14 operate the capacitor discharges through relay 22 to operate the contacts 23 and 26 which latch up mechanically and connect the subscriber to the line. Once connected to the line, the low impedance of the calling subscriber's loop causes the other capacitor, or capacitors, of the other subscribers on the line to discharge. Other subscribers therefore on raising their handsets operate the cradle switch contacts 13, 14, but cannot operate the relay 22. When the calling subscriber dials the break impulses are insufficiently long to allow another subscriber to break in since the capacitors 20 must charge over high resistances 21. To release the connection cradle switch contact 14 connects up magnet 25 when a subscriber hangs up and releases the mechanical latch on the contacts 23, 26 of relay 22. To call a party on the same line a subscriber dials the wanted number of the party line subscriber and operates a switch to operate contacts 27, 28, to disconnect the bell set and to break the calling subscriber's loop over a capacitor 29 so that he can listen in and restore contacts 27, 28, when he hears the called party reply. Instead of charging capacitor 20 in series with resistor 21 in shunt with transmitter 7 it may be charged in series and by shunting resistor 21 with contacts of relay 22 the charge taken up by capacitor 20 as contacts 14 fall back may be used to operate magnet 25 in the release circuit, see Fig. 2, not shown. Contacts of the relay 22 may be provided to short-circuit the receiver of a sub-station when idle. The relay 22 and its release magnet 25 together with the switch with contacts 27, 28 may form a unit connected to the remainder of the sub-station by means of a four-wire cable, Fig. 3 (not shown).