Patent GB-1501704-A (GB)
Title: Telephone installations and telephone sets
IPC: H04M3/58
Filed: 1975-04-09
Abstract: 1501704 Automatic exchange systems TELEFONAKTIEBOLAGET L M ERICSSON 9 April 1975 [11 April 1974] 14709/75 Heading H4K Connections between subscriber stations of a small exchange are completed using a common line pair HL situated in a central unit CVE such that only one pair of subscribers may communicate at a time and the stations are given access to a main exchange line via a further common line NL. Visual display means are provided at each station whereby a first lamp signal is given at each station when the external line is busy and a second lamp signal is given when the external line is placed on hold. Internal stations are selected by depressing a button H (Figs. 2 and 3, not shown) a number of times equal to the number of a required station, a register KR, detailed in Fig. 6 (not shown), being used to count the number of loop breaks to select the called station. In the following summary of operation, figures and references in brackets are not shown. Basic operation.-When a subscriber Tstn goes off-hook the signalling line 3n, 4n is looped via a resistor (R22, Fig. 3) such that a loop current flows which is insufficient to operate an external line relay (LR) in the corresponding line circuit TShn. For an external call the subscriber depresses a first button (N) to loop the line 3n, 4n via a smaller resistor (R21) such that the external line relay operates and is held via one of its own contacts. A lock-out relay (NBR, Fig. 5) comes up in a linehold circuit OVC to prevent the line relays of any other line circuit from operating and a power source in the hold circuit supplies power to a lamp (L) at each station via a conductor LS and wires 3n, 4n. The operated line relay couples the station's speech wires In, 2n to the external line NL and the station proceeds to dial an external subscriber in known manner. An incoming external call rings a common bell and any of the stations can accept the call by going off-hook and depressing button (N). For an internal call a button H is depressed, which causes the signalling loop to be broken for a period of time which is limited by means of a circuit comprising a diode bridge, a capacitor, a thyristor and a transistor (GLI, C20, T1, T6, Fig. 3), the diode bridge being blocked during a period in which capacitor (C20) is charging and which conducts to short out the button (H) when the capacitor voltage reaches a predetermined voltage. The loop-disconnect pulse so produced is passed via the line circuit TSLn to the register KR where it steps a counter. Further depressions of the button H likewise cause the counter to step, the total number of such depressions indicating the required subscriber. The counter stages are coupled via gating circuits and wires BRSn to respective ring relays in the line circuits TSLn, such that after a predetermined period allowed for signalling the counter stage corresponding to the number keyed causes the operation of the respective ring relay to supply an internal ringing signal to the appropriate station. Call hold and transfer.-Depression of button (N) during an external call operates a Hold-relay to place the external line on Hold. The lamps at each station are supplied with a 1 Hz winking supply and the internal subscriber can then make an internal call. Either of the internal subscribers on the interim call can then pick up the external line by depressing button (N). Faults.-A voltage supervision relay SBR connects one of the stations to the external line if the local power supply fails. Printed ciruit cards are used for mounting the various circuits, which utilize discrete components such as Zener diodes, capacitors, transistors, thyristors and resistors.