Patent GB-1518537-A (GB)
Title: Line circuit for telecommunications exchange using tdm
IPC: H04Q11/04
Filed: 1975-10-14
Abstract: 1518537 Line circuits, key telephones INTERNATIONAL STANDARD ELECTRIC CORP 14 Oct 1975 [21 Oct 1974] 41985/75 Heading H4K A line circuit suitable for use in a key telephone system includes a hold latch adapted to be set in response to a hold signal from any one of a plurality of stations connected to the line and to signal the hold condition to all said stations, and a monostable circuit adjusted to be triggered so as to set a second latch, adapted to store a calling signal for transmission to the stations connected to the line, in response to an incoming call. The figure shows part of the line circuit which is designed as a key telephone line circuit for use in a time division exchange of the type described in Specification 1,455,799 but which can also be used as a line circuit for a conventional telephone. When a keystation not detailed initiates a call it initially earths the A or hold wire then loops the speech pair. For use with a conventional two wire telephone the A wire can be strapped to earth as indicated at 301. Operation of the system is generally along the lines set out in specification 1,455,799. Closure of the line relay make contacts 102 upon looping of the line applies earth to one input of a NAND gate 402 and via gates 306 and 324 to free/busy and onhook/off-hook gates 332 and 328 respectively. These gates 332 and 328 each have a gating input which is enabled during the control time slot (see Specification 1,455,799) allocated to that line circuit, one time slot in each frame being allocated for scanning free lines (one per frame) for calling condition. When a calling line is detected by the exchange processor a free register is allocated and dial tone signals in the register time slot are demodulated and fed to the caller, the exchange being equipped to receive pulse and multi-frequency dialling signals. If when dialling is complete the called line circuit is free, the calling and called line data is transferred to a local link for processing the call, the local link periodically enabling speech gates connecting the line circuit to PAM speech highways in TDM fashion. In the originating line circuit the earth input to NAND gate 402 turns on LED lamp 404 and turns on a transistor Q 109 which turns on the line lamp at every station having access to that line. In the terminating line circuit the called line is tested over the free/busy output F/B of gate 332. If the line is idle leads TLST, TLSO are activated enabling gate 422 to transmit during the allocated-register time slot a pulse via a gate 426 to trigger a monostable 350. The monostable remains triggered for more than a control time frame, is periodically retriggered throughout the call and restores itself after the end thereof. The triggered monostable enables the busy/free gate 332 and sets a calling signal latch 450. Ringing relay RR responds completing the ringing path and connecting a ring trip circuit 460 to one wire of the line. An output 346 of the monostable enables via a gate 452, a gate 462 which couples lamp flash signals LF to the lamp 404 and via transistor Q 109 to the line lamps of the stations connected thereto. When the called station answers transistors Q 108 and Q 109 respond providing via gate 402 steady lamp signals and resetting the latch 450 to terminate ringing. If the called line is unavailable, the processor sets a latch 508 enabling a transistor gating unobtainable tone from a source ROT to the caller, while if the called line is on lockout a latch 506 is set to transmit a barred tone BT to the caller. This latch also enables the line circuit lamp 404 to flash in inverted fashion. Holding.-To put a line on hold the station connected to the line presses a hold button which removes the ground potential from the A lead allowing a transistor Q 111 to release. Gate 532 responds triggering the hold latch 308. The output of the hold latch maintains the busy signal output of gate 332 to the processor and via gate 544 applies lamp wink signals to lamp 404 and transistor Q 109 controlling the station line lamps.