Patent GB-1116735-A (GB)
Title: Improvements in or relating to telephone trunk circuits
IPC: H04Q11/04
Filed: 1965-11-25
Abstract: 1,116,735. Automatic exchange systems- WESTERN ELECTRIC CO. Inc. 25 Nov., 1965 [25 Nov., 1964], No. 50132/65. Heading H4K] A trunk circuit has a multi-state register which can be stepped from one state to another by signals from a control unit in the local exchange and also by signals from the distant exchange to which the trunk circuit gives access; the states of the register determining the functional modes of the trunk circuit. Used in a PBX system as described in Specification 1,045,521, with TDM switching as described in Specification 1,029,204, the trunk circuit has a cut-through relay (CT), a dial tone relay (DT), and an M-lead signalling relay (M); these relays being selectively operable by the combined binary condition of three bi-stable circuits. Tip, ring, and E and M supervisory leads extend to the distant PBX, signalling from the distant PBX being repeated in the trunk by a relay (E). The trunk circuit is controlled locally by a common control unit which is instructed, on the basis of loop conditions detected in the switching unit, to produce seize pulses on a lead (S) and release pulses on a lead (R). The trunk is supervised, therefore, for all purposes including outpulsing, by the signals on leads (S), (R) and (E). Incoming call.-The (E) relay is brought up by the distant PBX over the E wire and presents a loop in the trunk circuit for detection by the switching unit which instructs the control unit to apply a seize pulse to the trunk on lead (S). The seize pulse moves the three bistable circuits in the trunk from their normal released state, represented by 000, to a state 001 in which dial tone is connected by the trunk by relay (DT). In this condition the trunk does not respond to any further seize pulse unless dialling has begun. Relay (E) follows dial pulses from the distant PBX and on the first break the trunk state goes to 101 to remove dial tone and remains in this state until the digits are fully registered in the control unit, the digits being repeated by way of the trunk and switch unit. Once in receipt of all the digits the control unit pulses the trunk on the seize lead to achieve the state 110 in which the trunk is cut through to give passage to ring-back or busy tone from the switching unit. If the wanted party answers, the control unit pulses the seize lead once more to produce the state 111 in which relays (M) and (CT) are operated, the speech path remaining cut through and potential being applied forward on the M lead. The trunk remains in this state immune to further seize pulses until off-hook conditions appear. Should dialling have been started before the connection of dial tone the trunk moves to state 100 in which dial tone is connected continuously from the switch unit until the distant party goes off hook to release relay (E). Outgoing call.-On demand by the switch unit the control unit applies a seize pulse to the trunk to dislodge it from the released state 000 to the state 111 with relay (E) unenergized. Relay (M) is operated to apply potential to the M lead forward to the distant PBX and the trunk is cut-through by (CT). Operation of (M) triggers a so-called wink circuit which operates relay (DT) for 200 ms. to signal the switch unit that (M) has operated and allow outpulsing to proceed under control of the control unit. Outpulsing proceeds by the successive application of release and seize signals on leads (R) and (S) to switch the circuit between states 111 and 000 and thereby cause (M) to pulse the M-lead accordingly. When the called party replies relay (E) responds to a signal on the E-lead and connects a D.C. loop to the switch unit to indicate reply. Release.-On-hook at the distant PBX causes (E) to release in the trunk and the loop to the switch unit is broken. The control unit delivers a release pulse after 1À5 secs., the delay being necessary to discriminate between release and flashing signals. A release pulse is similarly delivered in response to release from the local PBX but with relay (E) still energized in the trunk its state is changed from 111 to 110 in which (CT) is operated and the trunk is in a condition to detect flashing signals. With subsequent release from the distant PBX, relay (E) falls back and the trunk moves to its rest state without need of pulsing from the control unit.