Patent GB-845791-A (GB)
Title: Improvements in or relating to automatic and semi-automatic telephone systems
Inventor: GALVIN JOHN LESLIE
IPC: H04Q1/446
Filed: 1958-01-30
Abstract: 845,791. Automatic - exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd. Jan. 14, 1959 [Jan. 30, 1958], No. 3062/58. Class 40 (4). In a system employing alternating current signals of a single frequency for the setting- up, control, and supervision of calls over both-way junctions terminated at each end by a similar both-way relay set, an idle junction may be taken out of service (e.g. for maintenance) by a condition applied to either relay set, the application of such a condition to one of the relay sets resulting in the application of a busying condition to its local access path, in the transmission of a continuous single-frequency signal over the junction, and in the setting of means within said one relay set to enable said one relay set to respond to a singlefrequency signal of a duration indicative of a release guard condition eventually to be received over the junction, while the reception of the continuous signal by the other relay set effects the busying of its local access path and conditions said other relay set for the eventual transmission backwardly over the junction of the release-guard signal to the exclusion of any other signals. The drawing shows a both-way relay set at one end of a junction LK. Incoming call.-A seize signal, consisting of voice-frequency alternating current lasting about 40-60 ms., received over junction LK is detected by receiver RCVR which causes the operation in sequence of relays X (not shown), A, B, and BR, and the commencement of the slow operation of relay BG. Relay BR disconnects relay AK from the P-wire of the incoming selector IC and removes idle marking battery from the P-wire of the preceding selector multiple PSM. When the seize signal ends, relays X and A release, bringing up relays EA and SP, which switches junction LK from static modulator SM to incoming selector IC via transformer TR. The release of relay A also loops the speech wires to the incoming selector IC, which is thereby seized and applies earth to its P-wire. Relay BG now completes its delayed operation. Call-routing digits, consisting of trains of V.F. impulses, are received over junction LK. The first impulse re-operates relays X and A, releasing relay SP, breaking the loop to incoming selector IC, and bringing up slow-releasing relay CD which prevents re-operation of relay SP during impulsing and provides a low-resistance loop for impulsing incoming selector IC. Thus the routing digits are repeated by relay A to incoming selector IC. Between digits, relay CD releases and relay SP is energized so that supervisory tones may be returned over junction LK. When the called subscriber answers, the reversal of potentials from incoming selector IC operates relay D which, by means, not shown, brings up relay SX so that the answer signal is sent back over junction LK. At the end of the call, the both-way relay set at the originating end of the junction sends the clear forward signal, and when this ceases the both-way relay set at the remote end of the junction sends the release-guard signal, enabling the relay sets to restore to the idle condition. This releasing operation is described in detail below. Junction to be taken out of service backward busying signal. This is effected either by applying earth to the P-wire of the incoming selector IC or by inserting a U-link in the test jacks TJ. Relay AK operates, followed by relays BR (which disconnects the idle marking battery from the P-wire of the preceding selector multiple PSM), RG, SG, and SX. The last-named relay applies battery to static modulator SM which causes the single-frequency V.F. signal on leads VFS to be applied to junction LK. Relay SX also prevents operation of relay A when relay X responds to the V.F. signal. This signal is maintained continuously as long as junction LK is required to be kept out of service. To end the busying signal, the earth connected to relay AK, either from the P-wire of incoming selector IC or via test jacks TJ, is removed so that relay AK releases, connecting earth to release-guard alarm lead 10. Thermal relay TH operates after a delay of 6-8 sees., breaking its own circuit so that it releases after 25-30 sees. and releasing relay SX to remove the V.F. signal from junction LK. The remote exchange returns a release-guard signal (as described below), bringing up relays X, A, and VF so that relays SG and RG fall back, disconnecting earth from release-guard alarm lead 10. When the release-guard signal ceases, relays X, A, VF, and BR fall back, and idle marking battery is restored on the P-wire of the preceding selector multiple PSM. If the remote exchange fails to return the release-guard signal, relays X, A, and VF do not operate, so that when thermal relay TH completes its delayed release it brings up relay SX again so that a clearing signal is transmitted during the 6-8 sees. taken by TH to re-operate and thereby release SX. Effect at remote exchange of busying signal received over junction LK.-Relays X, A, B, BR, and BG pull up as on the reception of the seize signal described above. But in this case the signal persists, so that the operation of relay BG brings up relay CD to release relay B, breaking the path to incoming selector IC and energizing relay EA which holds BG operated. When the busying signal ceases, relays X, A, and CD fall back, bringing up relay SX to send a V.F. (release-guard) signal over junction LK. Relays EA and BG release slowly in turn, ending the release-guard signal by the release of relay SX and also releasing relay BR to restore idle marking battery to the P-wire of the preceding selector multiple PSM. Specification 709,399 is referred to.