Patent GB-1157761-A (GB)
Title: Improvements in or relating to Telephone Signalling Systems
Inventor: WILLIAMS PHILIP JOHN
IPC: H04Q1/453
Filed: 1967-01-05
Abstract: 1,157,761. Automatic exchange systems. GENERAL ELECTRIC & ENGLISH ELECTRIC CO's. Ltd. 2 Feb., 1968 [5 Jan., 1967], No. 698/67. Heading H4K. In a fully compelled signalling system employed between exchange registers a receiver on cessation of a received signal inhibits itself from detecting any received signal for a period which is chosen in relation to the transmitter and receiver response times and the transmission path propagation time so as to reduce errors produced by splits in the received signal. The invention can be applied to an acknowledging type compelled signalling system but the system specifically described is of the demand type in which a demand signal is transmitted from a transit register back to an originating register which responds by forwarding a digit in 2-out-of- 6 multi-frequency code, the backward demand signal being removed after receipt of the digit which continues to be sent until cessation of the demand signal is detected, a fresh demand signal being sent on detection of the end of the digit. As shown in Fig. 2 the receivers employed in the system comprise receiving equipment 4 the output of which is tested by a 2-outof-6 gate 8, output of any kind producing output from OR gate 9. Given a valid code a gate 10 is opened to start a make timer 11 to check the persistence of the signal before it is allowed to be registered on a set of bi-stables 6, a gate 12 being opened when the timer matures. As soon as a code is registered, and supposing it satisfies the 2-out-of-6 gate 16, the bi-stables 6 are disconnected from the received signals by inhibition of gate 12. The code on bi-stables 6 also opens a gate 17 to prime a gate 13 which, when the received signal ceases, opens to start a break timer 14 which tests the persistence of the no signal condition. When timer 14 times out the break, an inhibit timer 15 is started up and by inhibition of the gate 10 prevents response to any received code for a fixed period. Transient signals, which do not persist beyond the time required by make timer 11, since they cannot result in a registration on bi-stables 6 with consequent activation of gate 17, do not start the break timer and produce no subsequent inhibition. Instead of inhibiting the gate 10 the inhibit timer may directly disable the receiver 4. Complete protection against false interpretation of received signals subject to single splits in either or both directions of transmission can be obtained by additionally employing a demand signal length timer so that demand signals made to appear plural to an originating register due to splits in the backward transmission from a transit register, which splits also prevent transitions from one signal to another from the originating register being detected at the transit register, are recognized as demand signals of unreasonable length. If the demand signal timer matures it brings in a restart timer so that after a period beyond which the originating register may be presumed to have exhausted possible responses, the whole signal sequence may be re-initiated and whatever digits may have been accumulated are discarded. The response times, between detection of the presence and absence of a signal and the consequent sending of or ceasing to send a reply signal, are comprised of the response or decay times of filters in the receiver, of the persistence checks applied by make and break timers, and the time taken by the registers once called upon to act to provide the required response. Taking into account these time constants of response as well as the propagation time between originating and terminating registers, consideration is given to the possible effects of splits in either direction or simultaneously in both directions of transmission (Figs. 3 to 17, not shown). It is demonstrated that, at the expense of prolonging the signalling process, errors of reception from any one line split can be eliminated. Additionally the amplitudes of each of the two frequencies of the multi-frequency code may be compared and the signal be rejected if they are of widely different amplitudes. With long propagation times between registers the restart procedure may be called into operation excessively and a discussion is given of setting up break, inhibit, and demand signal length timers, in order to produce a workable compromise.