Patent GB-869373-A (GB)
Title: Improvements in or relating to electrical signalling systems
Inventor: CLARK THOMAS HAROLD
IPC: H04M15/06
Filed: 1958-05-12
Abstract: 869,373. Automatic exchange systems. BRITISH TELECOMMUNICATIONS RESEARCH Ltd. April 24, 1959 [May 12, 1958], No. 15203/58. Class 40 (4). The difference between metering pulses of one duration and a calling line identification signal of another duration is detected by a magnetic drum register which tests for the signal periodically and keeps a score of the number of successive tests which indicate the signal present or absent, the score for signal present not being abandoned unless the score for signal absent exceeds a given number. To be recorded a metering pulse must last for at least 6 counts and be spaced by at least three counts while calling line identification is initiated for a count of at least 16. A count of over 32 operates an alarm circuit. Interruptions of the signals giving rise to counts of less than 3 are ignored. The system to which the invention is applied is shown in Fig. 1 where a subscriber S seizes a trunk relay set TRS over line circuit LC and group selector GS whereupon a register-translator RD of magnetic drum type, over a scanner SC2, causes operation of contacts HV to revert a positive potential in demand for calling line identification. Detecting this potential over scanner SC1 a metering drum MD, which also carries subscriber's addresses, transfers the calling number and its class of service to RD by way of a buffer store BS. Register translator RD then stores the called subscriber's address, decides on the charging rate, and times the call. At the end of the call RD transfers to automatic accounting equipment AA calling and called parties addresses with call duration and meter charge. A control track of the metering drum MD is shown in Fig. 3 and comprises twelve storage elements synchronized with clock pulses TA1 to TA12. The gating circuits of Fig. 4 with bi-stable circuits MAN, MBN, govern the control track. Inputs SL, SL, indicate a 1 or a 0 read out, respectively, while SA and SB indicate a 0 or a 1 written respectively. Elements 1 and 2 form a counter b which with no signal, represented by input PAM, cycles idly. When a signal PAM is detected in the preceding period TA12 counter b is set to zero, a 1 is written in element 3 to mark the track busy and one is added to the counter a in elements 4 to 9. Should the signal PAM cease the counter b starts from zero but if the signal PAM reappears before b counts to 3 the counter a is not disturbed and continues to count during the interruption by virtue of the busy mark in element 3. If the signal PAM is absent long enough for counter b to reach 3 the element three is erased and the counter a is read out. If the count is between 6 and 15 in a, by time TA7 bi-stable circuit MAN has its upper output active and this will indicate a metering pulse. If the count in a is 16 to 31 bi-stable circuit MAN has its lower output active by time TA8 and this indicates a calling line identification signal. If the count has gone as far as 32 then MAN has its lower output active in time TA9 and this produces an alarm condition. So long as elements 8 and 9 are recording 0 and a count between 6 and 15 occurs, 1 is written in element 10 or, if it already carries 1, this is erased and 1 is written in element 11. The record on elements 10 and 11 causes the addition of one or two units to the drum track registering the meter count which registration may be by means of a binary count or by a binary count over initial elements followed by elements having equal values.