Patent GB-1486943-A (GB)
Title: Telephone exchange metering system
IPC: H04M15/34
Filed: 1974-09-24
Abstract: 1486943 Metering VIDAR CORP 24 Sept 1974 [10 Oct 1973] 41520/74 Heading H4K An arrangement for metering trunk calls, i.e. calls to the public network, in a PABX includes a store for each outgoing trunk of the exchange for storing the calling and called subscribers numbers, a further location of the store being used to measure the call duration by accumulating therein 1 second timing pulses throughout the call. Fig. 1 shows the general arrangement of the system as applied to a PABX, 6. Internal and external calls are assumed to be processed in known manner and all the equipment shown to the right of the exchange 6 is only concerned with metering the external calls, and in no way affects the normal operation of the PABX. General operation.-A control processor 9 scans the exchange's 150 trunk circuits by cyclically supplying BCD trunk addresses TA over bus 28 to trunk interface 10 (Fig. 3, not shown), which includes for each trunk circuit, a unit, comprising differential amplifiers, transistors, a diode and a bi-stable circuit, which is coupled to the R, T and S leads of the trunk and to a reference voltage VREF supplied over wire 44 and circuit 45 from the exchange power source. If the trunk has been seized for an outgoing call the bi-stable circuit is set and when the corresponding unit of interface 10 is addressed a busy indication SBZY is gated to the processor over wire 39 every time the unit is scanned. Having detected a newly seized trunk on a last look basis, the processor 9 supplies a line search command LSRCH to the line interface storage buffer 8 when the trunk is next scanned to cause an identification signal ID to be gated to the trunk sleeve lead via wire 27 and the trunk interface 10. A line interface encoder of any known tree circuit form receives the ID signal on one of the 1800 subscriber sleeve leads and provides a line identification in BCD form to the line interface storage buffer 8 (Fig. 2, not shown), for subsequent transfer via processor 9 to a store in memory 14 associated with the seized trunk circuit. The processor then assigns both a dial pulse receiver (Fig. 6) and a multifrequency tone receiver (not described) to the trunk and subsequently supplies the address of the trunk circuit over bus 32 to multiplexor 11 and the addresses of the receivers over bus 36 to the receiver block 13 when the corresponding trunk is scanned. The multiplexor 11 (Fig. 4, not shown) which includes circuitry comprising differential amplifiers and FET's for distinguishing between different analogue signals supplied to it over 150 analogue lines extending from the trunk units of interface 10, gates dialling information DP to bus 37 and hence to the receivers, one of which is dropped after the first digit has been detected. Subsequent dialling information is transferred from the outgoing trunk to the receiver each time the trunk is scanned and once the whole number is received and has been transferred via the processor 9 to the store of the associated trunk the receiver is dropped. An answer supervision receiver (Fig. 7, not shown) or a ring back tone receiver (Fig. 8) is then allocated in dependence on whether exchange 6 is of the type which has answer supervision or not. If answer supervision is used, the answer supervision signal is supplied by the multiplexor 11 to wire Ans which extends to an allocated receiver when both devices are addressed over buses 32 and 36 respectively. Where ring back tone is used, its presence is indicated over wire 34 which extends, via a gating arrangement of FET's 12 (Fig. 5, not shown) addressed with receiver addresses over bus 36<SP>1</SP>, to the allocated ring back tone receiver. When an answer supervision signal is detected or ring back tone ceases, the processor starts a count by incrementing the contents of a storage location associated with the trunk using the circuit of Fig. 11 (not shown), the count being incremented every second. When the call ends the busy signal on wire 39 does not appear for the particular trunk. The count is stopped and the contents of the trunk store i.e. the calling and called subscribers' numbers and the call duration are supplied to input/output circuitry 16. Further figs. 9, 10, 12-14, 16 and 17 (not shown) illustrate various logic circuits comprising the processor 9. The dial pulse receiver, Fig. 6.-The counter 185 counts valid dialling pulses for each digit and the contents of the counter are read out to the processor when the inter digit timer 186 indicates an interdigital pause. The receiver is initially seized by the processor over wire AS to set bi-stable circuit 182 which enables the gate 181 after a delay of 191 milliseconds to enable signals received on wire DP to be fed to the timer 192. The receiver functions to receive signals each time it is addressed over bus 36 by the central processor. Ring back tone receiver, Fig. 8.-The receiver not only indicates that ring back tone is present but it also provides a signal to the processor if busy tone is received. A received signal sample is passed through filters 204-206, a detector 207 and an integrator 208, whose output is connected directly to a first threshold detector 213 and via an amplifier 209, a detector 210 and integrator 211 to a second threshold detector 212. The threshold detector outputs are NOR'ed to a measuring circuit 215 which gives an output on wire 40-1 of bus 40 to indicate whether ring back tone is present or not. The output of threshold detector 213 is coupled to a line busy measuring circuit 216 to give a trunk busy indication on wire 40-3 and a called line busy indication on wire 40-2.