Patent GB-997343-A (GB)
Title: Improvements relating to validity checking arrangements adapted to perform validity checks in respect of code signal conditions and to the use of such arrangements in telephone systems
Inventor: DRAKE BERNARD
IPC: H04M15/08
Filed: 1962-02-05
Abstract: 997,343. Automatic exchange systems. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Jan. 21, 1963 [Feb. 5, 1962], No. 4356/62. Heading H4K. In an arrangement for checking the validity of a signalling condition existing on a certain number out of a larger fixed number of code signalling wires, the said wires are combined to provide a check voltage indicative of the signalling condition, and this is compared with two reference voltages, the condition being a valid one if the check voltage falls between them. The arrangement is applied to the checking of signals representing the identity of a calling line in an STD system having centralized accounting equipment. The register sender RS includes three stores NS, CS and CKOS controlled by an identifier CI. The store NS comprises 21 relays such as SR, one identifying the exchange and four sets of five recording the calling number in a 2-out-of-5 code. The store CS records the class of service of a special rate subscriber on two sets of five relays, while the store CKOS records if the calling subscriber is not identifiable for any reason, e.g. a call from a multi-party line. The output wires 1 . . . 31 from the stores NS, CS are connected through rectifiers MR1 and similar resistors R1 to a common point, and the output wire 32 from the store CKOS is connected thereto through a rectifier MR32 and a smaller resistor R32 (=R1/9). For validity checking the common point is connected to the emitter of transistor VT1 of which the circuits are such that a check voltage of - 13À6 volts should be obtained at its collector. Reference voltages of - 12-9 and - 14-S volts are provided at the base of transistor VT2 and at the emitter of transistor VT3, and, if the check voltage falls outside these limits, one of these transistors conducts and disables transistor VT4 so that relay RFC fails to energize. A first check is made for the case of a special rate subscriber, when earth should be present on 13 of the wires 1 ... 31 but not on wire 32. If the check is satisfactory relay RFC energizes and the sequence control circuit SCC starts the transmission of the stored digits to the toll centre. But if RFC fails to energize, SCC operates relay SA which inserts resistor R42 in the collector circuit of VT1, and a second check is made for the case of an ordinary subscriber, when earth should be present on 9 of the wires 1 ... 31 or on wire 32. On the energization of RFC, the stored digits are transmitted, or, if only the store CKOS is operated, a signal is sent to route the call to a manual board. If, however, RFC still fails to energize, SCC resets the stores and a fresh identifying operation takes place. The digits are transmitted to the toll centre in a 2-out-of-6 frequency code, and the validity of each digit code is checked by the connection of wires 33 ... 36 through suitable resistors R36 to transistor VT1, relay SB being operative. During this check, transistor VT5 is normally conducting, and transistor VT6 conducts in response to negative voltage peaks corresponding to at least two signalling frequencies appearing on wire 39, thus periodically disabling VT5 in order to allow the operation of relay RFC if the check is satisfactory.