Patent GB-940034-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: ALLUM ROBIN DEVENISH
IPC: H04M15/06
Filed: 1960-11-17
Abstract: 940,034. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd., and STANDARD TELEPHONES & CABLES Ltd. Oct. 30, 1961 [Nov. 17, 1960], No. 39498/60. Heading H4K. [Also in Division G4] Individual stores on a magnetic drum record individual subscriber's meter totals and when a metering pulse identifying a particular subscriber is detected his total is increased by one, the stores being arranged in groups of tracks and corresponding tracks in each group being scanned simultaneously with the other groups and sequentially through the group, each track being provided with a test meter store, the total in which is increased by one at each scan and is compared with the total in the corresponding test store in another group. Since a discrepancy detected in a comparison of test totals in two groups does not locate the fault in one group or the other, comparisons are made between each group and two others. Thus a fault involving groups 1 and 3 and groups 3 and 4 locates the fault in group 3. Each register track of the magnetic drum serves 100 lines or junctions and the tracks are arranged in groups of eight which are scanned sequentially to serve corresponding groups of 800 lines or junctions. Each group of eight register tracks has a carrier track and a meter check track in common. As shown in Fig. 3, the write head of a meter track is placed the length of two meter stores ahead of the read head and the read head of the carrier track is placed two meter stores ahead of the write head. If a metering instruction is received it is synchronized with the appropriate metering total store by means of a library track which identifies the line or junction involved. The total existing in the store is read out over the track switching circuit and meter track read circuits to be written by way of carrier track write circuit on to the carrier track. After a delay of 98 meter storage lengths the carrier track record of the total is passed from the carrier track read circuit to an add 1 circuit enabled by the metering instruction and from which the increased total is written back on to the meter track. in the same position as the original total. At the same time as the increased total is written on to the meter track the original total from the carrier track is written on to a meter check track or this total is increased by one in a second add 1 circuit. Two store lengths later both the meter track total and the meter check track total are read out to a meter path checking circuit which will detect any discrepancy to signal a fault. A further test is carried out by means of the test store set aside in each track for simulating the reception of a metering instruction during each scan. The totals accumulated in the test stores of the tracks tested simultaneously in each of the groups should therefore keep in step and by comparing the totals read from the groups taken in pairs by means of the circuit labelled Compare Test Meters discrepancies can be detected. Totals proving to comprise all 1's and 0's are possibly due to fault and are signalled by a detector in the meter check track read circuit. The presence of a metering instruction for more than the probable number of consecutively metered stores is detected direct from the instruction lead as being a further possible fault. A fault detector for the track switching circuit looks for write current in the tracks not involved with scanning in a group. When a fault occurs the meter track write circuit of the faulty group is disconnected and replaced by spare equipment while all the test stores are reset to zero. Since to scan and test a store two revolutions of the drum are necessary, and since read circuit amplifiers need to be connected up a half-revolution before output is needed from them, each group of tracks has four meter track read circuits, each of which is shared by two tracks;, see Figs. 5 and 6, not shown. A detailed description of the circuits shown schematically in Fig. 3 is given using circuit script. The magnetic drum may be of the kind described in Specification 717,687 and the invention may be applied to an exchange as described in Specification 940,016.