Patent GB-1415762-A (GB)
Title: Multiple stage telecommunications switching network
IPC: H04Q3/52
Filed: 1973-06-14
Abstract: 1415762 Automatic exchange systems INTERNATIONAL STANDARD ELECTRIC CORP 14 June 1973 [20 June 1972] 28287/73 Heading H4K In a multi-stage switching network in which each stage comprises at least one matrix, end markings on the network cause a crosspoint in the first stage associated with the marked input to become conducting for the duration of a scanning interval, during which interval crosspoints in the second stage are scanned successively to find the link emanating from the operated crosspoint in the first stage thereby enabling completion of a path from the marked input to an output of the second stage. Fig. 1 shows a 100 line PABX system with primary (P), secondary (S), tertiary (T) and quaternary (Q) stages. The P and S stages are grouped, each PS group serving the line circuits for 25 lines. Similarly the T and Q stages are grouped, each of two TQ groups being connected to 27 junctors or trunks. Each P group contains five 5 Î 5 matrices Fig. 3, while each S group contains five 5 Î 6 matrices Fig. 4. Corresponding outputs (SH) from PS groups 1 and 2 are paired together with half of the resulting 30 links extending to TQ Group 1, and the other half to TQ Group 2. Similar output pairing is adopted for the groups 3 and 4. Each T group contains five 6 Î 6 matrices Fig. 5 (not shown), while each Q group contains three 10 Î 9 matrices Fig. 6 (not shown). For each TQ group, half the T outputs (TV) are connected to respective Q inputs (QV) of the same group while the other half are connected to respective Q inputs of the other TQ group. Each matrix may be built up from 3 Î 6 IC unit matrices wherein each crosspoint element is a thyristor. The links leaving the P, S and T stages are permanently biased at a voltage level below the negative reference level of one terminal of a crosspoint thyristor. Hence to fire a crosspoint two conditions are necessary: (a) a marking voltage on the appropriate input greater than a pre-set level; (b) a trigger voltage applied to the thyristor gate. Each gate has an enabling lead, and these leads are connected to primary, secondary and tertiary control leads -Figs. 3, 4 and 5 (not shown). In the quaternary stage each crosspoint gate terminal is permanently enabled by being grounded and hence input and output (QV, QH) threshold-exceeding marking voltages are necessary to fire a crosspoint. When the network is end-marked (PH, QH) the P and S controls, Figs. 7 and 8 (not shown) and T control, Figs. 9 and 10 (not shown) are actuated. The PC leads-Fig. 3-are enabled in sequence, each remaining enabled for one scanning interval. During each scanning interval all the SC leads are enabled, in 6 successive sub-intervals. Hence if a free primary crosspoint is available for a marked PH inlet the crosspoint will fire at the beginning of a scanning interval, thereby extending a marking voltage to a n SV inlet. During the same interval a secondary crosspoint having access to the marked SV inlet will fire, completing a path to the T stage. The output QH mark is sent, via the T control, to enable simultaneously all T crosspoints having access to the marked QH output. Providing there is a T crosspoint having access both to the marked T input (TH) and Q output, it will fire, thereby providing a marking voltage for a Q input (QV). Hence a Q crosspoint is fired and a path through the network is completed.  Each fired P crosspoint is held by a circuit HC-Fig. 4-only for the duration of a scanning interval. If no path has been found through the network at the end of the fifth scanning interval the scanning process is repeated and continues until the QH marking is removed. At this point the P, S and T controls are re-set to their original positions. An alarm signal is produced in the form of a lamp indication in two circumstances. Firstly, if the counters (130, 132) which control the scanning interval and sub-intervals fail to operate when the PS control receives a start signal intitiated by the end markings. Secondly, if the hold circuit fails to release a fired P crosspoint at the end of a scanning interval during which no path has been completed to an output of the S stage.