Patent GB-1480327-A (GB)
Title: Telecommunication switching system
IPC: H04Q3/00
Filed: 1974-09-04
Abstract: 1480327 Automatic exchange systems POST OFFICE 4 Sept 1974 [5 Sept 1973] 41777/73 Heading H4K The crosspoints of a reed relay switching matrix are actuated and held by sets of three bi-stable devices and associated groups of gates which are enabled by selective combinations of the bi-stable outputs. The bi-stable devices and the gates are also used, e.g. for scanning lines, accessing network-link availability and actuating cut-off relays independently of their crosspoint-control usage, by means of control pulses whose duration is less than that required to operate a reed relay. Figs. 1 and 2 show the use of three bi-stables 200-202 and eight AND gates for controlling a block of eight relays R1-R8 in an nÎm matrix. (It is acknowledged that the 000 condition is not useful so that in practice one of the relays would be dispensed with.) Fig. 6 shows part of the command logic for controlling the access of 64 lines to eight trunks. The common control 500 selects one trunk for use by continuously pulsing leads 601, 602 and applying a particular series binary coding to leads P1, P2, P3 i.e. each lead in turn is pulsed 1 or 0. Consequently bi-stables 606-608 adopt this code which is then decoded by a coupling field into gates 612-619. One of eight sets of gates 603, 604, 605 is thus enabled. The required line is then selected by applying another particular series binary coding to leads P4-P9 whereby bi-stables 620-625 adopt the code. These bi-stables are arranged in two sets each having a respective set of AND gates 633-640 and 641-648. The outputs of these two sets of gates are further combined such as to enable a unique gate, e.g. 651, corresponding to that reed relay 652 which gives access between the selected trunk, e.g. 1, and the calling line, e.g. 1. A long duration pulse signal WF1 applied to gate 632 now permits operation of relay 652. The repetition rate of WF1 is high enough to prevent 652 from releasing during its absence. In order to scan the lines the above selection process occurs except that a short pulse PS is applied instead of WF1. The pulses on 601 enable the relevant one of gates 633-648 so that gates 654 and 658 are enabled. The latter passes the condition of the line, free or busy, to common control. The cut-off relay K is turned on or off by setting or resetting bi-stable 655 with short pulses PKO or PKR following a similar selection process and during the absence of WF1. A free trunk for use by a calling line is found by pulsing each line's common PI lead during the absence of WF1. The application of command pulses is effected during a 1 msec. repetitive cycle (Fig. 7, not shown). Thus during any form of scanning or interrogation of a particular line, the code for that line is set up in the bi-stables 620 &c. just as if a relay selection were taking place. However during the next following cycle, a cancelling code is set up so that a crosspoint relay associated with that line code does not have its condition altered. During free-trunk scanning, the code for the line 1, trunk 0 is set-up in bi-stables 620. In conjunction with the WF1 and PI pulses a monostable 665 is therefore set. When WF1 ceases, a trunk pulse in a time position unique to trunk 0 is passed via 661 whereby in conjunction with the output of the monostable, central control is informed of the availability of this trunk. During the next cycle, the code is cancelled so that the original condition of the relevant crosspoint is maintained. If in fact the relay was originally operated, the cancelling code is actually an operational code. Furthermore if the relay was so operated, the relevant bi-stable 620 &c. would have been producing an output for more than one cycle. This output would therefore have exceeded the delay time of a delay circuit 663 and would thus be serving as an inhibit on gate 661. Consequently during trunk scanning the trunk identity pulse could not penetrate gate 661 and thus central control would recognize that this trunk was busy.