Patent GB-1377242-A (GB)
Title: End mark controlled switching system and matrix
IPC: H04Q3/52
Filed: 1972-01-31
Abstract: 1377242 Transistor pulse, gating and switching circuits; automatic exchange equipment WESCOM Inc 31 Jan 1972 [3 Feb 1971 25 Jan 1972]. 4521/72 Headings H3T and H4K An end marked switching matrix for use in a telephone exchange employs 4-layer thyristor devices which are switched on when the anode and cathode terminals are marked and an enabling. bias voltage is present on the gate terminal which is connected to the region adjacent the anode. A gate control circuit 117 is provided which on receipt of the mark at the cathode terminal immediately supplies an enabling voltage to the gate circuit and after a short delay raises this voltage to an inhibiting level. In the matrix shown the gates of the thyristors are biased by positive voltages through a voltage divider 71 including resistors 81, 82 and the voltage source is preferably a few volts  higher than the anode marking voltage applied at 54. The cathode end marking is applied by making transistor 83 say conduct and a thyristor which already has its anode marked (at 54  55, 56) will now conduct to provide a speaking connection from inlet 51 to the respective outlet (54, 55, 56) of the matrix. In a modification the potential divider used to set the initial gate voltage may have a transistor which when conductive short circuits resistor 81 to clamp the gate to the bias source and thus provide the inhibiting voltage. This transistor is controlled by the gate control circuit, Fig. 7 (not shown). Alternatively a field effect transistor can be used to provide a varying potential at the gate, Fig. 10 (not shown). The switching network shown in Fig. 12 comprises a plurality of substages each consisting of a switching matrix similar to that of Fig. 2. In the network, one outlet of each J substage forms an inlet to a different K substage so that when all outlets of a K substage are marked only one outlet of each J substage is marked. The outlets of the K substages are marked sequentially by scanner 618 so that only one inlet 601 has access to the L-stage inlets at a time. The crosspoints in the J substages remain operated for longer than those in the K substages due to the capacitors 613 so that they revert to a non-conductive state only after a path has been established.  In the gate control circuit shown in Fig. 5 the Darlington pair transistors 221, 222 are normally conducting to develop a voltage across resistor 229 thus causing transistor 225 to conduct and hold transistor 233 non-conductive. Transistors 237 and 226 are held non-conductive. When the cathode end mark is applied the transistors 221, 222 conduct less and lower the voltage across resistor 229 thus switching transistor 225 to its non-conductive state. This switches on transistor 223 and thus lowers the voltage at the gate of the thyristors to an enabling level. After a brief delay transistor 237 switches on and when saturated conduction occurs transistor 226 switches into conduction thus switching off transistor 223. When an end mark is applied, in the alternative, Fig. 9 circuit, the double emitter transistor 426 conducts and causes transistor 427 to conduct. However transistor 426 limits the collector/emitter voltage drop across transistor 427 and thus prevents it saturating. Transistor 439 conducts until the time constant of the RC circuit 435, 437 allows transistor 436 to conduct when transistor 439 switches off. In a modification of this circuit the size of resistors used may be reduced by employing Zener diodes. The components preferably form an integrated circuit.