Patent GB-1216473-A (GB)
Title: Electronic switching circuits
Inventor: PURVES RICHARD FRANCIS
IPC: H04Q3/52
Filed: 1967-01-31
Abstract: 1,216,473. Semi-conductor switching circuits; automatic exchange systems. POST OFFICE. 30 April, 1968 [31 Jan., 1967], No. 4693/67. Headings H3T and H4K. Conduction of a thyristor SCR2, Fig. 1 (SCR1, Fig. 3, not shown) upon simultaneous occurrence of signals at input terminals T1, T2 of a switching circuit, causes a switch VT1 to turn on to connect the T1 input to an output T3. Instead of being a transistor VT1, the switch could be another thyristor (SCR2, Fig. 3, not shown). In Figure 1, a pulse at T1 turns on a further thyristor SCR1 provided a T2 signal occurs, and the output of SCR1 turns on SCR2 whose anode voltage drop due to a resistor 270 then causes SCR1 to turn off. VT1 remains open until the end of a pulse at T2, to pass all pulses occurring at T1. Further transistors VT2, VT3, may be controlled by SCR2, cross-talk being reduced by a capacitor (not shown) across R2. The resistor R2 could be replaced by a reed relay. In the case where the switch is a thyristor (SCR2, Fig. 3, not shown) the positive pulse at T2 must be accompanied by a complementary negative pulse (at T2<SP>1</SP>) applied through a resistor to the cathode of SCR2. This cathode is also connected to the base of a transistor VT11, whose collector is here connected to T2, and which again passes the pulses from T1 during the T2 pulse. A matrix switch for a t.d.m. or p.c.m. telephone switching system, incorporates the switching circuit of Fig. 1 at some (Fig. 2B, not shown) or all (Fig. 2A, not shown) of the crossovers of a set of information pulse lines (1FP1 to 1FPY), and a set of lines from call units (CU1 to CUX or CU1 to CU100), the switching circuits (S11 to SXY) having terminals T1 connected to the 1FP lines, terminals T2 to mark pulse lines (MP) from the call units, and terminals T3 to received pulse lines (RPL1 to RPLY) to the call units. When any one circuit (SXY) has been selected, the drop in potential at T2 due to SCR2 conduction prevents selection of any other circuit in the same row (its SCR1 cannot conduct); and conduction of VT1 to the negative supply at T3 prevents selection of any other circuit in the same column (T1 is held negative). In the modification of Fig. 3 (not shown) one. transistor (VT11) serves for each row, but again unique selection is ensured by the voltage levels at T1 and T2.  To avoid possible selection of two switching circuits in different rows due to coinciding mark pulses (MP) from the call units, a guard circuit is employed for each row (Fig. 4, not shown). Pulses are supplied cyclically from a source (PS) to the control electrode of a controlled rectifier (SCR3) in each guard circuit, coincidence of which with a mark pulse (MP) to the same circuit (at A) turns on the rectifier (SCR3). This causes: a transistor (TR1) to connect the mark pulse to the associated row (at B); a transistor (TR2) to conduct so as to earth a line (ML) preventing any other guard circuits from operating; and a third transistor (TR3) to conduct when an information pulse arrives (at D) from the selected switch circuit, whereupon the information pulse appears at the output (C) and the thyristor (SCR3) is turned off by the fall in its anode potential caused by conduction of the third transistor (TR3).