Patent GB-1489577-A (GB)
Title: Solid state circuits
IPC: H04Q3/52
Filed: 1973-10-02
Abstract: 1489577 Two-wire crosspoint semiconductor switch; automatic exchange systems PLESSEY CO Ltd 30 Sept 1974 [2 Oct 1973] 45881/73 Headings H3T and H4K A crosspoint circuit arrangement has a first transistor affording an outward transmission path switching element and a second transistor affording a return transmission path switching element, the transistors each having emitter, base and collector electrodes, the collectoremitter paths forming part of the transmission paths and bias means coupled to their bases for causing them to be set in an OFF state or in a non-saturated ON state. As shown there are two channels, via T1, T2, controlled by a common switching element of SCR form, which may however be a dual transistor equivalent T5, T3, which acts as a memory to hold the switches open or shut according to the memory trigger T4 which is used to turn them on, or to the cessation of supply volts which turns TR3, TR5 and hence Tl, T2 off. The input 1 may be connected to a subscriber or another crosspoint, the input 3 from a re-entry switch or another crosspoint. The output 4 may be connected to the subscriber via a line and hybrid unit (Fig. 3, not shown) or to another crosspoint while the output 2 is connected to another crosspoint or a re-entry switch. These crosspoints can be used in matrices of rows and columns. In each row the emitters of the first transistors are interconnected, as are the collectors of the second transistor; while in each column the emitters of the second transistors are interconnected as are the collectors of the first. In operation a mark input applied to any subscriber input such as S2, Fig. 2A will be detected by the corresponding mark detector which then injects current into the diode chain D12, D13, D14 and provides base current for the inputs 6 of the crosspoints 9 associated with that input. The current in the diode chain also generates bia across D9, D10 and D11 allowing four mark forward generators MG1-MG4 to operate as current sinks and to pass on the marks. If no crosspoint 9 is already selected all four mark generators operate, but if one or more crosspoint is ON then inhibiting of mark forward takes place, for example by T28 being ON and turning T30 off. The bias for T28 is obtained from the SCR type of memory element within the crosspoint 9 via the current limiting resistor R2, Fig. 1. The mark forward signals at the collectors of T20, T23, T26, T29, Fig. 2A pass through the voltage level translators VT5-VT8 before passing to the next stage. The collector signal is also used to indicate that that output is handling current, for example a signal at C from VT5 biases T6 ON and provides bias to the input 5 of the SCR type element of every crosspoint associated with that output, these do not operate until a hold signal is returned to the matrix. This is received, say, along E7 where it is detected by a hold detect circuit HD4 which consequently injects a current into the input 8 of all the crosspoints in that column to fire one crosspoint in the column. The diode chain D12, D13, D14 forms a priority system in that the crosspoints at the top have a higher base bias, the highest marked takes all the current from HD4 and the others do not turn on. Selection of a crosspoint turns OFF both the mark detector MD associated with the row and the appropriate hold detector HD4. The The mark forward current is still maintained by the crosspoint selected at output 4 and passed to the voltage level translators.