Patent GB-1248132-A (GB)
Title: Automatic telecommunication exchange
IPC: H04Q3/52
Filed: 1969-09-11
Abstract: 1,248,132. Automatic exchange systems. INTERNATIONAL STANDARD ELECTRIC CORP. 11 Sept., 1969 [13 Sept.. 1968], No. 44940/69. Heading H4K.  A path through a 4-wire network of four-layer diodes is set-up by marking the required end points of the network and then successively marking an intermediate point of each of the possible paths between these points until a free path is found as indicated by the breakdown of the diodes in the path. This method ensures that the only diodes fired during the whole of path finding are those in the first free path (thus saving on the power supply requirements for a fan-out path finding method) and also vitiates the problem of rate effect firing of diodes. The network comprises three stages each of two, 2 x 2 four-zone diode matrices. A free link circuit, e.g. Alott 1, is automatically selected in response to a calling line or trunk, e.g. trunk 1. The link and line circuits apply positive and negative battery respectively to the corresponding outlet and inlet of the network. These potentials pass via OR gates 4, 6 and 1, 3 and differentiators DC2 and DC1 to a so-called array scanner control (Fig. 3, not shown). The latter, in response to both potentials, applies stepping pulses to a so-called array scanner which has as many outputs as there are intermediate matrices (Fig. 5, not shown). Each stepping pulse (generated by a monostable circuit in the control) causes the scanner to apply an enable pulse MGP1 or MGP2 via an amplifier Al or A2 to the outlets of a corresponding matrix. In the present example, if both outlets of the calling primary matrix CM1  are free then the negative potential in line circuit 1 is also effective via diodes CR1, CR2 to charge capacitors C1, C2 and via OR7, OR8 to prime AND 1, AND 2. Capacitor C6 is charged by the positive potential from the link circuit. Assuming pulse MGP1 occurs first, AND 1 is opened to charge capacitors C3, C4 if the paths 5, 6 corresponding thereto are free. (Path 6 is of course irrelevant here). With path 5 free, diode D9 breaks down, the charge on capacitor C4 goes positive, thus allowing D5 to breakdown and similarly C1 goes positive allowingD1 to fire. With the path thus completed, the link circuit informs the scanner that its services are no longer required. The scanner does not reset but rests on the MGP output to which it is currently set. If path 5 had been busy C4 would not charge negatively and the MGP1 pulse would have been of no effect. The MGP2 pulse, when it appears, would then charge C10 and diodes D11, D17 and D2 would then fire. If path 1 had been busy, C1 would not be charged and, more importantly, AND 1 would not be primed so that the MGP1 pulse would not even be applied to paths 5 and 6. If all the available intermediate stage matrices have been scanned by their respective MGP pulses without effect, a central control causes. a new link circuit to be allotted and a new scanning sequence in respect thereof is started. It should be noted that the array shown in Fig. 2 is for speech transmission in only one direction. The array for the other direction (Fig. 6, not shown) is of exactly the same form except that it contains none of the control logic. The necessary control signals for the other array are supplied thereto from the points marked A, B, C, D in Fig. 2.