Patent GB-1321471-A (GB)
Title: Switching arrangement
IPC: H04Q3/00
Filed: 1970-10-05
Abstract: 1321471 Automatic exchange systems TELEFONAKTIEBOLAGET L M ERICSSON 5 Oct 1970 [3 Oct 1969] 47284/70 Heading H4K A crosspoint switch for transmitting pulse width modulated speech signals is controlled by a pair of interacting bi-stable circuits that are set by end-markings applied via the speech wires or via separate control wires. Fig. 3 depicts the latter case wherein the horizontal d wire is marked with a "0" during path finding but otherwise carries a " 1 " marking. A marking 1 or 0 on the c wires denotes as free or busy respectively a crosspoint at the intersection thereof. All the gates shown are of the NAND type. In operation, with the c wires marked as free by a " 1 ", a " 0 " applied to the horizontal d wire causes bi-stable pair G4/G3 to produce " 0 " and " 1 " outputs respectively, the latter after inversion to " 0 " at gate G5 being passed to the vertical d wire. This marking is transmitted in practice via two further stages of crosspoint matrices (Fig. 2, not shown) to a pre-allotted register REG (marked with " 0 " at its input H) whereby all free paths between the calling subscriber (who it is assumed is connected to the horizontal a, b, c, d wires) and the register are prepared for selection. Bi-stable G18/G19 in the register is thereby effective to apply " 0 " to the vertical c wire whereby the bi-stables G8/G9 in all the prepared crosspoints are set. Consequently the " 0 " marking is transmitted onwards to the horizontal c wires and the speech gates G11, G12 for the two directions of transmission (i.e. an effective 4-wire system) are opened. The change of marking on the c wire does not affect the now set G3/G4 bi-stables. When a complete path has been set up in this way, the marker (not shown) restores the normal " 1 " marking to the vertical d wire via input I in the REG. Removal of the " 0 " marking from the horizontal d wire resets G3/G4 so that the marking applied to the inputs of gate G14 is then 101 (reading left to right). This resetting has no effect on G8/G9 which can only be reset by appropriate marking of the vertical c wire. Fig. 4 (not shown) depicts an arrangement in which the a speech wire is used for transmitting the initial path finding signal. The significant differences are that: gate G2 is tied additionally to the horizontal c wire so as to render it unresponsive to speech; gate G14 (whose use is mentioned below) is dispensed with, reidentification taking place over the b wires via gates G12 and G13; and the a and b wires are interconnected via gates G1, G11 in series and G12, G13 in series respectively. Release.-In either embodiment, in order to reset G8/G9 which effectively controls the crosspoint G11/G12, it is only necessary to place a free " 1 " marking on the vertical c wire. System operation (Fig. 1).-A calling subscriber Ab is identified during a cyclic scanning process to the marker M which allots a free register REG by marking its H input (Fig. 3) and applies a path finding mark to the subscribers d wire. A path is thereby set up as described above. In order that only one path is selected the marker artificially busies all except one of the vertical c wires in the A and B stages during path finding. In response to dialling tone, the first digit is dialled into the register which, in dependence on the digit value, either immediately calls in the marker to set up a new connection to an outgoing relay set FDA or awaits reception of further digits whereafter it calls in the marker to set up a new connection to a feedbridge SNR to which the called party has previously been connected. In either case the marker applies a " 0 " to the vertical d wire thereby opening each crosspoint gate G14 (which now has its inputs marked 111) so as to pass the " 0 " marking on to the d wire of the next stage. The appearance of the " 0 " marking at the subscriber's line circuit is effective on the identifier ID which thereby supplies the marker with the identity of one end of the connection which it has been instructed to set-up by the register. Thereafter the connection is cut through in the above described manner utilizing a separate pulse width modulator and demodulator for each direction of speech transmission.