IMPROVEMENTS IN OR RELATING TO TELEPHONE SYSTEMS

1,218,567. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd. 25 March, 1968 [21 March, 1967], No. 13289/67. Heading H4K. Path tracing across a marker controlled twostage relay cross-point switching network is effected by altering the holding potential on the H-wire and searching amongst the marking wires for those wires exhibiting a corresponding change of potential. The network described is a group selector RRGS giving 48 access relay sets RARS served by four 12 x 20 A stage switches AS1-AS4 full access to 200 outgoing trunk relay sets OTRS by way of 80 links and twenty B stage switches BS1-BS20. Mounted on the same rack as the group selector the path tracing equipment, Fig. 1c, has for forward path tracing two manually controlled selector switches MSA, MSB and associated lamps OLP, LLP, by means of which a search is conducted amongst the marking wires of the A and B stages, respectively, a flexible cable being plugged at one end into a test jack to gain access to a known inlet and being plugged at its other end into the forward path test jack FPTJ of the path tracing equipment. Path tracing can also be effected in the backward direction by plugging the flexible cable into a test jack giving access to a known outlet to a trunk relay set and into the backward path trace jack BPTJ on the test equipment, manually operated selector switches MSC, MSD, being used in conjunction with indicator lamp ILP to apply a potential to all group selector inlets to determine that inlet which produces a response to the known connected outlet. As shown in Figs. 2a and 2b, where only one access relay set RARS and one outgoing trunk OTRS are shown, a connection is established by a marker CCE by operating one of the 20 A relays associated with an inlet the holding contact of the selected relay extending the H-wire forward over a link to one of the four inlets of a B switch where one of ten B relays is selected. The path is held by earth from a set RARS to negative 50 volts in the set OTRS. Forward path tracing.-Given the location of a set RARS in a path, either existing or set up from the set for observation, a key KFP is pressed to operate relay CPT in dependence on the stage register REG not requiring the network for a connection. At the end of any interruption of tracing a delay DE holds the relay CPT back for a short period to allow full release of the register and marker. Contacts CPT1 and CPT2, in series to reduce the risk of fault due to either sticking