Patent GB-1218567-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: RIGBY DONALD FRANK
IPC: H04Q3/00
Filed: 1967-03-21
Abstract: 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, connect 22 volts to the H wire connected over the flexible cable TLA. Since the identity of the A switch is known the twenty position switch MSA is stepped manually over the marking leads of the A stage to connect them to a voltage discriminator VCA set at 15 volts which is about midway between the 22 volts appearing on leads of disconnected paths and the 8 volts present on the path marked for tracing. A relay PTA, lighting lamp OLP, operates for voltages below 15 volts and in consequence the lamp OLP is extinguished when the marked outlet of the A switch is found. The ten position switch MSB is similarly stepped over the marking leads of the B stage a voltage discriminator VCB, set at 1À5 volts mid-way between the 8 volts present on a disconnected outlet and the negative 7 volts present on the path marked for tracing, controlling relay PTB and lamp LLP. The position of MSB at which LLP extinguishesgives the identity of the outlet from the B switch the identity of which is determined from the MSA switch. In order to compensate for variations of impedance from one path to another, the 8 volts obtained from the marking lead of the A stage is used in a converter DCS to obtain the 1À5 volts setting the threshold of discriminator VCB. Backward path tracing.-The four banks of manually operable switch MSD give access to corresponding ones of the H-wires at the inlets of the four A switches. The twelve banks of manually operable switch MSC give access to all the H-wires at the inlets of selected ones of the A switches. When a key KIB is latched down and a key KBP is pressed, with the flexible cable TLB plugged between the test jack TJO of a known group selector outlet and the jack BPTJ, relay CPT responds and brings up CPR to connect 33 volts to the wipers of MSC. In the zero position of MSC the 33 volts is connected to the four wipers of MSD so that when MSD is stepped over its contacts corresponding H-wires at the inlets of the four A switches are connected to 33 volts. When one of the banks of MSD reaches the inlet connected to the outlet jacked into the trace equipment a voltage discriminator VCC fails to energize a relay PBT and consequently fails to light the lamp ILP. The setting of MSD indicates the number of the inlet but not the A switch involved. Stepping of MSC over contacts 1 to 4 by selecting all inlets of each A switch in turn causes VCC to discriminate when the appropriate A switch is found.