Patent GB-663037-A (GB)
Title: Determination of the class and/or condition of subscribers' lines
IPC: H04Q1/18
Filed: 1947-12-10
Abstract: 663,037. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. Dec. 10, 1947 [Sept. 25, 1943], No. 32561/47. Class 40 (iv). Several potentials characteristic of classes of subscriber's lines are fed to a comparison circuit which is fed successively with a series of reference potentials, the circuit responding when a predetermined relationship between the two potentials exists. The circuit, which is located in the register controller, is used to control the line finder and then to test to determine whether the line is an " absent subscriber " line or a "restricted service " line. After operation of the final selector the circuit determines whether the selected line is a dead line, a " changed number " line or an " absent subscriber " line; or, if busy conditions are encountered on a P.B.X. line, whether the line is a first, intermediate or last line of the group. The circuit may also discrimi. nate between lines busy in toll and local calls. Arrangements are provided whereby in a system in which the same final selectors are used for establishing both local and toll calls to transfer " last line conditions in a P.B.X. group to the last line busy in a local call when succeeding lines are toll busy, thereby ensuring that an operator may always offer toll calls to the P.B.X. while one of its lines is engaged in a local call. As described, single lines or intermediate lines in a P.B.X. group are characterized by a 19,000 # earth on the d-wire and first and last lines of a P.B.X. group by 4300 o and 1000 # earths respectively. Link circuits for establishing toll and local calls apply busy battery to the d-wire over 2400 # and 5700 # respectively so that the d-terminals of the abovementioned types of lines carry potentials of -42.7, - 30.8 and - 14.1 volts when toll busy and of - 36.9, - 20.6 and - 7.2 volts when busy in a local call. Absent subscribers have their d-wires temporarily strapped directly to earth. " Changed number " lines have their d-wires connected to battery. The d-wire corresponding to a dead number is open-circuited. Comparison of potentials is effected by means of a static switch wherein the class potentials and comparison potentials are applied to the centre taps of the transformers T1, T2, Fig. 1, so that when the potential of the former is higher than that of the latter, rectifiers Re1, Re2 conduct to pass alternating current to trigger the tube Tu and operate Ftr. When the register controller and a finder/1st group selector link have been seized in response to closure of the calling subscriber's loop, as described in Specification 641,819, relay Utr (not shown) operates to send the sequence switch PM to position 1, wherein the line-finder LF is energized. When the calling line, marked by a reduced potential on its c-terminal is found, Ftr operates, stops the finder, and moves PM to position 2. If the line is a restricted service line the rectifier Re3 prevents operation of the static switch and relay Rsr is brought up to effect the necessary restriction and send PM to position 3. Otherwise, operation of Ftr moves PM to position 3. A further test on the calling condition of the line is then made and Ftr moves PM to position 4. The potential of the d-wire is then checked against a reference voltage of - 5 to ensure that the line has not meanwhile been seized by a final selector. If the line is still free Ftr brings up Psr and PM goes to position 5. Otherwise PM stops on position 4 until the register is released by removal of the calling condition. In position 5, Ps connects resistance battery to the d-wire so that Ftr operates only if full ground is present thereon, in which case the absent subscriber relay Asr operates to direct the call to an operator who may then order the removal of the earth strapping if the subscriber so desires. Otherwise Ltr operates to enable the register-controller to receive the dialled digits and PM goes to position 6. When the final selector FS is set, Esr (not shown) operates to move PM to position 7 where the d-terminal potential is compared against a reference voltage of - 5 and if the line is idle or in the " absent subscriber " condition Ftr operates Sfr and PM goes to position 8 where a test for the " absent subscriber " condition is applied as described above. If full ground is present then Ftr brings up Asr to move PM to position 3 and divert the call to an operator. Otherwise Lbr operates and with Sfr also operated the link circuit is put in the ringing condition and the register released. If the line is busy PM goes to position 10 wherein the d-terminal potential is checked against a reference voltage of - 17.4 to determine whether the line is the last of a P.B.X. group. If so, Ftr brings up Sbr which moves PM to position 11 in which a reference voltage of - 10.9 is used to determine whether the line is busy in a local or a toll call. According as Ftr operates or not Lbr or Tbr comes up to put the link circuit in a local busy or toll busy condition and release the register. If the line is not a last P.B.X. line PM goes to position 12 in which a reference voltage of - 34.2 is used. If the line is a first P.B.X. line Ftr fails to operate and Pbr comes up to move PM to position 17 in which P.B.X. hunting is initiated using a reference voltage of - 17.4. When Ftr operates to stop the switch PM goes to position 18, where a reference voltage of - 5 is used to determine whether the switch is stopped on a free line or on a busy last P.B.X. line. In the former case Ftr brings up Sfr and Lbr to initiate ringing and release of the register. In the latter case Sbr operates to move PM to position 19 where discrimination between local busy and toll busy conditions takes place, as described above. When PM is in position 6 if the line is not a first P.B.X. line PM goes to position 7 where a test is made with reference voltage of - 46.1. If the line is a busy single line or intermediate line of a P.B.X. group Ftr brings up Sbr which moves PM to position 8 where discrimination between toll and local busy takes place as described above, using a reference voltage of - 40.5. If Ftr does not operate in position 7, PM goes to position 9 where Ftr only operates if the d-wire is connected to the - 48 volts exchange battery. According as Ftr operates or not Cnr or Dlr operates to route the call to a " changed number or " dead line " operator's position. Advancing the " last line " conditions on a P.B.X. group, Fig. 4. The d-terminals 2 ... 5 of a P.B.X. group are normally connected to earth through resistances of 4300 #, 19,000 #, or 1000 #, characteristic of their positions in the P.B.X. group (relay Lar has a 1000 w winding). Marginal relay Lar operates only if the lower resistance toll busy battery is connected to the d-wiper thereby shunting relay Lbr across the 19,000 # resistance connected to terminal 4 so as to reduce the resistance between it and earth to 1000 w. Similarly as positions 4 and 3 become toll busy, last line conditions are advanced to positions 3 and 2, respectively. Specification 176,911, [Class 40 (iv)], is referred to.