Patent GB-807126-A (GB)
Title: Improvements relating to selector systems
Inventor: WARMAN BLOOMFIELD JAMES
IPC: H04Q3/42
Filed: 1956-03-26
Abstract: 807,126. Automatic exchange systems. SIEMENS EDISON SWAN Ltd. March 15, 1957 [March 26, 1956], No. 9360/56. Class 40 (4). In a testing circuit for a selector the test terminals of the outlets are connected in turn to a test point (e.g. a wiper of the selector), the path between the base and emitter electrodes of a transistor being connected in series between the test point and a source of reference potential and the arrangement being such that when the test terminal of a free outlet is connected to the test point the transistor conducts and the flow of collector-emitter current causes the operation of a relay which stops the selector on the free outlet. In Figs. 1 and 2 only the test wiper P and test terminal bank W are shown for each of two selectors designated respectively by the suffixes 1 and 2 appended to the reference letters of their components. The test terminals in banks W1, W2 are multipled to testing resistances Rt via which a free marking potential (+ Vt in Fig. 1 and - Vt in Fig. 2) is applied to the test terminals. As either selector of Fig. 1 scans the outlets the test terminals are connected in turn to the emitter of a point-contact transistor T in the associated test circuit TC. The base electrode of this transistor is clamped to a positive reference potential Vr less positive than Vt, the positive biasing potential Vb rendering the rectifier MR conductive. When a free outlet is tested, the transistor conducts, the resulting collector-emitter current operating the control relay RL which stops the selector. This current flowing in the base resistor Rb causes the base electrode and hence the emitter also to excurse negatively thus reducing the potential on the test terminal of the seized outlet to a busy value insufficiently positive to cause the transistor of a second testing circuit to conduct. If the two selectors reach a free outlet simultaneously the transistors T1 and T2 will each attempt to conduct and to prevent conduction of the other, but due to minor differences in the transistors only one of them will conduct and the other will be prevented from so doing. In the Fig. 2 arrangement junction transistors T<SP>1</SP> are employed, each having its base electrode connected via a shunt resistance Rs to a point more positive than the marking potential - Vt (e.g. to the grounded positive terminal of the voltage source providing the potential - Vt), and its emitter electrode connected to an intermediate point X of a potential divider 1R, 2R connected across a voltage source such as that the point X assumes a reference potential lying between - Vt and the potential of the outer end of Rs. The collector electrode is in each case connected to a suitable negative potential through the primary winding of an output impulse transformer TX the secondary of which is connected to the control electrode of a cold-cathode tube CC having a control relay RL in its anode circuit. As each test terminal is tested the shunt resistance Rs of the test circuit concerned forms with the test resistance Rt a potential divider which if the outlet is free applies to the base of the transistor a potential more negative than the reference potential at X. The transistor conducts and the output pulse triggers the tube CC, whereupon the relay RL operates and stops the selector. The relation between the values of the shunt resistances Rs1 Rs2 and the test resistances Rt is such that if an outlet has been seized by one selector, say selector 1, and is then tested by the second selector (2) the parallel combination of the two shunt resistances Rs1 and Rs2 connected in series with the test resistance Rt produces a potential at the base of the transistors T<SP>1</SP>1 and T<SP>1</SP>2 which is less negative than that at X. T<SP>1</SP>2 does not therefore conduct, and although T<SP>1</SP>1 ceases to conduct temporarily this has no effect on the conduction of tube CC1. If the two selectors test an outlet simultaneously neither is permitted to seize it. Application to an automatic exchange employing register-controlled motor uniselector switches in the group and final selector stages-Fig. 3.-A first group selector, a second group selector, and a final selector are represented at W, X and Y, the +, - and P arcs having their usual significance and the c arcs representing the contact banks to which the potentials for'reverting impulses to the registers are connected. These potentials are so connected that for the group selectors a pulse is reverted at the beginning and end of each group of outlets, and for the final selector a pulse is reverted at every outlet, the pulses being alternately positive and negative. Each second group selector X when in its home position has - 40 volts on its private wiper, and each final selector Y - 20 volts. If a second group selector is off-normal its P-wiper potential may have any one of six values: (a) - 20 volts if the contact engaged by the P-wiper is connected to a final selector P-wiper in its home position, (b) ground potential if the final selector is off-normal and engaged in a connection (contacts ec closed), (c) about + 10 volts (depending on line conditions) if the final selector is off-normal but is not engaged in a connection, that is, it is awaiting restoration to normal after the cessation of a call, (d) about - 6 volts if the final selector is engaging the P- wire of a calling subscriber, (e) - 15 volts (via RXF) if the final selector is engaging the P- wire of a first or last line of a P.B.X. group, or (f) + 50 volts via RRF if the final selector is engaging its penultimate outlet having failed to test-in to a wanted free line. The private wiper of a first selector searching a wanted group of outlets is connected via coa normal to the testing circuit including transistor T7 which responds only to a potential more negative than - 35 volts, and thus relay RB operates when the first free second group selector at home is tested and the first selector is stopped. If all the outlets of the group are busy the pulse reverted on the + wire from the first outlet of the next group cuts the drive circuit and closes contact b1 to bring up NS whereupon busy tone is reverted. Assuming a free outlet to have been found, a drive circuit for the chosen second selector is extended from the register whereafter contact coa is changed over to connect up the test circuit including T8, which responds to potentials more negative than - 15 volts, and a search for a free outlet in the wanted group is made. Should no final selector be available, closure of contacts b2 brings up NS; but if the search is successful a driving circuit is extended to the chosen final selector and contact cob is changed over to connect up the remaining test circuits. When the final selector reaches the outlet immediately preceding the wanted one, a stimulus is applied over WD to prime tube C6 and the next pulse received over the + wire fires this tube, thus priming tubes C3 and C5 and cutting off the normally conductive transistor T4. The fall in the collector potential of T4 renders T5 conductive, the consequent fall in the emitter potential thereof being clamped to ground by Rf2. Thus, if the line is free the + 10 volts present on the private wire causes Rf1 to conduct and the resulting pulse in the secondary of TR4 is amplified by T6 and applied via TR5 to fire C whereupon RF is operated to cut the drive circuit to the final selector. However, if the line is busy, or calling, the private wire is at ground potential, or - 6 volts, respectively, so that Rf is backed off and no pulse is produced by TR4. The final selector is therefore driven on and the next pulse reverted over the + wire fires C5 which extinguishes C6 thus restoring the + 15 volts potential at the emitter of T5. Any further free lines encountered are ineffective on the test circuit, but when the final selector reaches the penultimate outlet the + 50 volts present on the private wiper causes Rf 1 to conduct and the selector is stopped. The firing of C5 brings up RE which operates NS to revert busy tone to the caller. Selection of a free line in a P.B.X. group.- Tube C6 fires when the final selector reaches the wanted outlet as for an ordinary line, but this outlet being the first of a P.B.X. group has - 95 volts present on its private wire, which causes T3 to conduct and apply a pulse via TR3 to fire C3 and bring up RC which prevents energization of NS by the subsequent operation of RE when C5 fires to the next pulse reverted on the + wire. The rise in the cathode potential of C3 keeps T4 cut-off when C6 is extinguished by the firing of C5, and thus maintains the emitter of T5 at ground potential. The final selector therefore continues to hunt for a free line until halted, when one is found, in the same manner as for an ordinary line. If all lines of the group are busy, the - 15 volts on the private wire of the last line causes T3 to conduct again and the resulting pulse fires C4, thus extinguishing C3 and bringing up RD which causes NS to revert busy tone. The emitter of T5 returns to + 15 volts and the final selector is driven to the penultimate outlet as for an ordinary busy line.