Patent GB-592856-A (GB)
Title: Improvements relating to electrical selective signalling systems
IPC: H04Q1/18
Filed: 1944-12-15
Abstract: 592,856. Automatic exchange systems. SIEMENS BROS. & CO., Ltd., and STALLWORTHY, F. A. Dec. 15, 1944, No. 25245. [Class 40 (iv)] In a selector of the type in which each outlet of group of outlets is marked by a discriminating electrical condition, the wire which is used to apply the discriminating condition is also used to apply busy test and if the outlet is free subsequently to apply a holding condition for the switch. As described, a digit switch DS, set by incoming impulses, applies a discriminating condition to the p-wire and a latch-controlled motor-driven group selector GS searches for an outlet having a complementary condition on its p-contact. The conditions on the p-wire of the outlet may be represented by different resistances, potentials or alternating currents of different frequencies. Resistance-marking. The usual relays A, B and C operate upon seizure, and B removes the group discriminating resistance Y and marks the switch busy by putting - ve potential on the p-wire. A repeats incoming impulses to the digit switch magnet DSDM to set the digit switch to a contact appropriate to the outlet required and wiper ds1 connects -ve battery through an appropriate tapping of resistance Z to the p-wiper of GS. On release of C latch magnet LM energizes and connects up the driving motor of GS which searches to find an outlet whose earthed marking resistance Y, together with the tapping of Z, produces a potential sufficient to fire the gas-filled valve V which operates T. The resistances Y successively decrease as the switch goes from group to group, but are the same for all outlets of a particular group. T opens the circuit for latch magnet LM which stops switch GS. H (coil i) operates in series with LM and extends the + and - wires through to the next switch whose B relay operates to put busy - ve battery back on the p-wire to which H (coil ii) locks. A relapses followed by 13 and C, valve V is extinguished and switch DS goes home. On release, holding battery is removed by the succeeding selector from the p-wire. H relapses and LM is energized to drive GS home. If an outlet is busy, the p-wiper encounters - ve battery through resistance X and V does not fire. Off-normal contacts GSON1 prevents testing into the switch GS during homing. All outlets busy. To prevent false operation in the event of all outlets of a group being busy due to the possibility of the switch continuing to search in the next group, a blank position is provided at the end of each group, the p-contact carrying resistance Y A equal to the value of the discriminating resistance for the group and the - contact is earthed. The switch therefore stops at this position, and busy tone relay BT operates to send busy tone back and disconnect H. BT is sufficiently slow not to operate during transit of the wipers over the - contacts. For dead numbers, corresponding contacts of the digit switch ds5 are connected via a ccontact to relay NU, which operates on relapse of C to send NU tone back, and disconnect H. Frequency marking. In the arrangement, relevant parts of which are shown in Fig. 2, the digit switch tunes the transformer TR to a characteristic one of a set of frequencies by means of condensers K1 ... K0. Contacts of the p-bank carry either the discriminating frequencies or earth if the outlet is busy. When the p-wiper encounters the frequency at which the tuned circuit is resonant, T operates to earth the p-wiper, so busying the outlet. LM is de-energized, and H operates in the usual manner. All outlets busy. In this arrangement the outlets of the various groups may be indiscriminately mixed with one another and the final position of the switch is used for giving busy tone for any group. In this case, in addition to the busy tone relay BT, test relay T operates over a second coil to the earth on the - wire to stop the selector.