Patent GB-689031-A (GB)
Title: Signalling system with pbx subscribers
IPC: H04Q1/18
Filed: 1947-12-30
Abstract: 689,031. Automatic exchange systems. STAATSBEDRIJF DER POSTERIJEN TELEGRAFIE EN TELEFONIE. Dec. 30, 1947 [Oct. 18, 1945], No. 34845/47. Class 40 (iv). When a final selector finds the first line of a PBX group busy, a potential characteristic of the group is transmitted over an additional bank to the register, a device therei recording this potential, and the selector is then caused to hunt in the additional bank for further potentials equal to that recorded. The invention is described with reference to a group containing both numbered lines to which the selector can be set directly and unnumbered lines which the selector can seize only during PBX hunting. As shown the tens digit which is registered on relays XA ... XD is transferred to the relays YA ... YD which connect a potential characteristic of the wanted tens group to the comparison device B1, B2 and the final selector EK hunts over its e-bank for the same potential. When this is found relay R in the comparison circuit operates to cut the drive. The units digit is now transferred (by means not shown) to the relays YA ... YD and the selector is set again in accordance with the units digit. Busy testing is effected over the c-wiper. A free line is indicated by earth potential which, with an earth reference potential being applied by a sequence switch to the cathode of valve B3, causes P to operate, thereby to control switchthrough. The resistances W1 ... W6 are so chosen that in the case of a line being busy the c-bank potentials are - 2V for an ordinary line, - 24V for a first PBX line, - 22V for an intermediate PBX line and - 20V for a last PBX line. If P fails to operate a sequence switch is stepped to apply a - 20V reference potential to the cathode of B3. If then the busy line is an ordinary line relay P operates and initiates the reversion of busy tone. If, however, the line is a first or numbered PBX line, P fails to operate and the sequence switch is stepped to close contact x which connects up the device NA ... ND (not shown in detail) which records on contacts na ... nd the PBX group potential found on the f-terminal. These contacts set relays YA ... YD and the sequence switch then opens contact x and changes over contact y. Relay R then re-operates and the selector hunts over the f-bank for a free outlet or a last outlet characterized by the PBX potential now registered on YA ... YD. When this is found both R and P operate and the sequence switch is again stepped to replace the earth on the cathode of B3. If the line is free P remains operated and switch-through takes place. If the line is a busy last line, P releases to initiate reversion of busy tone. In the above embodiment the switch hunts when set to a busy line irrespective of whether the line is a first or an intermediate numbered PBX line. However, by providing a further device similar to NA ... ND which is capable of discriminating between - 22V and - 24V hunting after an attempt at connection to a busy intermediate numbered line can be prevented. By the use of suitable switching arrangements it may be arranged that the same device NA ... ND may be used both for this discrimination and also for determining the PBX group potential existing on the f-bank of a first PBX line.