1,216,233. Automatic exchange systems. INTERNATIONAL STANDARD ELECTRIC CORP. 4 April, 1968 [7 April, 1967], No. 16164/68. Heading H4K. In an exchange in which the sub-stations are divided into classes, calls involving certain calling class-called class combinations are intercepted and an indicator is operated at an operator's position. The system described is incorporated in a P.B.X. for hotels or hospitals. In the hotel P.B.X. the sub-stations are divided into three groups: guests, administration and services. Calls from guest to service sub-stations and between sub-stations of the same group are automatically established under dial control of the calling sub-station as outlined below while other calls are inhibited and a warning light at the operator's position is caused to flash. The operator speaks to the calling line and if necessary the called line (which can be obtained automatically in response to the closure of a single key as the number of the called line as derived from the dialled designation is still stored in the register) and decide whether the call can be allowed. Closure of an appropriate key causes either the automatic connection to be completed or the line to be cleared. The register (Fig. 2, not shown) receives an earth marking on one of three wires to indicate the class of the calling line by operating a corresponding marking relay. The register also receives the class of the called line as a marking on three further wires each of which splits into three branches which each pass through a contact of a different one of the marking relays. The nine branches correspond to the nine different calling class-called class combinations and are individually connected either to an allow relay or an intercept relay. Special services.-The operator's position includes a key which can be closed say at night so that for calls to guests the class comparison arrangement is by-passed and the intercept relay operated causing the warning light to flash. The operator intercepts the call and determines if it may be allowed. The line circuits of guest sub-stations include individual non-disturb relays which can be set under the control of the guest who dials a " 5 " if he wishes his relay to be set and a " 6 " when he wishes it to be released. When the relay is set calls to that line enable the intercept relay. Instead of the individual non-disturb relays being set under control of the guest they may be set by the operator. To reach the operator a guest dials a " 9 " which lights a lamp at the operator position and causes the number of the calling line to be displayed to the operator. The operator can check that the guest is calling from his own sub-station. The operator has a keyboard through which may be entered the number of the sub-station at which the condition of the non-disturb relay is to be modified. Establishment of a connection between subscribers.-The Figure shows a first sub-station 551 (assumed to be a calling station) permanently associated via its line circuit LC1 with an associated terminal section TS1 which connects to 50 such line circuits. Any two terminal sections, of which there are 20 may be interconnected via one of a number of local connecting circuits LCC1, each terminal section being connectible to the local connecting circuit by one of a number of primary selector circuits PS1. The local connecting circuits which incorporate the feed bridges and the ringing tone generator G can also be connected via a register finder RF1-2 to any one of a number of registers and to at least one operator's position. The connection is controlled by a marker comprising a preselect control circuit PCC and a selection control circuit SCC. Those terminal sections which have one or more of the associated line circuits in the calling condition signal to the preselection control circuit which selects one such section which has access to at least one free primary section and selects from the free primary sections one having access to a free local connecting circuit which has access to a free register. The preselection control circuit, while inhibiting operation of the selection control circuit, operates crossbar electromagnets controlling horizontal and supplemental bars giving access to the calling line as in Specification 1,123,460. These electromagnets operate additional contacts such as 1B2 and 14B2 in the terminal section to supply the earth potential on one (R) of three wires to the register indicating the class of the calling line, which indication is used as described above. After the register has been seized the electromagnet Csr controlling the vertical hold bar of the primary section PS1 and the vertical operate bar of the terminal section TS1 is operated to complete the connection of the calling line SS1 to the local connecting circuit LCCI. After the calling line and the register have been interconnected the number of the called subscriber as dialled by the calling subscriber is entered into the register. Those registers which have stored therein such a designation signal to the selection control circuit which selects one and receives the dialled designation from it and translates it into a designation of the terminal section and particular line wanted. The selection control circuit, while inhibiting operation of the preselection control circuit, operates the electromagnets of the horizontal and supplemental bars giving access to the called line. For a call to the line SS50 shown contacts 13B3 and 14H3 of the electromagnets close to supply earth on a line R indicating that the called line is of the same class (guest) as the called line. Since this is an allowable call the allow relay Aur is operated as described above. If the subscriber busy relay Sbr of the selection control circuit indicated that the subscriber is free, a primary selector PS1 is selected to connect the called line to the local connecting circuit which transmits ringing tone to the called subscriber.