Patent GB-771485-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: CURTIS GERALD
IPC: H04M3/58
Filed: 1955-05-05
Abstract: 771,485. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. May 5, 1955 [May 19, 1954], No. 14652/54. Class 40 (4). A private automatic branch exchange is provided with facilities which enable a first local subscriber having a connection with a main exchange line to make an enquiry call to a second local subscriber while holding the main exchange line and then to either revert to the main exchange line or transfer the main exchange line to the second local subscriber by clearing from the exchange. The connection for an enquiry call and the transfer of the main exchange line to the second local subscriber is made over a trunk finder switch such as BTF, Fig. 1, paired contacts of which are connected to paths NP and EP emerging from an exchange line circuit ELC. The main exchange line MEL connects with local subscribers over path NP and a trunk finder switch such as ATF or by a direct connection to a group selector GS, while enquiry calls are made by linking the path NP or the direct connection via GS, to the path EP in the circuit ELC and setting up the call over the trunk finder BTF. Transfer involves the release of the switching stages connected with trunk finder ATF, or of the connection via GS, while trunk finder BTF steps once to disconnect the second local subscriber from the path EP and establish a connection with the path NP, giving access to the main exchange line and providing for further enquiries over the path EP. Local call. In the trunking diagram of Fig. 1, subscriber SB initiates a call to subscriber SA by marking an idle hunter circuit FHC, causing switch SH to hunt for an idle group selector GS. When GS is seized, a line finder LF connects with the calling subscriber SB who may dial three digits to establish connection with subscriber SA over a final selector FS. Incoming call. Both way lines MEL from the main exchange terminate at ELC and ring- ing tone from the main exchange flashes lamp CL at the manual board MB on which the operator actuates a key to mark the relevant circuit ELC on the line finder ELF, which having found the line connects the operator with MEL, lights lamp SL, and trips out lamp CL. Having ascertained the local number required, the operator applies a holding condition to the line MEL and makes a connection with path NP which has access to subscribers over a trunk finder such as ATF. When the wanted subscriber answers, the operator may clear from the connection by restoring the key marking the circuit ELC. Before clearing, the operator may restore a connection with the line MEL, while applying a holding condition to the wanted local line, by actuating an exchange talk key. Outgoing call. There are two classes of subscriber to the branch exchange the privileged one of which, by dialling " 0," can gain direct access to an idle exchange line circuit ELC over the tenth level of the selector GS. The other class of subscriber must dial " 9 " to take into use an assistance circuit AC over the selector GS to mark line finder ELF and light a lamp AL on the manual board MB. The operator MB connects with the caller over ELF and, having ascertained the number calling and the number required, asks the caller to clear. The operator then operates a key to mark an idle exchange line circuit ELC and sets up the required connection over a line MEL to which the operator applies a holding condition when an answer is received. The operator proceeds to set up a connection to the local subscriber as if he were receiving an incoming call. Enquiry facility. A subscriber having a connection with a main exchange line may initiate an enquiry call, as described above, by pressing a push-button which holds the line MEL and makes a connection with the path EP which has access to local subscribers, and to the operator over the ninth level of a group selector TGS. The exchange connection is restored by pressing the push-button again, and is transferred by clearing from the connection.