Patent GB-718914-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: CURTIS GERALD
IPC: H04M3/58
Filed: 1952-03-14
Abstract: 718,914. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. Feb. 25, 1953, [March 14, 1952], No. 6617/52. Class 40 (4). In a private automatic branch exchange provided with facilities which enable a calling party to set up a local connection while holding a main exchange connection and to transfer the main exchange connection to the line called in the local connection, the setting up of the local connection while the main exchange connection is held is effected by means of a switch train which is separate from that over which local connections are normally set up and additional switching means controlled from the final selector of this separate switch train are adapted to transfer the main exchange connection to the line called in the local connection. As shown in Fig. 1, a P.A.B.X. subscriber A makes a local call over a line finder LF, a group selector GS, and a final selector FS. The main exchange is reached over the tenth level of group selector GS, an exchange line relay set ELC, and an exchange line EL. If subscriber A wishes to call another local P.A.B.X. subscriber B while holding the main exchange connection, he dials the digit "1" which changes over contacts CC while holding the exchange line. The local connection is set up over a "call back" line finder CBLF, a group selector CBGS, and a final selector TFS. At the end of the local call, subscriber A may return to the main exchange call by again dialling the digit "1" which restores the exchange line relay set ELC to its original condition and releases the local "call back" connection. Alternatively he may hang up, thereby causing the exchange line relay set ELC to send a signal through the finder CBLF and group selector CBGS to the final selector TFS, which initiates the operation of a transfer finder TF and a transfer allotter TA to associate the final selector TFS with a transfer control relay set TCRS which operates to restore the exchange line relay set ELC to its original condition. The called subscriber B is then in connection with the exchange line EL by way of final selector TFS, finder TF, and relay set ELC, the rest of the equipment being released. The exchange line relay set ELC may also be controlled from an operator's position in the P.A.B.X. The circuits of the final selectors FS and TFS are identical, and the line finders LF and CBLF and the group selectors GS and CBGS are of standard types. Detailed operation: local call. A final selector FS, Figs. 2 and 3, is seized by a loop from a group selector GS, relays A and B being energized. The first dialled impulse train received is repeated by relay A to vertical magnet VM, relay C holding during impulsing. When C releases at the end of the train, relays E and IR are energized. A wiper-switching relay WS may or may not operate at this stage, according to the level of the group selector over which the final selector has been reached. It will be assumed that WS does not operate. The final dialled impulse train is repeated by relay A to rotary magnet RM, and the release of C at the end of the train completes a test circuit over wiper P1. This encounters battery if the called line is idle, and relays H and HA are operated. Relay E now completes its slow release, followed by IR, and ringing current and tone are applied from leads 16, 18 and 11 respectively. The reply of the called subscriber operates relay F which trips the ringing and completes the connection. Battery feed is provided over relay D which operates, bringing about battery reversal on the incoming line and energizing relay C. The equipment is arranged for last party release. Breaking in. If the called line is busy, relays H and HA are not operated, and the release of relay E applies busy tone from lead 12. The caller may then break into the established connection by dialling "O". Relay A responds to the impulses, and relay C operates at the first break to complete a charging circuit for a condenser QA. At the end of the impulse train, relay C releases and the condenser QA discharges, bringing up relay PF which locks to the busying earth on the called subscriber's P-wire and completes a talking circuit to the busy line. A statistical meter is operated over lead 13, and relays C and E are energized. The caller may now request the engaged subscriber to hang up. If this is done, relays PF and C fall back and the test circuit over wiper P1 is completed, the call then proceeding in normal fashion as described above. Call from P.A.B.X. subscriber to main exchange subscriber. The caller dials "O", raising the wipers of the group selector GS to the tenth level where they hunt for an idle exchange line relay set ELC, Figs. 4 and 5, which is marked by battery on the P-wire. The relay set is seized by a loop over the group selector multiple GSM and relays 1A, AA, 1B, BB, and BA are energized, completing a loop ME to the main exchange. The calling subscriber receives dial tone and dials the wanted number, the impulses being repeated to the main exchange equipment by means of relays 1A and AA. The impulsing loop is improved by the operation of relay IC during each impulse train. When the called subscriber replies, battery reversal brings up relay 1D and the call proceeds in normal fashion. Release is controlled by the P.A.B.X: subscriber, relay RB preventing the application of idle marking battery until the main exchange line has been cleared. Call back. If the P.A.B.X. subscriber wishes to call another P.A.B.X. subscriber while holding the main exchange connection, he dials the digit "I". Relays 1A and AA respond, and their release brings up relays W and 1C. When 1A and AA re-operate, relay X is energized and 1C is released, bringing up relay SW which releases W and X and extends earth over a start lead 30 to a call back line finder CBLF. Relay SW also connects idle marking battery to the P-wire leading to the finder CBLF. The finder hunts and stops at the marked P-wire, relay K operating in series with the test relay in the finder to extend the calling subscriber's line through the finder CBLF to the associated call back group selector CBGS and to complete a loop holding the main exchange connection. The calling subscriber dials the required number, setting the group selector CBGS and a final selector TFS which then returns earth over the D-lead to operate relay P. When the called subscriber answers, battery reversal brings up relay DD to open the impulsing loop, and the local connection is complete. At the end of the conversation, the calling subscriber may return to the main exchange call by again dialling the digit "1". Relays W and X operate as before and the release of 1C at the end of the digit brings up relay 1CO to release W, X, and SW, followed by K and DD. The main exchange connection is restored and the local connection is released, the removal of earth from the D-lead allowing relay P to fall back, followed by 1CO. Transferring call. Alternatively the calling subscriber may transfer the main exchange call to the subscriber called in the local connection. To do this he hangs up, releasing relays 1A, AA, 1B, and BB to extend battery over the D-lead through the line finder CBLF and the group selector CBGS to the final selector TFS, Figs. 2 and 3, where relay TF operates. This applies a marking potential to the test lead KL of the transfer allotter TA and extends earth over start lead ST to the transfer control relay set TCRS, Figs. 6 and 7. There relay ST operates and energizes the latch magnet LMA of the transfer allotter TA which hunts for the marked lead KL from the final selector TFS. When this is reached, relay FK operates to stop the switch and bring up relay 1BB. Relay ISW operates, and ST and FK fall back, completing a circuit for the latch magnet LMF of the transfer finder TF which begins to hunt. In the final selector TFS, Figs. 2 and 3, relay KL operates, in series with FK in TCRS initially, and removes earth from the D-lead to the group selector, also bringing up relay AL which removes the earth from start lead ST. In the exchange line relay set ELC, Figs. 4 and 5, the removal of earth from the D-lead releases relay P, thereby extending marking battery over the M-wire to the transfer finder TF, Figs. 6 and 7. When TF finds this marking on bank TF4 or TF8, relay FK operates in series with 1CO in the relay set ELC to stop the switch and bring up relay RL which extends earth over wiper AL7 or AL15 and lead RL to operate relay HF in the final selector TFS, Figs. 2 and 3. This releases relays AL and TF and extends the local called subscriber's line through the transfer finder TF to the exchange line relay set ELC. Here (Figs. 4 and 5) the operation of relay 1CO releases the connection to the local calling subscriber over group selector GS, and the extension of the local called subscriber's line energizes relays 1A, AA, 1B, BB, and BA. Relays SW, K, DD, and 1CO fall back, initiating the release of the transfer control relay set TCRS and completing the connection of the local called subscriber to the main exchange through final selector TFS, transfer finder TF, and exchange line relay set ELC. Safeguards. If a P.A.B.X. subscriber in calling back dials a digit other than "1", the second release of relay 1A brings up relay G which prevents the operation of relay SW and the seizure of a line finder. The calling subscriber receives no dial tone and may try again. If a calling subscriber attempts to transfer a main exchange call before the local called subscriber has answered, i.e. if he hangs up and releases relays 1A, AA, 1B, and BB before relay DD has operated, relay R is energized and causes the calling subscriber to be rung and reconnected to the main exchange. This also happens if the calling subscriber attempts to transfer a main exchange call to a subscriber who is barred from making or receiving such calls, indicated by earth on the D-lead of the line circuit. The release of relay C in TFS at