Patent GB-673249-A (GB)
Title: Improvements in or relating to arrangements for private branch telephone exchanges
IPC: H04Q3/00
Filed: 1950-04-12
Abstract: 673,249. Semi-automatic exchange systems. TELEFONBAU UND NORMALZEIT GES. April 12, 1950 [Dec. 4, 1948; Dec. 4, 1948; Dec. 4, 1948; March 7, 1949; Sept. 19, 1949], No. 9001/50. Class 40.(iv). In a semi-automatic PBX a separate call indicating device is provided at the manual position for each subscriber's line and the device of a line is actuated in a different manner according as a predetermined number for calling an operator has been dialled or as another predetermined number for calling a public exchange has been dialled over a subscriber's line not entitled to be automatically connected to the public exchange. In the embodiment described internal connections are set up automatically and privileged subscribers can obtain access to exchange lines by dialling but non-privileged subscribers on dialling the public exchange number initiate a calling signal to the operator who completes the connection manually. Incoming calls are completed manually. A subscriber may set up an interim connection to another PBX subscriber while holding an exchange line and may transfer the exchange line to the other subscriber if desired. PBX subscriber initiates call to public exchange. When a subscriber at station N (Fig. 1) lifts his receiver his relay T is energized over its left-hand winding and operates over its first step to close contact tl and to cause a free call finder AS to hunt for the calling line. On finding relays C, V, W, M and A pull up and dialling impulses are repeated by A to a magnet F of a group selector GW. This magnet controls the selection of a set of wipers. On the first movement contacts ko operate and at the end of the train of impulses V relapses and during the release time of W (which follows) a magnet S is operated which renders the wiper set chosen operative. As soon as W is back, F is again energized and releases the wiper carriage which falls freely over the selected decade under the influence of gravity or like stored energy until P pulls up to a free outlet. Assuming that the digit 2 is dialled to obtain access to the public exchange the group selector GW tests in the second decade to which connecting links AU of exchange lines AL (Fig. 2) are connected by terminals la, lb, lca and ld. The test relay P pulls up in series with VA of the link AU, switches through to operate A (Fig. 2), and busies the link by shortcircuiting its left-hand winding. Testing class of calling line. In the case of privileged subscribers the call indicating lamp L is connected by the jumper VAB directly to the negative pole, but for non-privileged subscribers it is connected by HAB to a second-step contact ts of relay T which extends it to the negative pole over a contact tr common to all the subscribers' lines. On the operation of VA circuits are completed for TR and VH, and TR operates to disconnect the supply voltage from the lines of all non-privileged subscribers. VH can thus be energized only if the calling line is that of a privileged subscriber, in which case VH switches through and brings up the engaged relay BR which disables VB. Completion of call from non-privileged subscriber. VA brings up VB, which in turn releases VA and the test circuit over wire lca is opened to release P. Relay A (Fig. 1) reoperates and brings up X over winding I to connect an interrupter U to lead d and flash the call indicating lamp L. This indicates to the operator that a non-privileged subscriber desires connection to the public exchange and if lines are available she inserts the plug ST (Fig. 2) of an idle line into the subscriber's jack K (Fig. 1) without entering the connection and actuates key ET (Fig. 2). Relay VH is thus operated followed by BR and relay C (Fig. 1) is short-circuited to release the switches AS and GW. Test relay P (Fig. 2) pulls up in series with the right-hand winding of T (Fig. 1) and switches the line through. Relay A (Fig. 2) operates to the loop and brings up VA and the connection to the public exchange is completed. All call finders busy. In this event the calling subscriber operates his key E to flash his lamp L and call in the operator who may complete the call manually if desired. Interim connections to other subscribers. To effect a call to another subscriber while holding an exchange line the subscriber momentarily operates his key E. Relay X (Fig. 2) responds followed by RA and RB and the subscriber's line is switched over to an internal enquiry connection RF (Fig. 2) while the line AL is held. A free call-finder AS hunts for the connection RF and the subscriber controls selectors GW and LW (Fig. 3) to reach the wanted line. On the reply of the wanted party relay Y (Fig. 3) pulls up, switches through and disconnects M (Fig. 3) which was operated while the connection was being set up. During the release time of M ground is connected to terminal ld1 and thence to ld2 to bring up K (Fig. 2). Transfer of exchange line to second subscriber. Having set up an interim connection, if the original subscriber replaces his receiver relay A (Fig. 2) relapses followed by VA and VH. The subsequent operation of VB brings up HZ of the connecting link and AB common to all exchange lines. Relay RA falls away releasing the interim connection and HZ connects ground to ld2, thus bringing up UM (Fig. 3) to release P (Fig. 3) and hold V and Y. Relay T (Fig. 1) of the second subscriber has its right-hand winding de-energized and restores its contacts ts, but remains operated over its first step over the left-hand winding. Another free call-finder therefore hunts for the second subscriber's line and the heavy current in the test circuit resulting from the connection of the right-hand windings of V (Fig. 3) and T (Fig. 1) in parallel brings up PU (Fig. 1) as well as T. V being differential relapses and releases the selector LW. Relay PU transmits an impulse to magnet F and on the subsequent operation of S the group selector hunts over the first decade, all exchange lines appearing in this decade on terminals la, lb, lcu and ld (Fig. 2). GW then finds the exchange line whose relay HZ is up, whereupon P (Fig. 1) and VA (Fig. 2) pull up. Relay AB being up, relay TR (Fig. 1) is disabled, so that VH comes up independently of the class of subscriber and switches through. Relays HZ, AB, and RB relapse. If VH is not energized because the second subscriber is not entitled to call the public exchange, TR (Fig. 2) pulls up slowly and releases BR, whereupon the line AL is freed. All outlets of group selector busy. GW steps on to the last contact of the decade, the test contact of which is connected to battery over resistor Wd. Relay PA comes up as well as P and removes the short-circuit from XII which operates and connects ground to wire d to light the lamp L steadily. The operator earths the sleeve of jack K to shunt down C (Fig. 1) and release the finder AS and answers the subscriber. If free lines (other than those in the banks of GW) are available she then completes the desired connection. Calls to the operator. By dialling a particular digit, say 3, a subscriber obtains access to outlets of GW all wired in the same manner as the last contacts of the decade giving access to exchange lines. Relays PA and X come up as above and lamp L is lighted. Allowing automatic access to exchange lines by non-privileged subscribers. During light traffic periods the operator may operate key TT to disable relay TR. Premature release and faulty circuits. If a subscriber making an interim connection hangs up before relay K has operated, relay HR pulls up and lights the lamp LA. Again, if the switch-over on an interim connection is not completed a time delay relay TB (Fig. 2) comes up and energizes HR. Incoming calls from public exchange. Relay AR responds to ringing current and switches in HR to light the calling lamp. The operator replies and effects the desired connection using plug ST. Night service. Key GNS (Fig. 2) is opened so that HR cannot lock up and lamp LA is thus lighted only as long as ringing current is being received.