Patent GB-283973-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04Q3/00
Filed: 1928-01-21
Abstract: 283,973. Telefon-Apparat Fabrik E. Zwietusch & Co., Ges. Jan. 22, 1927, [Convention date]. Semi-automatic exchange systems.-A single trunk line terminating in a main exchange leads to a number of small sub-exchanges, and all calls are set, up by an operator at the main exchange. A chain of relays E1 - - E10 at each sub-exchange is adapted to respond to two trains of impulses transmitted from the main exchange, the first train serving to determine the desired sub-exchange by energizing a tens relay Z thereat in order to render the second train effective. Impulses are sent over the b-wire of the trunk, while the a-wire is used for the transmission of supervisory signals. Means are provided for recognizing and dealing with faults on the subscribers' lines and on the trunk line,. Call from sub-exchange to main exchange. When the subscriber 10, for example, removes his neceiver, line relay TO energizes and locks in series with relay D, which brings relay Ab(I) (previously short-circuited) into the circuit. These relays open the starting circuit 121. Feed relay R energizes over the calling loop, and at the main exchange, calling relay AR energizes over the a-wire in series with relay A (I), operating call indicator ASZ over the b-wire in series with relay J at each sub-exchange. Sluggish relays V1, V2 energize. The operator answers by plugging into jack VK1, connecting relay AB to the b-wire, so that it energizes and connects up sluggish relays AV1, AV2, which disconnect call indicator ASZ. Sleeve circuit AC energizes and disconnects relay AR. Relay A(I) is de-energized when the answering key is thrown, and the call is set up by the operator in the usual manner. Switching relay K(I) energizes and locks at the sub-exchange of the calling subscriber, in which relay D is energized, but not at the other sub-exchanges. Upon release, feed relay R falls back, short-circuiting relay D and connecting relay A(I) to the a-wire to operate a clearing signal. The operator withdraws the plug, releasing relays AB. AV1, AV2 at the main exchange and sending a releasing impulse over the b-wire to relay J at each sub-exchange, which causes a momentary operation of relays U, E1, P, JC in a manner which is described in the next paragraph. The locking circuit of relay K(I) is opened, and relays V1, V2. TO relapse, relay A<b>(II) being held until relay JC falls back. Call from main exchange to sub-exchange. The operator tests the trunk with the tip of plug WSt, and if it is idle, inserts it in jack WK1. Sleeve relay AC energizes and disconnects calling relay AR. Relay AT energizes and connects the b-wire to jack WK1. Relays J, V1, V2 energize at each sub-exchange. Two sets of impulses are now transmitted by means of the dial NS to select the wanted subscriber, the first digit serving to select the required sub-exchange. Assuming the wanted number to be 10, at the first deenergization of relay J, relay U energizes and connects up relays E1(I), JC. The latter being slow-to-release holds during the impulse train, and operates relay Ab(II), which opens the starting circuit 121. Relay E1(II) locks in series with relay P, which connects up relay A(II). At the subsequent energization nf relay J, relay U falls back and relay St(I) energizes. The first impulse train being completed, relay JC relapses. and at the first sub-exchange, tens relay Z energizes over contact 59 and locks in series with relay ZH. Relays E1, St, P fall back, and relay WW energizes and locks. At the other subexchanges, contacts 62, 64, &c. of relays E2, E3, &c. respectively are connected to tens relay Z, the remaining contacts at all the sub-exchanges being connected to relay Sp(I). In the present case, therefore, relay Sp(I) energizes at the other sub-exchanges and locks over (II), disconnecting relay V2 as well as relays E1, St, P in order to render the second digit ineffective. At the first sub-exchange, the first impulse of the second train results in the energization of relays E1, JC, P, Ab(II), St(I). At the second deenergization of relay J, relay St(II) holds in series with relay G(I), which locks over (II). and relay E2(I) energizes and locks over (II) in series with relay P. At the subsequent energization of relay J, relay St falls back (both windings being disconnected), and opens the locking circuit of relay E1(II). At the next impulse, relay U energizes again, with the result that relay St re-energizes in place of relay G, while relay E3 energizes in place of relay E2. After the ten impulses of the series have been received, relays G, E10 are energized. Relay JC now falls back, and switching relay K(II) energizes, locks over (1) and connects up line relay TO of the wanted subscriber, which locks first in series with relay D and then in series with relay Ab(I). Relay Sp(II) energizes and disconnects relay V2. which in turn releases relays Z, ZH. WW. P. E10, G. The operator inserts a calling plug into jack VK1, and removes the selecting plug WSt, and operates the ringing key. Relay W energizes and connects up relay F. which short-circuits condensers C1, C2 to extend the ringing circuit to the wanted subscriber. On reply, during a pause in the ring- ing current when relay F is back, feed relay R energizes and disconnects relay A(I) from the a-wire. When the called subscriber hangs up, a clearing signal is given by the reconnection of relay A (1) to the a-wire, and all the relays are released upon removal of the calling plug. Local call in the same sub-exchange. A calling signal is given at the main exchange, and the operator tells the subscriber to hang up. The wanted subscriber is then selected as alreadv described, and both subscribers are rung. If the calling subscriber replies first, be is asked to hang up until the called subscriber replies. Call between sub-exchanges. The wanted subscriber is selected as before, and ringing takes place. At the sub-exchange of the calling subscriber, switching relay D is de-energized as the result of dialling, but relay Ab(I) remains held, while at the sub-exchange of the wanted subscriber, switching relay K is energized, so that at these sub-exchanges, relays W, F operate and extend the ringing circuit. When the calling subscriber replies, relays R, D energize and connect up switching relay K. Failure of either subscriber to hang up is indicated by the operation of call indicator ASZ when the operator withdraws her plug in response to a clearing signal. Trunk engaged. Relay V1 is energized, and the starting circuit 121 is open at contact 89. If a call is initiated, self-interrupting relay Bes operates to give a busy signal. Faults. A calling condition may be simulated by a loop or earth on the b-wire of a subscriber's line. If the operator answers and obtains no reply, she dials the digit 10 to energize relays E10, P at each sub-exchange. A circuit is thereby closed through the fuse Si corresponding to the faulty line, and when this melts the contacts such as 140 are opened, and a clearing signal is given. To determine the sub-exchange which contains the faulty line, the operator dials. the tens digit of each sub-exchange in turn, and inserts a plug connected to an indicator which responds to battery on the a-wire, which is supplied at the sub-exchange in question over busbar BB when tens relay Z energizes. A subscriber whose a-wire is earthed may initiate a call while another call is proceeding, in spite of the opening of the starting circuit 121. To determine the faulty line, the test clerk inserts a special plug, the tip of which is connected to a head-set and a generator, and tests each subscriber's line. In the case of the faulty line, the current flows directly to earth, and a signal of greater strength is heard than for the remaining lines. A calling condition may also be simulated by earth on the a-wire of the trunk, but, in this case, a clearing signal is givers when the operator plugs in. If the b-wire of the trunk is earthed, relay J at each sub-exchange is shortcircuited and relay K cannot operate. A clearing signal is therefore given as soon as the operator answers a call.