Patent GB-176911-A (GB)
Abstract: 176,911. Western Electric Co., Ltd., (Western Electric Co., Inc.). Dec. 22, 1920. Automatic and semi-automatic exchange systems.-In a final selector adapted to extend connection both to single lines and to lines of multi-line groups, a single test relay performs successive tests upon a line engaged by the switch, to determine its character and control the operation of the switch accordingly. When connection is required with one of a group of lines leading to, say, a private branch exchange, the appropriate number is dialled and the first line in the group is engaged in the usual manner. A first test is made when the test relay 106 is connected to the test contact 148 in position 6 of the sequence switch 117. If the first line is free the relay 106 responds, the sequence switch moves to position 7, and the call proceeds in known manner. Between positions 6 and 7 a resistance 149 is included in the test circuit and a second test is made, but the inclusion of this resistance has no effect. If the first line is busy, the relay 106 remains inert when the line is first tested, the line relay 100 is energized, and a circuit is completed for the power magnet 135, which rotates the brushes until an idle line is found and the test relay responds, a second non- effective test then being made as before. The test contacts of single lines and the last lines in multi-line groups carry such a potential that on the first test the test relay 106 responds whether the line is free or busy, in order to break the circuit of the power magnet 135 in the case of last lines, and prevent its energization in the case of single lines. On the second test, when the sequence switch passes between positions 6 and 7, if the line is busy the inclusion of the resistance 149 in this case causes the relapse of the test relay 106, whilst if the line is free the relay 106 is held energized and the call proceeds. The switch may be adapted for single lines only, by opening a contact 152. If a wanted line is free the test relay responds on the first test and the call proceeds, but if the called line is engaged the test relay remains inert, the energization of the power magnet, however, being prevented by the opening of the contact 152. The inclusion of the switch when used for single lines only, and for both single lines and multi-line groups, is described in greater detail below. In Fig. 1 the sequence switch contacts indicated by a large triangle are permanently connected to the spring which they are shown touching. Connection to single lines only; contact 152 open. The final selector extends the calling line to the contacts 163, 164, 148 of the wanted line in known manner, the sequence switch 117 moving to its 6th position and subsequently to its 7th position in which a circuit (118, 119, 134) is completed to drive it to position 9. In passing between positions 6 and 7 a short circuit is removed from the resistance 149 which is now included in the test circuit. The potential of the test terminals of ordinary subscribers, however, is such that this has no effect on the testing operation. If the wanted line is busy the test relay 106 remains inert and the switch 117 is arrested in its 9th position, in which the relays 100, 142 are energized and busy tone current is transmitted to the calling subscriber from the source 175. Release is effected in the usual manner when the calling subscriber hangs up. If the wanted line is free the test relav 106 is energized in the 6th position of the switch 117 and on the 9th position being reached, a further circuit 118, 119, 140, 120 is completed for the switch, which moves to its 11th position. (In position 9 the relay 114 is energized over contact 155 and the third conductor 144). In position 10 the line relay 100 is energized and locks up over its contact 109, a circuit 118. 119, 116, 109, 111. thereby being completed to drive the switch from its 10th to its 11th position, in which the relay 114 is energized over the interrupter 156 and the switch moved to position 13, where the relay 114 is again energized over a second interrupter 157 and the switch driven to its 14th position. Between positions 12 and 13 a continuous ringing current is applied to the called line from the source 158 which is replaced by an intermittent current over the interrupter 161 when the switch 117 arrives at its 14th position. When the called subscriber replies, the relay 142 responds and deenergizes the line relay 100 which drives the sequence switch into the talking position 15. Release takes place in the usual manner on the calling subscriber hanging up. Connection to both single lines and multi-line groups; contact 152 closed. The final selector extends the calling line to the first line in a PBX group in known manner, and if this line is free the test relay 106 is energized in the 6th position of the sequence switch 117 which again energizes in circuit 131, 132, 111 and moves to position 7. Between positions 6 and 7 the relay 106 remains energized in spite of the inclusion of the series resistance 149, and the call proceeds as described in the previous paragraph. If the first line, however, is engaged, the test relay 106 remains inert and a circuit is completed in the 6th position of the switch 117 for the line relay 100 (over the contacts 151, 152, 150). Relay 100 opens contact 132 to hold the sequence switch in position 6, and closes a circuit 111, 109, 136 for the power magnet 135, which rotates the brush carriage until a free line is engaged, when the test relay 106 energizes and breaks the circuit of the line relay 100, de-energizing the magnet 135 and arresting the rotation of the brushes. The sequence switch is also driven into position 7 and the call proceeds as before. In order to prevent the brushes from passing the last PBX line if all the lines in the group are engaged, the potential of the test terminal of the last line is raised by a connection to it from battery through a resistance, in parallel with the cut-off relay 146 and the metering relay 147, so that in the 6th position of the switch 117 the test relay 106 responds to arrest the brush rotation whether the last line is free or busy. In passing between positions 6 and 7, if the line is idle the inclusion of the resistance 149 will not de-energize the test relay 106 and the call will proceed as described above, but if the last line is busy the insertion of the resistance 149 will de-energized the test relay, and a circuit will be completed for the line relay 100. which will hold the sequence switch 117 in its 9th position, where busy tone is transmitted to the calling subscriber. Single lines are equipped in the same way as the last lines of groups so that on the first test in position 6 of the sequence switch 117, the test relay 106 responds whether the line is busy tor idle, but when the resistance 149 is inserted in the test circuit between positions 6 and 7, the relay 106 relapses or is held according to the condition of the line.