Patent GB-557800-A (GB)
Title: Telephone systems
IPC: H04Q3/42
Filed: 1942-09-04
Abstract: 557,800. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. (Western Electric Co., Inc.) Sept. 4, 1942, No. 12510. [Class 40 (iv)] In a system in which subscribers are provided with reed transmitters for generating a combination of frequencies for each digit, direct current is removed from the calling subscriber's line while the register-sender is in use. If the subscriber abandons the call during this time, a disconnect signal consisting of a single frequency is sent to release the register-sender. If he hangs up momentarily, however, the disconnect signal causes the transmission of a " re-order " tone to instruct him to initiate the call afresh. Other features of the system described consist in the adjustment of the intensity of dialling tone in accordance with the resistance of the calling line and the connection of high frequency current to the line during sending to keep its resistance to a value comparable with that when direct current is source 224 is connected across the line at this time. The digits received by relays 400 ... 404 are transferred to relay registers, Fig. 5, and the release of relay 406 at each digit restores those relays after the transfer has been effected. When the call has been set up in accordance with the stored digits, relay 212 operates and brings about the relapse of marginal relay 138 to advance the sequence switch of the selector. In the talking position, relay 207 energizes and causes the release of the sender. A disconnect signal while the sender is in use is received by one of relays 400 ... 404 and brings about the operation of relay 208 which opens relay 202 to reconnect battery and ground to the line. If the loop is open, line relay 200 fails to energize and the relapse of sluggish relays 217, 218 connects up relays 230, flowing. The subscriber's transmitter comprises five reeds 100 which are plucked in pairs by the depression of code bars 102 under the control of keys 142, the pick-up coil 101 being connected to the line over springs 107 which are actuated by a common bar 104. For the sending of the disconnect signal, the switchhook is coupled to one of the code bars through a lever 109 which also serves to open the line at springs 117 after a short interval determined by a fan-governor 115. When a call is initiated, these springs are closed, the line is extended to a finder-selector link F, S and a registersender is seized over a link S4, S5. Line relay 200 energizes and connects up relay 202 which removes the battery and ground from relay 200 so that it falls back and operates sluggish relays 217, 218 to extend the line to a hybrid transformer 300. Before relay 202 pulls up, negative potential dependent on the resistance of the line is applied to the grid of a valve 314 the anode of which is connected in parallel to four gas-discharge tubes 315 ... 318 arranged to ionize at progressively increasing voltages. As a result, one or more relays such as 323 operate to connect appropriate impedances across the dialling tone transformer 319 which feeds the line through the hybrid transformer 300. The signal currents generated at the substation pass by way of the transformer 300 to five parallel circuits comprising filters 303 ... 307, detectors, 309 ... 313 and relays 400 ... 404. A high frequency 207, which bring about the operation of marginal relay 134 to restore the selector and release the sender. If the loop is found to be closed, however, line relay 200 energizes and re-operates relay 202, whereupon re-order tone is sent, and the subscriber must hang up and start the call again. The switch contacts 419, 422, 423 and relays 417, 418, 420 enable a particular frequency or any one of three frequencies to be used for the disconnect signal.