Automatic dialing and electronic selection on railroad message and dispatcher circuits

Low-level tone-signal techniques are used for automatic calling and selection on multiparty single-line facilities. The subscribers, equipped with transistorized transmitters and receivers, transmit single tone signals for seizure, dialing, and release. These signals are propagated along the wires to the terminal equipment, where they are converted to d-c pulses and stored for interpretation. This interpretation is accomplished by relay logic in order to decide whether the call is for a party on the same line, or for a party within the PBX (Private Branch Exchange) to which the line is terminally connected. In the first case, the called number is converted into an amplitude-modulated tone, where the frequency of the carrier and the modulation correspond to the first and second significant digits dialed, respectively. This tone is then retransmitted for selection of the desired party. At the called subscriber's station, the electronic selector receiver, which takes the place of the electromechanical selector, responds only to the particular carrier corresponding to its number. In this receiver, the envelope is detected and the recovered modulation tone is amplified directly to drive one out of a maximum of five tuned ringers. A total of 60 stations can be used on one pair of wires having a maximum attenuation of 30 db (decibels).