The remarkable characteristic of an automatic telephone system with the 500-switch, as the name implies, is that the mechanical switches have a capacity of 500 lines. Lines enter the switch in 25 multiple frames with wires that are arranged vertically in the switch rack in groups of 20 lines. The lines are bare bronze wires. The selectors are driven by electric motors, and the wiper arm can move in both a circular movement to select the correct contact field and a radial movement to find the correct contact wire in the field. With 25 multiple frames and 20 groups of wires in each switch, a selection can thus be made among 500 lines.
Briefly, a call is connected in a 500-switch as follows.
When the subscriber lifts the telephone receiver, a line finder identifies the number and the line is marked as busy. From the line finder, the call is routed on to a register, and the calling subscriber hears a dial tone in the receiver.
The subscriber dials the number and the digits are stored in the register.
The register starts the first group switch. The switch turns to the correct multiple frame and then the wiper arm extends into the frame to the multiple wires that belong to the desired group of subscriber numbers. Other group switches function in the same manner as the first to connect to the 500 number to which the subscriber being called belongs.
The register is released.
The final selector finds the subscriber number and tests to see if the line is engaged. A ring signal is sent to the number being called and a ring tone is heard in the telephone to which the call is connected. When the subscriber answers, the ring tone is terminated and the number is marked as busy. If the subscriber is busy, the caller receives a busy signal.
When the call has been terminated and both receivers placed on hook, the wiper arms are withdrawn from the frames. In the final selector, both the wiper arm and the selector plate return to their original positions.
In addition to the 500-switch, the system includes a serial converter that produces contact combinations to control the switches, registers that store the impulses from the subscriber’s telephone dial and control the switch movements, batteries, signaling machines, electric motors, call meters, etc.
Author: K V Tahvanainen