Rotary Dial System

The switching mechanisms used in this system consist of finder switches, selector switches, sequence switches, and relays.

Rotary Finder Switch. The driving mechanism of the rotary finder switch consists of two flat gears. The driving gear is mounted on a continuously rotating shaft, and the driven gear is mounted on the center shaft of the finder switch. The driven gear is flexible and is held out of mesh with the driving gear by a downward pressure of the helical spring on the armature of the control magnet. The operation of the control magnet removes this pressure from the driven gear, which springs upward and engages the driving gear.

Rotary Selector Switch. The selector switch is similar in construction to the finder switch. The selector has two control magnets. One controls the meshing of the brush carriage driving gear, and the other controls the meshing of the driving gear to what is known as the trip spindle. The trip spindle has ten fingers which are arranged so as to unlatch one of the ten sets of brushes corresponding to the desired level or group of lines.

There are two types of selectors; first, the group selectors, and second, the final selectors.

Sequence Switch, The sequence switch is actually a power-driven relay which permits 18 different switching conditions with one complete revolution of the contacting disks. This switch is used to control ringing, busy signal, and closing of the talking circuit. The sequence switches are associated with the selectors.

References 24 and 25 are suggested for further details on the rotary dial system.

Panel Dial System.^ref.4,19,26 The panel dial system was developed by the Bell System in about 1920 for use in large exchanges. The switching in this system is accomplished on large frames on which are mounted several hundred terminals arranged in a number of groups, or banks. Connections are made to these terminals by brushes which are moved up and down on vertical rods, or elevators.

Except for the line finder, each of the major switching frames, or panels, has five of these banks. Thirty elevators may be located on each side of the frame, each having access to all the lines or trunks connected to the banks. At the bottom of the elevators are flat strips, called racks, which are controlled by magnetic clutches. When the elevator is to be driven upward the associated rack is pressed against continuously rotating electric motor-driven cork rolls.

Figure 30. Schematic diagram of the panel dial system.

When a call is initiated, an idle finder in the line-finder frame moves upward to connect to the calling line (see Fig. 30). The other end of the line finder terminates on a district-selector frame. The operation of the line finder is directed by the associated start and trip circuit, link circuit, and the line circuit of the calling line. At the same time an idle sender also is connected through the link circuit. The sender transmits dial tone to the calling party to indicate that the equipment is ready for the dialing. The sender proceeds to direct -the completion of the call as soon as dial impulses are received. An elevator on the district-selector frame selects an idle trunk line to the proper office. In central offices of large cities, another group of frames, called office-selector frames, are necessary at this point to multiply the possible number of outgoing trunks. Each incoming trunk from other central offices is terminated on an elevator of the incoming-selector frame. The banks of the incoming-selector frame are connected to elevators of final-selector frames. The incoming-selector elevator, therefore, must select a trunk to an elevator on the particular final frame to which the called line is terminated.