Step-by-Step Dial Systems

Figure 26. The linefinder makes connection to the calling line, and the connector selects the called line.

Figure 27. A step-by-step system for a 10,000-line exchange. This uses a line switch instead of a linefinder as in Fig. 26.

In a telephone system having more than 100 lines, a similar plan is used, except that one or more groups of selector switches are used between the line finders and the connectors. As an example, a 10,000-line system is divided into 10 groups of 1000 lines each. Each 1000-line group is subsequently divided into 10 groups of 100 lines each. This requires two sets of selector switches as shown in Fig. 27. Suppose calling party 1563 desires party 7861. When the receiver or handset is raised, the line switch selects a vacant first selector, and dial tone is sent out. The party calling 7861 then dials the digit 7. This causes the wipers of the first selector to step up to the seventh level and then to rotate until a trunk to an idle second selector is found. The party then dials the digit 8, and the wipers of the second selector step up to the eighth level and rotate until a trunk to an idle connector is found. Dialing the third digit, 6, causes the wipers of the connector to step up to the sixth level, and dialing the last digit, 1, causes them to rotate to the first contact of that level; the desired telephone, 7861, is then rung.