Group-start method of subscriber's line identification

The method applies to step-by-step equipment working into sleeve control switchboards, and a brief description of its application to the B.P.O. director system is given. The subscribers' lines are grouped in 100's, each group having a group-start relay and uniselector, to hunt for the calling line. The relay indicates the thousands and hundreds digits, while the final position of the uniselector indicates the tens and units digits. The number indicator, mounted on the keyshelf of the operator's position, consists of a number of identical units. Each unit consists of 2-ph. motor having a rotor with projecting teeth bearing the numerals 1-0 on its periphery. The teeth are attracted by two pairs of magnets, known as pulse and hold magnets respectively. The nominal operating speed is 10 impulses/sec with min. pulses of 20 msec. In operation, an identification key associated with the operator's position circuit, is thrown. This causes a start signal to be transmitted over the junction, causing the o./g. relay set to be connected to a free storage and sending circuit. When the control circuit is free, an identification marking signal is applied to the third wire of the subscriber's line circuit. This marking potential operates the relevant group-start relay and the uniselector hunts for the marked line. Information relative to the subscriber's number is then passed to the storage and sending circuit. The information is then transmitted as 50 c/s a.c. impulses, to the receiving circuit, where it is converted to d.c. impulses to operate the number indicator. Only one line in an exchange can be marked for identification at any one time. The max. holding time of the control unit is 1 sec.