The relationship between subscriber equipment number and subscriber directory telephone number is programmed into the TEX2 exchange by threading a cable for each subscriber through a series of pulse transformers. As each new customer is added another wire must be threaded.

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The Calling Number Generator

 As a result of relay LR operating in the line circuit, the identity of the calling line in directory number form is generated and stored, so that, subsequently, the line circuit can be identified, or marked. On operation of relay LR in the line circuit, a current pulse passes in a jumper wire threading an array of pulse transformers (Fig. 5). This array, known as the calling number generator, consists essentially of four rows of pulse transformers, known as cores, each row representing, respectively, the thousands, hundreds, tens and units parts of the directory number. 

There are 10 transformers in each row, each of these representing a digit 0-9. The jumper is connected to the equipment number (EN) tag from the line circuit and passes through a start core, the appropriate thousands, hundreds, tens and units cores and is terminated on a similar EN tag at the bottom of the field. Where the local exchange uses 5-digit or 6-digit directory numbers, the last four digits only are generated, as these are sufficient to identify the calling subscriber. When the circuit is idle the capacitor C1 is charged to 50 volts (Fig. 4). Operation of relay LR in the line circuit causes the capacitor to discharge around the loop of jumper wire between the EN tags. The jumper forms a single-turn winding on each core through which it passes, and the output signals from the secondary windings of the cores are amplified and stored. To reduce the storage required for the calling-line identity, each decimal digit is coded into a 2-out-of-5 form. The K-relay contact performs two functions; on originating calis it ensures that when the LR relay is dis connected from the line, the capacitor cannot be charged, so that on clear-down any bounce on the switch-hook contacts 

connexion is wired, pass into a temporary high-speed store. From there the signals pass rapidly into a second high-speed store, and the process of selecting a free register is started. When selection is complete the calling line's directory number passes into the register store. The use of two high-speed buffer stores has two advantages; calls originated in quick succession can be distinguished, and time is allowed to select a free register, thus pern1itting the use of inexpensive reed relay access and storage in the register (Fig. 7). This arrangement can handle calls arriving at not less than 100 us apart.