Patent GB-1231418-A (GB)
IPC: H04Q3/545
Filed: 1968-11-22
Abstract: 1,231,418. Automatic exchange apparatus. TELEFONAKTIEBOLAGET L. M. ERICSSON. 22 Nov., 1968 [23 Nov., 1967], No. 55626/68. Heading H4K. Translations from directory number to equipment number and vice versa are obtained from groups of storage compartments arranged to be addressed stage by stage in a tree formation, each stage of the tree being a progression from the digits of highest to the digits of lowest significance, the compartments of each stage of the tree except the first and last having addresses of the compartment in the preceding stage contaming the associated preceding digit and also having the addresses of a sub-group of compartments in the following stage containing the associated subsequent digits for which translations exist, the sub-group compartments of the last stage having the address of the associated compartment in the preceding stage and the compartments of the first stage having the addresses of associated sub-groups in the following stage. The invention is described by the example of Fig. 1 in which a directory number 2843 is used to address the storage compartment stages A, B, C, and D, to uniquely identify the equipment AU associated with a storage compartment in the D stage. The first digit 2 is compared with each of the thousands digits in stage A and when matched at address a2 this address is placed in register REG A and the address b5, of the first of a sub-group of hundreds digits in the next stage B for which translations exist, is obtained from associated location A2. Directed by address b5 the hundreds digit 8 is compared with each of the hundreds digits in the sub-group. A match is seen to occur at the first compartment, its address b5 being stored in REG B and a composite address a2 + cp being obtained from associated location B5. The portion cp of this address gives the sub-group of compartments in stage C holding tens digits for which, following digits 28, a translation exists. To obtain cp the address a2 in REG A is subtracted from the composite address in circuit AR1. The tens digit 4 is now employable in comparison with the digits in the sub-group the first compartment of which has the address c p  and a match can be seen to be found in the second compartment with address c(p+1). A further composite address b5+d(r+2) is obtained from the associated location C(p+1) the forward part of which is obtained by subtracting the address b5, which has been stored in REG B, in circuit AR2. The address d(r+2) now indicates the sub-group of compartments in stage D containing the units digits which follow digits 284 and for which a translation exists. A match is seen to occur in compartment with address d(r + 3) which is associated uniquely with the equipment AU and directly accessible from the equipment number. The reverse translation, from equipment number to directory number, is effected by addressing the compartment d(r + 3) the address of which is the equipment number. This compartment contains the units digit of the directory number. The address d(r+2) of the first compartment of the sub-group is obtained and placed in REG D, Fig. 2. A location D(r + 2) in stage D gives the address c(p + 1) of the tens digit in the tens sub-group of stage C with which the units sub-group in stage D is associated. Address c(p+ 1) gives the tens digit of the directory number and from associated location C(p+ 1) the composite address b5+d(r+2) is obtained the backward part b5 of which is obtained by subtracting in circuit AR1 the address d(r+2) from REG D. The address cp of the first compartment of the tens sub-group is obtained and placed in REG C, the address b5 being used to access the hundreds digit of the directory number in stage B. In stage B from associated location B5 the composite address a2 + cp is obtained the backward part a2 being obtained by subtracting cp in REG C. The address a2 gives access to the thousands digit in stage A. All equipment numbers inevitably produce directory numbers, but if at any stage of processing a directory number no match can be found between a directory number digit and the digits in the storage compartment stages, a no translation available signal is produced. An abbreviated dialling process is suggested where the called subscriber has the same first two digits as the calling subscriber. A signal from 2843 that abbreviated dialling is to be employed allows translation of directory digits 28 up to address b5 of stage B. Dialled digits, such as 26, are now used to complete the translation and obtain the equipment number of subscriber 2826.