Automatic Trouble Diagnosis of Complex Logic Circuits

In November, 1960 Bell Laboratories started its field trial of an experimental electronic telephone switching system in the town of Morris, Illinois. This system (extensively described elsewhere)1 was one of the first attempts to introduce electronics on a large scale into telephone switching and as such, brought us face-to-face with a new class of problems, especially in the field of maintenance of centralized telephone equipment. The problems of maintaining an electronic telephone switching system are formidable, but as will be seen presently, the tools naturally available for this maintenance are powerful. This paper will deal with the problem of maintaining the most complex portion of the experimental electronic telephone switching system, the central control. 1177 1178 T H E BELL SYSTEM TECHNICAL JOURNAL, JULY 19G2 I I . RELIABILITY AND MAINTENANCE OBJECTIVES The character of a commercial telephone system as a whole imposes unusual maintenance objectives for its component parts. The vital role of a central office demands that it have an extremely low downtime. At the same time, since the telephone system is so widespread and cannot be concentrated in a few key locations, another objective is that it be maintainable by telephone system craftsmen. These are extremely difficult requirements. In order to maintain a sufficiently low downtime with devices currently available, it is necessary to provide some redundancy in the equipment so that single troubles do not cause the entire system to fail.