Methods of Interpreting Diagnostic Data for Locating Faults in Digital Machines

The problem acute with the panding use in techniques for of locating faults in digital systems is becoming more increased complexity of these machines and their exreal-time applications. The need for automatic testing locating faults by automatic programmed diagnostic 289 290 T H E BELL SYSTEM T E C H N I C A L J O U R N A L , FEBRUARY 1967 tests has been realized for some time and is being actively pursued in many areas. 1 , 2 This need has particular urgency for a system designed to provide uninterrupted service. Such systems require extensive subsystem duplication as well as facilities for rapid fault isolation, location, and repair. The Bell System's No. 1 Electronic Switching System (No. 1 ESS), designed to control telephone switching functions, is an example. 3 When machines were smaller and simpler, even the use of diagnostic tests to provide relevant trouble location symptoms was rare. An expert could usually locate the trouble by a quick survey of the behavior of the machine. This may be described as the "eureka" approach. With large complex machines, however, analysis of symptoms by mere observation is lengthy and costly. Strange behavior sometimes occurs which even the expert is hard put to explain. Even the addition of test points and special diagnostic tools such as programmed testing may not immediately clarify the situation. Further, the sheer quantity of test data necessary to isolate a trouble to one of the myriad of components comprising the machine often demands preprocessing before presentation to the maintenance craftsman.