Recent times have seen a rapid growth in the development of digital systems. The cost of the hardware has shown a remarkable decline allowing designers to build systems which are extremely complex. This in turn has resulted in these complex systems being used in aplications which require a high degree of reliability. With the advent of a distributed processing environment a large number of quasi independent modules are now interconnected to form megasystems. The real time applications in which these systems are used require that their designee build into them efficient means of failure detection and in most cases correction mechanisms. Circuit level fault diagnosis , correction techniques are used widely in building digital systems. However the complexity , the interconnections of the various modules involved in the structure of these systems require fault diagnosis at a system level too. The importance of system level diagnosis has been recognized by many in the recent past , has encouraged a lot of research which has now reached quite an advanced stage.