The manager's first step following the decision to proceed with the SCA is to select an organization to perform the analysis. Should the system contractor possess his own analysis capability, it is likely that the effort will be per- formed "in-house." If this is not the case, the manager may arrange with an outside organization to provide the SCA. (For additional information concerning companies which perform this service contact The Office of the Assistant Secretary of the Navy (Shipbuilding and Logistics) Reliability, Maintainability, and Quality Assurance, Washington, DC 20360-5100.)
Selection of the SCA contractor is a critical decision and must be carefully weighed. Useful criteria for evaluation and selection of the contractor are provided in the NAVSEA guidance document (Reference 1). While these criteria will not be discussed in detail here, they include consideration of the validity, advantages, and known limitations of the procedure to be employed.
The contractual arrangements between the user and the provider of the SCA also require the manager's attention. Both the NAVSEA and RADC documents (References 1 and 2) provide considerable information to assist in preparing contractual agreements between the user and the SCA contractor, including sample statements of work for inclusion in a contract.
Assuring that the completed sneak circuit analysis will provide results that are satisfactory to the project requires that the manager participate in the initial basic decisions. Decisions concerning three major analysis factors are of prime concern. These factors are:
Level of Analysis
The approach decided upon in each of these areas will exert a major influence on the outcome of the analysis. The effects will be felt jointly in the results achieved and in the time and dollars expended in the analysis. For this reason, it is essential that the manager understand and participate in these decisions. A summary of the effects of the decisions is presented below.
Level of Analysis
(System or Black Box)
Sizes the SCA in relation to depth of analysis and (System or black box) defines type of results, cost, and time reqmts.
Clarifies methods for examining interactions and further defines results.
Sets limits on the analysis by defining specific operational states to be considered and examined.
LEVEL OF ANALYSIS
The SCA may be conducted at two levels of detail. In order of increasing detail, these are
The level-of-analysis decision is the single most important control over the magnitude of the SCA effort. It must be made by the organization needing the SCA.
The system-level analysis is the less expensive and requires the shorter calendar time to complete. It is also the preferred level for the initial sneak circuit analysis because under normal conditions of system design, it is difficult to achieve a comprehensive and detailed understanding of the complex interactions among subsystems. This is one of the factors that can generate sneak conditions which have major system consequences. The results of the system-level SCA may be valuable in focusing the more detailed SCA in follow-on tasks into specific subsystem areas.
A system-level analysis considers all interfacing and input/output interactions between subassemblies within black boxes, and between the black boxes themselves. The black box (subsystem)-level SCA is a more detailed analysis in which the individual elements in the subassemblies are individually represented as combinations of switch, diode, and resistor analogs in network tree sets.
A key factor to be considered in the decision on level of analysis is the availability of design data. Since the black-box SCA requires in-depth information, it may be reasonable to do a system level analysis at an early stage of design development when the system is defined but all the detailed design data for the circuit cards are not available. The trade-off is early analysis versus problems missed due to lack of complete information.
The partitioning of a complex SCA task consists of deciding on the approach to segmenting the large analysis into more manageable subtasks. The choice may be one of the following:
Partitioning by hardware elements
Partitioning by function
Partitioning by hardware elements consists of analyzing each circuit board or black-box as an entity and then examining the interaction between the elements. Partitioning by function consists of segmenting the system into functional parts, in which major system input/output functions are traced through all circuitry, and the functional path is then analyzed.
The selection of the partitioning approach is driven to some extent by the characteristics of the system being examined. If the hardware elements perform relatively isolated functions, then either approach might be appropriate. If, however, there are significant and complex interactions between hardware elements in a multifunction system, then partitioning by function might be more efficient. These two approaches are not mutually exclusive and can be used in combination during sneak circuit analysis of a large system.
To perform the analysis of a partitioned system, it is necessary to select the operational states that will be examined. The operational states are called "time intervals" since they represent snapshots or slices in time of the circuit function. The initial network trees developed in the SCA have all switches closed and indicate all possible paths. The agreement on the time intervals to be examined permits the development of a "switching matrix" which indicates the relative position of each switch in the network tree for each time interval being considered. The time intervals focus and limit the SCA task and must be chosen in close consultation with, and with the agreement of, Navy engineers. Defining the time intervals forces detailed study and full understanding of the functioning of the system. This can contribute to bounding the size of the SCA and focusing the analysis effort on specific states of interest.
CHECKLIST FOR INITIATING SCA
Four basic conditions must be met before a sound decision to initiate a sneak circuit analysis can be reached. These conditions are expressed in the brief checklist presented in Table 6
TABLE 6. CHECKLIST
FOR INITIATING SCA
- Has the system been evaluated for the
potential benefits of SCA?
- Has the cost of performing SCA been estimated?
- Have the elements of the analysis task been
properly defined, including level of analysis, partitioning approach,
and time intervals?
- Is the design definition adequate for the