Reliability HotWire: eMagazine for the Reliability Professional
Reliability HotWire

Issue 35, January 2004

Tool Tips

*Can I have circular references in my BlockSim RBD?

Circular references are not allowed in BlockSim. The software uses a methodology called the Path Tracing Method in order to evaluate the reliability of the system in the reliability block diagram (RBD). The Path Tracing Method requires that each RBD has one starting block and one ending block. It then calculates all of the "paths" from the starting block to the ending block that would allow the system to function. Circular references are not allowed because they would result in an infinite number of paths through the system.

In some circumstances, it may be possible to work around the problem of circular references by using mirrored blocks. BlockSim's mirrored blocks can be used to represent a single item with more than one block placed in multiple locations within the diagram. Mirrored blocks can be used to simulate bi-directional paths within a diagram. For example, in a reliability block diagram for a communications system where the lines can operate in two directions, the use of mirrored blocks will facilitate realistic simulations for the system maintainability and availability. In the RBD shown next, the bi-directionality of Com Line 3 and Com Line 6 is modeled using mirrored blocks. (A mirrored block is indicated by a grey square in the upper left corner of the block.)

BlockSim RBD

* Where can I get information on the theory behind the calculations and analyses being performed in ReliaSoft's applications? 

In addition to the software User's Guide, ReliaSoft's standard software packages come with an additional reference book that explains the theory and background of the calculations being performed by the particular application. Many users have found these references to be an invaluable tool for increasing their understanding of the concepts behind the reliability analyses they regularly perform. These references can also be purchased separately from the software. For ordering information, see

Additionally, ReliaSoft provides free online versions of some of these reference books, which can be accessed from ReliaSoft's reliability engineering resources Web site at These include:


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