IBM SG24-7368-00 manual Block definition diagram

Traceability relationships can be deduced from the model using the structures created in operations analysis. Requirements information can also be deduced from joint realization tables.

Block semantics

As noted above, SysML provides a basic structural element called a Block, whose aim is to provide a discipline-agnostic building block for systems. Blocks can be used to represent any type of component of the system, for example, functional, physical, and human. Blocks assemble to form architectures that represent how different elements in the system co-exist.

Block definition diagram

The SysML Block Definition Diagram (BDD) is the simplest way to describe the structure of the system. It is the equivalent to the class diagram in UML. It is used to represent the system decomposition using for example associations and composition relationships. The BDD is ideal to display the features of a block, such as its properties, and operations. SysML allows blocks to own special types of properties: Block properties and distributed properties.

￿Block properties impose additional constraints on classic UML properties, and can for instance own a SysML value type. Value types are designed to hold units (for example, physical units) and dimensions.

￿Distributed properties let the user apply a probability distribution to the values of the property. SysML proposes model libraries for possible values of units, dimensions, and probability distributions.

In Figure 7-3we show a BDD for the RSW. For the sake of readability of the diagram, we do not render the associations between the sub-systems and the Rain Sensing Wiper element, although these associations exist in the model. Instead we use an illustrative box around each set of components (composite and external) and a black diamond shape over the composite component as a visual clue for composition. The main components of the RSW are an interface to actuate the wiper, an electronic control unit, a sensor and the windshield

element. Both the interface and the windshield can exist in the car with or without the RSW (In SysML they are so-called reference properties).

￿The properties and the operations for each block are visible in Figure 7-3.Properties (more precisely SysML block properties, shown using the stereotype <<blockProperty>>) are used to model the physical characteristics of the components. The operations (called sometimes services) represent the functional aspects of the system.

150Model Driven Systems Development with Rational Products