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Blocks are modular units of system description.

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ExampleBlock in a Block Definition Diagram (BDD)
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OMG Systems Modeling Language (SysML) 1.6
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About Object Management Group copyright in text extracts quoted from OMG specifications for educational purposes
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Block
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Blocks are modular units of system description.
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Each block defines a collection of features to describe a system or other element of interest. These may include both structural and behavioral features, such as properties and operations, to represent the state of the system and behavior ...
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Each block defines a collection of features to describe a system or other element of interest. These may include both structural and behavioral features, such as properties and operations, to represent the state of the system and behavior ...
Blocks provide a general-purpose capability to model systems as trees of modular components.
The specific kinds of components, the kinds of connections between them, and the way these elements combine to define the total system can all be selected according to the goals of a particular system model.
These include modeling either the logical or physical decomposition of a system, and the specification of software, hardware, or human elements.
Parts in these systems may interact by many different means, such as software operations, discrete state transitions, flows of inputs and outputs, or continuous interactions.
The Block Definition Diagram in SysML defines features of blocks and relationships between blocks such as associations, generalizations, and dependencies.
It captures the definition of blocks in terms of properties and operations, and relationships such as a system hierarchy or a system classification tree
The Internal Block Diagram in SysML captures the internal structure of a block in terms of properties and connectors between properties.
A block can include properties to specify its values, parts, and references to other blocks.
Ports are a special class of property used to specify allowable types of interactions between blocks ...
Constraint Properties are a special class of property used to constrain other properties of blocks...
Various notations for properties are available to distinguish these specialized kinds of properties on an internal block diagram.
A property can represent a role or usage in the context of its enclosing block. A property has a type that supplies its definition.
A part belonging to a block, for example, may be typed by another block. The part defines a local usage of its defining block within the specific context to which the part belongs.
For example, a block that represents the definition of a wheel can be used in different ways. The front wheel and rear wheel can represent different usages of the same wheel definition.
SysML also allows each usage to define context-specific values and constraints associated with the individual usage, such as 25 psi for the front tires and 30 psi for the rear tires.
Blocks may also specify operations or other features that describe the behavior of a system
SysML blocks are based on UML classes as extended by UML composite structures. Some capabilities available for UML classes, such as more specialized forms of associations, have been excluded from SysML blocks to simplify the language.
SysML blocks always include an ability to define internal connectors, regardless of whether this capability is needed for a particular block.
SysML Blocks also extend the capabilities of UML classes and connectors with reusable forms of constraints, multi-level nesting of connector ends, participant properties for composite association classes, and connector properties.
SysML blocks can be used throughout all phases of system specification and design, and can be applied to many different kinds of systems.
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