SysML Block Definition, Internal Block & Package Diagrams

Instructor: Martin Gibbs

Martin has 16 years experience in Human Resources Information Systems and has a PhD in Information Technology Management. He is an adjunct professor of computer science and computer programming.

In this lesson, we will cover the structural components of a SysML document. Package diagrams, block definitions, and internal block definitions are used to show objects and relationships within a system design document.

SysML Diagrams

In order to build anything, be it a desk or a complex computer system, you need some sort of diagram or schematic to understand what you are building. In addition, an understanding of the materials is key. How many boards? What kinds of nails or screws? Will the desk have a drawer?

SysML is a visual modeling tool that provides the meaning behind a system. It includes behaviors and objects (the boards and the screws) and how everything interacts. There are three types of diagrams that have to do with these objects: Package, Block, and Internal Block diagrams.

The parts of the SysML that we will be discussing are illustrated in blue below. They are part of the structure of the system, the parts that make it all work.

sysML diagram overview

Package Diagram

Let's start with the package diagram. In our example, we will be looking at parts of a train, a train car and its coupling and locking mechanisms. Although the example is specific, you can see how SysML can be expanded to describe the entire train and all its component parts.

A package diagram is an organizational tool that groups the objects you're working with. For example, you could create a package diagram that organizes the model by the type of diagram included (such as requirements, behavior models or object diagrams), or by system hierarchy (by system feature or at the enterprise level).

Another way to organize the package is by a view. This shows the system from the customer perspective. This is an example of a package model for our train car system, which includes the system components as well as a view.

sysML package diagram

Block Definition Diagram

Once we have the package diagram, it's time to start digging into the details of our structure. Just as a schematic for a bridge will have building blocks, so does a SysML document. A block definition diagram is like a box in a flowchart. It describes a fundamental construction component of a system or a building block. It can indicate hardware, software, data, procedures or people.

This block definition diagram shows examples of a few blocks. The TrainCar has two dependent blocks, a coupling block and a lock mechanism block (these are used to connect/lock the train cars together). There is also a block to indicate the controlling mechanism that the conductor uses.

SysML Block Definition Diagram

A few points about the diagram:

  • The block diagram is labeled with the package name and the identifier bdd (block definition diagram).
  • Every block is labeled with the keyword <<block>>.
  • There are relationships drawn between the blocks. The CarCoupling and LockMechanism blocks have a relationship with the TrainCar block. In fact, notice the filled-in diamond at the end of the line. It means that these two blocks can't exist without the train car.
  • The LockMechanism can be accessed by the conductor, so there is flow between these blocks.

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