Glossary · Systems engineering and MBSE
Sequence diagram
German: Sequenzdiagramm
In SysML and UML, a sequence diagram is a behavior diagram that shows the interaction between elements as a time-ordered exchange of messages along their lifelines.
- Systems engineering
- Data and interfaces
In one sentence
A sequence diagram shows how elements interact over time as an ordered exchange of messages along lifelines.
Example
A sequence diagram shows the MES sending an order to the line PLC, the PLC requesting a recipe from the batch server and confirming the start back to the MES.
How it applies
- Engineering: Sequence diagrams clarify communication between controllers, HMIs, MES and devices, including timeouts and error responses.
- Integration: They are a precise way to agree interface behavior with suppliers; each message should match the Interface specification.
- Documentation: Integration and communication manuals benefit from sequence diagrams for handshakes; the documentation team should keep them in sync with the protocol version they describe.
- Testing: Sequence diagrams translate directly into test cases for communication, including negative cases such as lost messages, late replies and restart during a handshake.
Sequence diagram vs. activity diagram
A sequence diagram emphasizes who sends which message to whom and when. An Activity diagram emphasizes the flow of actions and decisions.