Glossary · Distributed automation and IEC 61499
Service sequence
Also known as: Service sequence diagram, Time-sequence diagram
German: Dienstsequenz
In IEC 61499, a service sequence is a time-sequence diagram that specifies the behavior of a service interface function block as ordered exchanges of events and data between the application and the underlying service, for normal operation and error cases.
- IEC 61499
- Standards
In one sentence
An IEC 61499 service sequence is a time-sequence diagram specifying how a service interface block exchanges events with a service.
Example
The service sequence for a client block shows REQ from the application, a request to the network, the response and CNF back to the application, plus the timeout case.
How it applies
- Engineering: Service sequences describe what a service interface block does from the user's point of view: which input events it accepts, which interactions follow and which output events report results. They cover initialization, normal requests, indications and failures.
- Integration: Service sequences serve as a contract between application developers and device implementers. Deviations from them are a common source of interoperability problems.
- Testing: Each service sequence can be turned into a test scenario, including the error paths such as a Timeout.
- Documentation: Service sequences are a compact form of interface documentation, similar to sequence diagrams in UML or SysML. Include them in block reference topics, with a short text explaining each sequence for readers unfamiliar with the notation.
Service sequence vs. ECC
An ECC defines the internal state machine of a basic function block. A service sequence defines the externally visible interaction of a service interface block, without prescribing its implementation.