> For the complete documentation index, see [llms.txt](https://infosys.selmotech.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://infosys.selmotech.com/selmo-solution/selmo-solution-en/selmo-documentation/4-structure-of-the-machine-static/sequence.md). # Sequence **Goal:** Core logic\ **Content:** * What is a sequence? * State machine * Area of responsibility * Example sequence ### 1. Why are there sequences? In classic PLC programs, sequences, safety logic, operation and diagnostics are often mixed together.\ Logic arises implicitly in the code – not as an explainable model. This leads to typical problems: * unclear responsibilities * hard-to-follow sequences * high dependence on the programmer * lack of formal description of machine behavior **The sequence exists to solve this problem.** It separates **functional responsibility** clearly from: * Operation * Safety * implementation *** ### 2. What is a sequence? A **Sequence** is a **deterministic behavior model** for a clearly delimited functional task of a machine. A sequence: * describes **what** happens – not how it is programmed * is a **State machine** * has a clear start, run and end state * is self-contained and unambiguously responsible Examples of typical sequences: * clamping a workpiece * processing cycle * positioning an axis * inspection procedure * releasing and freeing > **One task – one sequence.** *** ### 3. Classification in the overall Selmo model The sequence is part of a clear hierarchical structure: * **Plant** – entire machine / plant * **hardware zone** – operating and responsibility framework * **Sequence** – functional sequence responsibility * **Zone** – connection to the real technology The sequence itself: * decides **not**, whether it may start * manages **no operating modes** * knows **not UI logic** It describes exclusively **functional behavior**. *** ### 4. Structure of a sequence A sequence is only fully defined when all relevant layers are described: * **Logic layer**\ describes the logical flow via states and transitions * **System layer (bit control)**\ defines the expected behavior of the zones in each state * **Parameter layer**\ contains temporal, position-related or quantity-dependent values > Only the interplay of these layers makes a sequence unambiguous, testable and deterministic. *** ### 5. Sequence and logic layer (overview) The logic layer describes the flow of a sequence as a state model. Essential principles: * A state describes **a situation**, not an action * transitions occur only when conditions are met * The sequence is uniquely determinable at any time Details on state types and logic elements are in the chapter **Logic layer** . *** ### 6. Sequence and system layer (bit control) The sequence itself defines **states** –\ the **behavior of the real technology** is described via the system layer. The following applies: * Each zone is evaluated in every state * The expected or allowed behavior is explicitly specified * There are no implicit assumptions The complete definition of the operands and their meaning is provided in the chapter **Bit-Control**. *** ### 7. Behavior of a sequence in operation A sequence is at any time in **exactly one state**. Regardless of the operating mode, the following applies: * States remain valid * Expectations remain active * Monitoring remains active In operation this means: * States are met or not met * Deviations are unambiguously localizable * Reactions occur deterministically A sequence knows **no special cases** for manual or automatic operation –\ it always works in the same state space. *** ### 8. Distinction from other elements To clarify: * A **Sequence** is **not a hardware zone**\ → it does not control operation or start release * A **Sequence** is **no zone**\ → it does not represent concrete technology * A **Sequence** is **not a safety function**\ → safety arises through modeled monitoring, not through the sequence * A **Sequence** is **not a code module**\ → it is a formal behavior model *** ### 9. Typical modeling errors in sequences Common mistakes are: * sequences with too large functional responsibility * technical states instead of logical states * mixing sequence and safety logic * operating mode logic inside the sequence * implicit assumptions instead of explicit modeling A good rule of thumb: > **If a sequence cannot be explained in one sentence, it is too large.** *** ### 10. Summary A sequence is: * the central functional unit in Selmo * a deterministic state model * clearly delimited and responsible * independent of operation, UI and implementation It forms the basis for: * clean sequences * automatic diagnosis * traceable documentation * safe extensibility > **Whoever understands the sequences, understands the machine.** --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. 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