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

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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.

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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.

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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.

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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 .

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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.

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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.

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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

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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.

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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.