Logic layer

Model the sequence

The logic layer describes the logical sequence of a sequence. It models states and transitions, however no technology and no signals.

The goal of the logic layer is to describe the sequence:

• understandably

• unambiguously

• deterministically

to describe.

The logic layer answers the question: In which logical situation is the process – and how does it continue?

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Basic principle of the logic layer

In the logic layer applies:

• a sequence is always in exactly one state

• transitions only occur when conditions are met

• states describe situations, not actions

• technology is not described in the logic layer

The logic layer is therefore:

• independent of hardware

• independent of PLC

• completely process-logical

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state

meaning

A state describes a logical situation in the sequence.

Examples:

• workpiece inserted

• clamping established

• processing completed

A state:

• contains no commands

• contains no actuator movements

• only defines, when it is considered fulfilled

A state describes, that something is the case – not, how it is brought about.

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When a state makes sense

• when a clear process situation exists

• when several conditions must be met together

• when the state can be named unambiguously

When a state does not make sense

• for short technical intermediate actions

• for pure actuator movements

• for PLC-internal auxiliary states

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timer state

meaning

A timer state is a state with a temporal condition.

It is considered fulfilled:

• when the underlying state is active

• and a defined time has elapsed

Examples:

• hold time after clamping

• waiting time before a check step

• cooling time

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When a timer state makes sense

• when time is part of the process

• when time logically belongs to the state

• when the sequence must be reproducible in a time-dependent way

When a timer state does not make sense

• to compensate for unstable technology

• as a substitute for missing feedback

• for purely technical delays

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decision

meaning

A decision describes a logical branching in the sequence.

It decides:

• which subsequent path is taken

• based on clear conditions

Examples:

• test result OK / NOK

• option activated / not activated

• repeat / continue

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When a decision makes sense

• when the process has multiple logical paths

• when the decision is technically justified

• when the decision must be explicitly traceable

When a decision does not make sense

• to map technical boundary conditions

• for error compensation

• as a substitute for clean state definitions

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repeater

meaning

A repeater repeats one or more states.

It is used to:

• loops

• repetitions

• test cycles

formally map.

Examples:

• multiple testing

• retry attempts

• iterative processing step

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When a repeater makes sense

• when repetition is part of the process

• when number or condition is defined

• when the sequence should remain structured

When a repeater does not make sense

• for uncontrolled endless loops

• to compensate for unstable processes

• when repetition is not technically justified

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jump

meaning

A jump consciously leaves the linear sequence and jumps to another state.

There are:

• conditional jumps

• unconditional jumps

Examples:

• abort paths

• targeted backward jumps

• exception handling

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When a jump makes sense

• for clearly defined special cases

• for abort or reset paths

• when the sequence would otherwise become confusing

When a jump does not make sense

• as a substitute for clean structure

• for normal sequence

• to circumvent modeling errors

Jumps are powerful – and should be used sparingly.

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sequence cross

meaning

sequence cross connects multiple sequences logically with each other.

It enables:

• synchronization

• dependencies between sequences

• coordinated transitions

Examples:

• robot waits for station

• station waits for transport

• parallel processes with synchronization point

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When sequence cross makes sense

• in modular plants

• with parallel sequences

• when sequences should remain logically separate

When sequence cross does not make sense

• for artificial splitting of simple sequences

• with unnecessary fragmentation

• when one sequence would be logically sufficient

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important distinction

The logic layer:

• describes not

  • signals

  • actuators

  • safety logic

  • operation

These aspects belong:

• in the system layer

• in zones

• in bit control

• in CMZ / MXIC

The logic layer describes the sequence – not its technical implementation.

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summary

The logic layer:

• is the logical backbone of every sequence

• describes the sequence in states

• separates process logic from technology

• enforces clarity and determinism

The logic elements are tools – their quality is shown in the conscious use.

A good logic layer is simple, explainable and free of technical details.