Hardware zone

Objective: Operating modes & control Content:

• Manual / automatic

• Start logic

• "Any sequence ready"

• Distinction from sequence

1. Why are there hardware zones?

In classic machine controls, operating modes, start logic and approvals are often implemented globally or distributed in the code. The consequence are typical problems:

• unclear responsibility ("Who is allowed to start what?")

• mixed process and operating logic

• difficult-to-control sub-systems

• unclean shutdown in case of errors

The hardware zone exists to clearly separate operation and responsibility from the process.

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2. What is a hardware zone?

A hardware zone is the overarching operating and responsibility area for one or more sequences.

A hardware zone:

• encapsulates a real part of the plant

• provides operating modes (manual / automatic)

• manages start and approval logic

• monitors overarching states

The hardware zone does not decide what happens, but whether it is allowed to happen.

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3. Classification in the overall Selmo model

The hardware zone is part of the hierarchical structure:

• Plant – entire plant

• hardware zone – operation & responsibility

• Sequence – functional process

• zone – technical significance

Important:

• Sequences operate within a hardware zone

• A hardware zone contains no process logic

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4. Area of responsibility of the hardware zone

The hardware zone is responsible for:

• operating mode of the contained sequences

• enabling or blocking of automatic mode

• central start behavior

• overarching shutdown reactions

It is not responsible for:

• process details

• state logic

• technical signal evaluation

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5. Operating modes of the hardware zone

A hardware zone typically knows two operating modes:

Manual operation

• Operator influences zones manually

• Sequences remain active

• Logic and monitoring remain fully valid

Automatic operation

• System drives the process

• Sequences run autonomously

• Transitions occur automatically

Sequences themselves do not know an operating mode – they are framed by the hardware zone.

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6. Automatic enablement

Automatic operation alone is not sufficient. In addition, each hardware zone requires an automatic enablement.

Characteristics of the automatic enablement:

• consciously activated by the operator

• time-defined (e.g. hold pressed)

• can be revoked at any time

The automatic enablement is revoked in case of:

• interlock errors in a sequence

• CMZ errors at HW zone or plant level

The hardware zone is the guardian of the automatic mode.

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7. Start logic in the hardware zone

The hardware zone takes over the central start decision:

• it recognizes which sequences are ready to start

• it starts sequences selectively or together

• it prevents inadmissible starts

Typical principle:

• "Any Sequence ready to start"

• Start occurs onlyif all conditions are met

Important:

The hardware zone coordinates – it does not interpret a process.

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8. Relationship hardware zone ↔ sequence

For clear separation:

• hardware zone

  • provides operating mode

  • allows or forbids start

  • stops on overarching errors

• Sequence

  • describes the process

  • knows its own state

  • reacts deterministically to deviations

No layer takes over the task of the other.

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9. Error and shutdown behavior

The hardware zone reacts to errors hierarchically:

• Sequence error → affects only this sequence

• Hardware zone CMZ → stops all sequences of this HW zone

• Plant CMZ → stops the entire plant

The following applies:

• no undefined intermediate states

• no partial continued running

• clear, reproducible reaction

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10. Hardware zone and operator

The operator interacts with the hardware zone via:

• selection of operating mode

• start

• reset / enable

The operator:

• influences no logic

• bypasses no monitoring

• controls no processes directly

The hardware zone is the operator interface to the machine, not to the logic.

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11. Distinction from other elements

For clarification:

• Hardware zone ≠ Sequence → no process description

• Hardware zone ≠ zone → no technical significance

• Hardware zone ≠ CMZ → CMZ is a monitoring function

• Hardware zone ≠ Plant → it is part of the overall plant

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12. Typical errors in dealing with hardware zones

Common mistakes are:

• process logic in the hardware zone

• hardware zones that are too large

• operating modes per sequence

• mixing safety and operation

• global start logic without structure

A good rule of thumb:

If a hardware zone knows the process, it is modeled incorrectly.

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13. Summary

A hardware zone is:

• the operating and responsibility frame of a part of the plant

• responsible for manual / automatic and approvals

• strictly separated from process and technology

• a prerequisite for scalable, safe machines

The hardware zone brings order to operation – so that the logic can remain clean.