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PLC Automation Consultancy & Training | Liam Bee

Learn PLC automation properly, through structured courses, deep technical guides, and industrial consultancy.

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CODESYS 3.5

  • CODESYS PLC Course – Structured Programming With DUTs

    CODESYS PLC Course – Structured Programming With DUTs

    Liam (Site Owner) Liam (Site Owner)

    Estimated Time: 4 Hours

    Difficulty: 🔴🔴⭕⭕⭕

    Number of lessons: 29

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In this lesson, we focus on output request management and why it is a critical part of building robust and scalable sequence based control systems. This lesson steps away slightly from pure code walkthroughs and first uses visual explanations to clearly show the problem that request handling is designed to solve.

The core challenge discussed is how to safely manage multiple sequences that need to control the same asset at the same time. Examples include filtration, wash, fill, and shutdown sequences all needing to request valve movements without overwriting each other or forcing sequences to stop unnecessarily.

You are shown why directly writing to a single valve command from multiple sequences does not work reliably in ladder or structured text, and how scan order can cause one sequence to unintentionally override another. This leads into the introduction of the request handler concept.

The request handler is explained as an intermediate layer that aggregates multiple requests into a single clean command for the asset manager. This allows multiple sequences or even subsequences to request control of the same valve without interfering with each other. It also provides a controlled place to apply interlocks, shutdown conditions, or project specific behavior without changing the upstream sequence logic.

The lesson then walks through real examples from the filtration and wash sequence managers. You see how individual requests such as fill mode, top wash, backwash, air scour, and shutdown are grouped into structured request interfaces. These requests are then interpreted by dedicated request handler logic to produce the final output commands.

Shutdown handling is also covered, showing how close requests override open requests in a controlled and predictable way. This ensures assets behave safely during normal shutdowns, fault shutdowns, and manual interventions.

The final part of the lesson explains how outputs from multiple sequences are brought together in a higher level output command handler before being passed into the device control layer. This extra layer allows filtration and wash sequences to coexist while still maintaining clear ownership of assets.

Throughout the lesson, the emphasis is on standardization with flexibility. Sequences and asset managers remain consistent and reusable, while request handling provides project specific control without breaking standard designs. This approach makes systems easier to maintain, scale, and adapt across multiple sites.

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Siemens TIA Portal – Asset Oriented Programming - Output Request Management
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Latest Tip

When a Function Block is called, input values are When a Function Block is called, input values are copied from the calling environment into the block. Any changes made to those inputs exist only inside the block during execution.

Outputs work in the opposite direction. The block writes to its outputs internally, and once execution finishes, those values are copied to the external variables connected to them.

This means outputs do not track external changes after execution. If another part of the program modifies the same variable later, the Function Block does not automatically see that change.

Recognizing this copy behavior helps prevent incorrect assumptions about data flow and avoids logic that appears to work inconsistently across scans.

Watch more about this in The Complete Guide To TIA Portal V20 on YouTube: https://www.youtube.com/watch?v=VLO4iY9zqvY

#AlwaysLearning #Siemens #TIAPortal #Automation #PLC #HMI #DoAndGrow

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PLC Automation Consultancy & Training | Liam Bee

Learn PLC automation properly, through structured courses, deep technical guides, and industrial consultancy.

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